Contents
Plenary

2-7

Symposium A

8-62

Symposium B

63-115

Symposium C

116-191

Symposium D

192-223

Symposium E

224-310

Symposium F

311-401

Symposium G

402-430

Symposium H

431-495

Symposium I

496-550

Symposium J

551-567

Symposium K

568-589

Symposium L

590-607

Symposium M

608-631

Symposium N

632-659

Symposium O

660-716

MMM 2018

1Plenary(Plenary)
Amorphous Materials on the Meso-scale: Achieving Experimental
Length and Timescales
Christopher A. Schuh
Department of Materials Science and Engineering, MIT, USA

The defining characteristic of metallic glass is disorder, with the fundamental unit of metallic
glass plasticity being the Shear Transformation Zone (STZ), a local cluster of 20-100 atoms
rearranging to accommodate shear strain. While the energy scales of STZs are wellunderstood, deterministic relationships between disordered atomic structures and theirrespective mechanical responses have proven elusive (in stark contrast to, for example, thepredictable response of a dislocation to a stress field). In lieu of such detailed deterministic
relationships, we turn to stochastic modeling based on the energetics of STZ activation. This
talk will review the development and current status of the class of meso-scale models
referred to as Shear Transformation Zone Dynamics. These models calculate STZ
activation rates by transition state theory with energy barriers modeled using Eshelby’s
continuum solutions for isotropic inclusions. In these models individual STZs interact
through their elastic fields, which are evaluated by the finite element method, and thesample is evolved under the control of a kinetic Monte Carlo algorithm. We particularly
review our most recent developments incorporating dynamic structural state variables andimproved numerical methods into a new generation of STZ dynamics simulations. With
these advances, STZ dynamics simulations are now approaching the level where they canbe compared one-to-one (both in terms of length and time scales) with physicalnanomechanical experiments.

2

MMM 2018

(Plenary)Multiscale Modeling and Realization of Photo-responsive Polymers
Maenghyo Cho
School of Mechanical and Aerospace Engineering, Seoul National University, KOREA

Liquid crystalline polymers which contain photochromic chromophores can show
macroscopic mechanical deformation under light irradiations. The light-induced shapechange of the photo-responsive polymers (PRPs) comes from the trans-to-cis, or
cis-to-trans isomerization of the mesogens, and it can be utilized to the microscale
opto-mechanical actuation device. However, it is difficult to analyze and precisely predict the
deformation because the theoretical approach requires a comprehensive knowledge ofbroad, interdisciplinary physical regimes that range from photochemical reaction kinetics tomanipulating continuum scale deformations. Here, we develop a new multiscale model
which integrates light input conditions, mesogen alignment, and continuum polymer
deformations through sequential multiscale framework combining the DFT(density
functional theory), MD(molecular dynamics), and continuum FE(finite element) method. In
addition, the multiscale approach is applied to design the photo-mechanical behavior of the
PRP nanocomposites with the consideration of the opto-mechanical coupling effect andmicroscopic interaction between the PRP matrix and fillers. This integrated framework canhelp to design the PRP and its composites.

MMM 20183

(Plenary)
Plasticity in crystals and glasses: from the atoms up
David Rodney
Institut Lumière Matière, University of Lyon, France

Plasticity in crystalline metals is probably the most classical multiscale process, aroundwhich the Multiscale Materials Modeling community first organized at the end of the 1990s.
Plasticity in glasses shares many similarities with their crystalline counterparts. In particular,
in both cases, plasticity starts at the atomic scale, involving the motion of dislocation cores incrystals and shear transformation zones in glasses. In both cases also, elementary plastic
events interact and organize at the mesoscale through elasticity. There are however specificchallenges. In crystals, there is currently a need for quantitative and predictive data, which
often require to start with a first principles description of atomic interactions. By way ofcontrast, in glasses, we are still in need of phenomenological information, which can beaddressed with less computationally intensive models. But we are then faced with the highcomplexity of the configuration space and the energy landscape of disordered materials,
making it difficult to characterize and predict simply even the most elementary plastic events.
The aim of this talk will be to illustrate the challenges, recent progress and opportunities in
the field of multiscale modeling of plasticity and to discuss through selected examples thelinks between atomic and mesoscopic descriptions of plasticity.

4

MMM 2018

(Plenary)
Grain Boundary Sliding, Fracture and Dislocation Motion in Ceramics
Yuichi Ikuhara
The University of Tokyo/Japan Fine Ceramics Center, Japan

Ceramics have been widely used for structural applications because of their superiormechanical properties at high temperatures. It has been known that the behavior of GB
sliding is strongly dependent on the GB characters such as misorientation angle between
two adjacent crystals and GB plane, however, such effect has not been clarified yet. In
addition, this effect in much influenced by dopant segregation at GBs. In this study, in orderto clarify the atomistic mechanisms of GB sliding and its dopant effect, bicrystal studies havebeen performed to find the relationship between the atomic structures, chemistry and GB
sliding behavior of Al2O3 ceramics. Several kinds of bicrystals including GBs with specificgeometrical configuration were fabricated, and some of them were doped by rare-earth
elements to enhance the GB segregation.
It has been reported that several oxides can be plastically deformed even at R.T. by
dislocation slip like metals. So far, many experimental investigations have been tried forunderstanding the dislocation-grain boundary interaction, but these experiments were
mostly carried out statically, and the fundamental processes are still not well understood yet.
In this study, the nanoindentation experiments were conducted for SrTiO3 crystals theirbicrystals inside TEM. Several kinds of TEM specimens for in situ nanoindentation
experiments were prepared, that are single crystals and bicrystals including various types ofGBs. The SrTiO3 single crystals were indented with the sharp diamond tip and successfully
observed the dislocation dynamics. In the case of the GBs, the interaction between the
introduced lattice dislocations and the GBs were directly observed. The dislocation-GB
interaction and its dependence on the GB characters will be discussed in detail.

MMM 2018

5

(Plenary)
Property optimisation of titanium alloys based on phase stability
evaluation and microstructure design
Rui Yang
Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China

Titanium alloys are extensively used in aircraft engines and have emerged as ideal
materials for a wide range of biomedical implants. Applications of titanium alloys in suchreliability sensitive components/devices require a thorough understanding and precise
control of properties which are determined by the rich variety of phase transformations andmulti scale microstructures. In many cases experimental investigations have already pointed
to one or several key processes or parameters that dominate the properties and service
performance of the alloys. Under such circumstances the computational and modelling
tasks are relatively well defined and the results can be critical in clarifying and solving the
problems. This talk will review work conducted in the past few years using such an
experiment plus computation approach, taking examples from near-alpha titanium alloys
and titanium aluminides for aero engine applications and from beta-type titanium alloys forbiomedical use. The topics to be covered include alloying effects on the alpha phase andtheir relations to creep resistance and cold dwell fatigue propensity, alloying effects, phasestability and deformation mechanisms of high strength titanium aluminides, and metastablephases and stress induced transformations in beta-type titanium alloys.

6

MMM 2018

(Plenary)Integrated Earthquake Simulation Enhanced with High Performance
ComputingMuneo Hori
Earthquake Research Institute, The University of Tokyo, Japan

Integrated earthquake simulation (IES) is a seamless simulation of earthquake hazard,
disaster and disaster response for an urban area. A set of numerical analysis of Earth
Science, earthquake engineering and social sciences are used to carry out this simulation,
together with analysis models of the target area that are automatically constructed by using
available data of undergrounds, structures, and social activities. High performancecomputing is essential for the simulation with higher spatial and temporal resolution and forthe evaluation of uncertainties related to a possible earthquake scenario, state of structuresand social activities; capability computing is used to solve a large-scale model of an urban
area, and capability computing is used to solve numerous cases and analysis models for theuncertainties. This lecture presents the current state of IES that uses K computer and othersupercomputers. The following topics are focused: 1) key numerical techniques that are
implemented into finite element method for scalable parallel computation; 2) particle
simulation for fluid and human being; ad 3) the automated model construction that takeadvantage of various distinct data sources. Several examples of IES that are made for
actual cities in Japan are presented. Discussions are made for the future use of higher
fidelity models of structures in order to make more reliable seismic response analysis.

MMM 2018

7

Symposium A(A1 invited)
Atomistic to continuum: coarse-graining in and out of equilibrium*Celia Reina1, Xiaoguai Li1, Peter Embacher2, Nicolas Dirr2, Johannes Zimmer3
1. University of Pennsylvania, 2. Cardiff University, 3. University of BathWe will describe various spatio-temporal coarse-graining procedures to understand the thermodynamic
behavior and evolution of general atomistic/particle models. We will first revisit the equilibrium setting,providing further insights into the thermodynamic potentials and the interplay between temperature andthe mechanical behavior. Then, we will move to the non-equilibrium setting, where we will discuss how tocompute macroscopic dissipative evolutions, in particular parameters. In all cases, the results will bevalidated with full resolution particle simulations.

8

MMM 2018

(A1 oral)
Finite-temperature Localized Stress and Strain for Atomic Models*Ranganathan Parthasarathy1, Anil Misra2, Lizhi Ouyang1
1. Tennessee State University, 2. University of Kansas

Homogeneous deformation of an ordered crystalline solid at finite temperature can cause non-affine
transformation of atomic trajectories, particularly when the inter-atomic potential is anharmonic. In such acase, continuum measures based on affine transformation of trajectories are insufficient to ensure
energetic equivalence between the atomic and continuum scales and non-classical stress measures are
required. These measures also need to be localizable to obtain continuum stress and strain spatiallyresolvable to atomistic scales. To this end, the total work done on an atom under deformation isdecomposed into the work corresponding to changing its equilibrium position and work corresponding tochanging its second moment about equilibrium position. Correspondingly, we define two kinematic
variables: a deformation gradient tensor and a vibration tensor, and derive their stress conjugates, termed
here as static and vibration stresses, respectively. The proposed approach is validated using MD
simulation in NVT ensembles for fcc aluminum subjected to uniaxial strain at high temperatures up to
0.9Tm. Highly non-linear and non-affine evolution of second moments are observed in the elastic portionof the high temperature-high tensile strain regime, in which the conjugate pair of vibration stress andvibration tensor contribute significantly to free energy change, particularly as the material approaches
elastic instability through violation of the Cauchy-Born rule. In the elastic portion of the compressive
regime, the non-affinity of the second moments point to anomalous phonon dispersion andnon-monotonic variation of average group velocities with strain, which resembles experimental
observations in certain crystalline solids. The results are strongly relevant for developing finite
temperature continuum theories based on discrete simulations or inter-atomic potentials.

MMM 2018

9

(A1 oral)
Uncertainty Quantification for Classical Effective Potentials*Sarah Longbottom1, Peter Brommer1
1. School of Engineering, University of Warwick

Effective potentials are an essential ingredient of classical molecular dynamics (MD) simulations. Little isunderstood of the errors incurred in representing the complex energy landscape of an atomic
configuration by an effective potential containing considerably fewer parameters. The probabilistic sloppy
model method [1] has been implemented in the potfit force matching code [2]. This introduces
uncertainty quantification into the interatomic potential generation process. Uncertainties in the effective
potential are propagated through MD to obtain uncertainties in quantities of interest, which are ameasure of the confidence in the model predictions.
We demonstrate the technique using three potentials for nickel: two simple pair potentials, Lennard-Jonesand Morse, and a local density dependant EAM potential. A sloppy model fit to DFT reference data isconstructed for each potential to calculate the uncertainties in lattice constants, elastic constants andthermal expansion. These can be used to show the unsuitability of pair potentials for nickel. In contrast,with EAM we observe a decreased uncertainty in the model predictions. This shows that our method cancapture the effects of the error incurred in the potential generation process without resorting tocomparison with experiment or DFT, which is an essential part to assess the predictive power of MD
simulations.
Further work is in progress to create a new potential, with uncertainty quantification, for siliconheterostructures. The potential is tailored to describe thermal transport in specific morphologies which
are of interest as thermoelectric devices [3, 4].
[1] S. L. Frederiksen, et al., Phys. Rev. Lett., 93(16), 2004.
[2] P. Brommer, et al., Modell. Simul. Mater. Sci. Eng., 23(7), 2015.
[3] H. Karamitaheri, et al., J. Appl. Phys., 113(20), 2013.
[4] H. Karamitaheri, et al., J. Appl. Phys., 115(2), 2014.

10

MMM 2018

(A1 oral)
The role of null-lagrangians in the continuum interpolation of thelinear chain with hyper-pre-stress*Alexandre Danescu1
1. Ecole Centrale de Lyon

The simplest discrete one-dimensional model including elastic bulk and surface energy is the linear chain
with hyper-pre-stress. It is known that when NN and NNN "geometrically incompatible" interactions aretaken into account the qualitative behavior of the chain can fall into three generic cases depending on theratio of the stiffnesses of the NN and NNN interactions. We adress here the general question of finding anexact continuum interpolant of the discrete solution. Guided by the qualitative aspects of thecharacteristic equation we chose the Mindlin model is an appropriate candidate. Starting from a generalform of the elastic energy in the continuum we show that null-lagrangians are mandatory in order toobtain an exact interpolant.

MMM 2018

11

(A1 oral)
Practical Time Averaging of nonlinear dynamics
Sabyasachi Chatterjee1, *Amit Acharya1, Zvi Artstein2
1. Carnegie Mellon University, 2. The Weizmann Institute of Science

This talk aims to understand and exploit the slow time-scale behavior of rapidly evolving
microscopic dynamics. The microscopic systems considered are posed in terms of systems of nonlinear ordinary differential equations, not necessarily containing an a priori split into fast and slow
variables. Such a question arises naturally and ubiquitously in efforts to understand macroscopic
dynamics, on engineering time-scales, of well-accepted models of microscopic dynamics that, however, are not amenable to practical computing over the much, much larger macroscopic time-scalesof interest. This is because there is a vast separation of time scales involved between the dynamics
of the macroscopic variables of interest and the microscopic dynamics, and evolving the microscopic
dynamics directly fails to address the question of the macroscopic dynamics.
The methodology employed involves a computational scheme based on fundamental mathematical theory that a) defines appropriate `coarse' variables corresponding to the microscopic dynamics
that evolve in a stable manner on the coarse time scale; b) determines the equation of evolution forsuch variables; and c) defines a practically useful strategy for accurately initializing short bursts of
microscopic runs for the evolution of the slow variables, without special requirements on the natureof the microscopic dynamics.
We will illustrate the theory with examples that violate ergodicity and include both conservative anddissipative behavior. Depending on research progress between submission of this abstract and theconference, the coarse graining of discrete dislocation dynamics to a pde-based plasticity model without
relying on constitutive assumptions will be demonstrated. This latter effort is ongoing joint work with
Giacomo Po (UCLA), Xiaohan Zhang (Stanford) and Nasr Ghoniem (UCLA).

12

MMM 2018

(A2 invited)
Bridging the chasm between phenomenological theories andelectronic structure*Anton Van der Ven1
1. University of California Santa Barbara

The first-principles prediction of non-equilibrium materials processes remains a major challenge. The
evolution of a solid out of equilibrium is affected by intrinsic thermodynamic, mechanical and kinetic
properties that are often difficult if not impossible to measure accurately in isolation. Nevertheless, there
are many successful phenomenological theories of phase transformations and microstructure evolutionthat have been formulated in terms of thermodynamic free energies and kinetic transport coefficients.
One approach to treating non-equilibrium materials processes from first principles, therefore, is to predict
intrinsic thermodynamic, kinetic and mechanical properties starting with electronic structure theory.
However, the crucial role of temperature and entropy in most materials processes requires a rigorous
statistical mechanics approach. In this talk I will describe our recent efforts at bridging the gap between
phenomenological theories of materials and first-principles electronic structure predictions. The
approach relies on statistical mechanics methods that utilize effective Hamiltonians expressed in terms ofmathematically rigorous local descriptors to treat configurational, vibrational and electronic degrees of
freedom. Effective Hamiltonians are capable of extrapolating first-principles electronic structure
calculations within (kinetic) Monte Carlo simulations, thereby enabling the calculation of equilibrium andnon-equilibrium materials properties at finite temperature. I will illustrate how these approaches can beused to predict the dynamic evolution of battery electrodes and high temperature structural materials.

MMM 2018

13

(A2 oral)
Two-component Dirac-Kohn-Sham calculation for multiscale modelingof materials*Koichi Nakamura1,2
1. Kyoto Univ., 2. E-JUST

It is known well that some specific properties and characteristics of materials cannot be exactly simulated
without considering the relativistic effect, whereas general first-principles electronic-state calculation
technique for material systems has been established on the basis of the Schrödinger equation under
non-relativistic limit. The most fundamental idea to treat the relativistic effect is the electronic-statecalculation based on the Dirac equation from the viewpoint of the relativistic quantum mechanics, andthe effective methodologies, such as multiconfiguration Dirac-Fock and Dirac-Kohn-Sham (DKS) methods,
have been developed for leading to exact electronic states. The relativistic calculation based on the Diracequation originally uses 4-component spinors, which consist of 2 large components and 2 small
components. However, the calculation with 4-component spinors is computationally too expensive toperform for material systems practically. To avoid the calculation with 4-component spinors, sometheoretical treatments of calculation with 2-component spinors, such as zeroth-order regularapproximation (ZORA), infinite-order regular approximation (IORA), relativistic scheme by elimination ofsmall components (RESC), and high-order Douglas–Kroll transformation, have been developed in the fieldof molecular physics and chemistry. For the application of these methods to material systems under theperiodic boundary condition, some special techniques should be required to adopt the elimination ofsmall components to the basis set by plane wave or projector augmented wave. In this study, examples of
the DKS calculation with 2-component spinors by ZORA, IORA, and RESC methods shall be presented forsome typical material systems. The details of techniques how to eliminate small components of spinors
and the differences in specific properties and characteristics of target materials according to relativistic
methodologies will be discussed in the conference.

14

MMM 2018

(A2 oral)
Combination of Kinetic Monte Carlo Method and First Principles
Calculation to Explore Stable Structure of Solute Cluster in Al-Si BasedAlloys*Kenjiro Sugio1, Hiroshi Mito1, Yongbum Choi1, Gen Sasaki1
1. Hiroshima Univ.

Stable structure of solute cluster in Al-Si based alloys was investigated by combining Kinetic Monte Carlo(KMC) method and first principle calculation. The new KMC code was developed to simulate the behaviorof the added impurities in aluminum at finite temperature. The size of KMC cell was 20a0×20a0×20a0 or
20a0×20a0×40a0, where a0 is lattice constant of aluminum. Aluminum atoms were arranged in FCC
structure and the impurity atoms of 3at.% or 4at.% was introduced. Diffusion of impurities was performedby swapping two neighbor atoms which was selected randomly. After the swapping, total energy of the
KMC cell was calculated and compared with the total energy before the swapping. The stable structure of
the impurity cluster was explored with repeating this process. The total energy was calculated by adding
up the atom energies which was calculated by dividing the total energy by number of atoms in the cluster.
The total energy of the cluster was calculated by the first principle method. Because the atom energy
calculated once was recorded in the database, the number of the first principle calculation was reduced.
Quantum ESPRESSO was used to carry out the first principle calculation. The pseudopotential file of
Becke-Lee-Yang-Parr function was used for Al, Si, Mg, Cu, Zn, Zr, Ti and Ag. The k-point mesh based on
Monk horst-Pack (4×4×4) was used to integrate Brillouin zone and the cut off energy of plane wave was
30 Ry. The size of supercell was 2a0×2a0×2a0, where a0 is 0.404 nm. The cluster consisted of a central
atom and the first neighbor atoms in FCC was arranged in the center of the supercell. The energy of an
isolated cluster was calculated. Calculations for binary system (Al-Si, Al-Mg, Al-Cu, Al-Zn, Al-Zr, Al-Ti,
Al-Ag), ternary system (Al-Si-Mg, Al-Zn-Mg, Al-Si-Cu, Al-Mg-Cu,Al-Mg-Zr) and quaternary system(Al-Si-Mg-Cu, Al-Si-Mg-Zr, Al-Cu-Mg-Ag) were carried out.

MMM 2018

15

(A2 oral)
Electronic structure analysis of Fermi level instability in Fe-rich Si alloy*Chen Ying1, Arkapol Saengdeejing1, Tetsuo Mohri2
1. School of Engineering, Tohoku University, 2. Institute for Materials Research, Tohoku University

Fe-Si binary alloy has a variety of applications due to its excellent magnetic and mechanical properties.
Some drastic change in mechanical properties as Si concentration increasing to 9-10at.% has been
intriguing since a long time ago. We have conducted an integrated study on elastic properties in Fe-rich Si
alloy based on the electronic structure calculations incorporated with phonon vibration effect andthermal electrons excitation for a Si-doped bcc-Fe alloy up to 12.5at.%Si [1,2], as a part of a multiscale
simulation of the mechanical properties of Fe-Si [3]. Our calculations reproduced a non-monotonic
change of the elastic properties with Si concentration, showed a ductile to brittle transition behavior as
the Si content increases beyond 9.4at.%, which is agree with the well know experimental results, The
calculation further revealed that the Si concentration dependence of the elastic properties is originatedfrom a combination of magnetovolume effect at a low Si concentration up to 8.0 at.%, D03 ordering on therecovery of elastic properties at Si concentration’s being over 10.9at.%, and the instability of density ofstates at the Fermi energy right at the Si concentration with drastic degradation of elastic properties. The
present work aims at a thorough understanding on this Fermi level instability. We calculated the Fermi
surfaces at 9.4, 10.9 and 12.5at.%Si, and found that at 10.9 at.%Si, the Fermi surface just touches theboundary of the Brillouin zone which leads to a decrease of the energy of the electrons, further deceases
the elastic properties. This founding suggests that the Fermi lever stability/instability is related to the local
atomic configuration of doped atoms, then the re-distribution of band structure and furthermore therelative position of Fermi surface to the Brillouin zone, which might be the electronic structural origin of
the drastic change of the mechanical properties at some subtle Si concentrations in Fe-rich Si alloy.
References[1] A.Saengdeejing, Ying Chen, Ken Suzuki, Hideo Miura, Tetsuo Mohri, Computational Materials Science,70, 100-106, 2013.
[2] S. Bhattacharya, M. Kohyama1, S. Tanaka1, Y. Shiihara, A. Saengdeejing, Ying Chen and T. Mohri,
Mater. Res. Express 4, 116518 (2017)[3] T. Mohri, Y. Chen, A. Saengdeejing, et al., npj Computational Materials –Nature 3, 10 (2017)16

MMM 2018

(A2 oral)
From first-principles defect chemistry to device damage models ofradiation effects in III-V semiconductors*Peter A Schultz1, Harold P Hjalmarson1
1. Sandia National Laboratories

An atoms-to-circuits assessment of radiation damage in microelectronics begins with understanding
atomic displacement damage in a semiconductor material, the subsequent evolution of theradiation-induced defect populations in the material, and propagating this understanding into
defect-aware damage models for continuum-scale device simulations. The foundation of a multiscalemodeling framework to understand long-time, device-scale response is a quantitative description of thedominant atomic processes using quantum mechanical modeling. We describe the development of acomprehensive radiation-induced defect reaction network in Si-doped (n-type) and C-doped (p-type)
GaAs using density functional theory (DFT), identifying mobile species and consequent defect reactions,characterizing defect properties needed to formulate the chemistry needed in defect physics models fordevice simulations. In turn, the device simulations solve a set of drift-diffusion equations to evolve amodel chemistry, and predict the electrical response of device of a specified configuration and operating
conditions. The upscaling bridge from the atomistic description given by DFT and experimental defectspectroscopies to a device description of radiation response proves to have many challenges. We
describe the successes and also the remaining outstanding challenges in standing up a full first-principles
motivated hierarchical multiscale model of radiation damage in electronic devices. --- Sandia National
Laboratories is a multimission laboratory managed and operated by National Technology and Engineering
Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S.
Department of Energy's National Nuclear Security Administration under contract DE-NA0003525.

MMM 2018

17

(A2 oral)
How to model ordering processes in metallic hydrides? A
Tight-Binding Ising modeling proposal and its application to Zr-H
Paul Eymeoud1, *Fabienne Ribeiro1, Remy Besson3, Guy Tréglia2
1. Institut de Radioprotection et de Sûreté Nucléaire/PSN-RES/SEMIA/LPTM, 2. Centre Interdisciplinaire deNanoscience de Marseille, CNRS, 3. Unité Matériaux Et Transformations (UMET), CNRS UMR 8207, Université de Lille,
59655 Villeneuve D'Ascq

Modeling metal hydrides is of prime interest in the field of materials for energy, due to these material
potential uses as hydrogen storage devices or to their potential formation in nuclear reactors. In
particular, zirconium alloys are used in water-cooled nuclear reactor and can be embrittled by hydride
precipitation. This motivates developing theoretical tools to model, understand and predict the behaviorof zirconium hydrides, and to precise their links to local stresses and mechanical properties. In order toexplore the Zr-H phase diagram and to clarify dissolution/precipitation mechanisms and kinetic, one
needs thermodynamic simulations such as Monte Carlo approaches. They have to be grounded on anenergetic model with a good compromise between precision and numerical cost, and in particular, which
is able to characterize order-disorder phenomena on the interstitial H-gap subnetwork of metal hydrides.
To this aim, using the Generalized Perturbation Method based on Tight-Binding Coherent Potential
Approximation, we have derived an effective Ising Model, which describes the order energy of the FCC
zirconium hydride systems as a sum of interactions of HH pairs. We present here this general approachthrough the example of Zr-H. The process revealed the predominance of pairwise interaction between
third-order hydrogen neighbors. Ability of the pairwise interactions model to characterizehydrogen-vacancy sublattice order has been then established by confrontation to first-principle
approaches.

18

MMM 2018

(A3 invited)
Challenges and gaps in length and time scaling of dislocation models*David L McDowell1
1. Woodruff School of Mechanical Engineering, Georgia Institute of Technology

We consider multiple crystalline plasticity model constructs that address evolution of dislocation
structures over a broad range of length and time scales, from atomistic modeling and coarse-graining
strategies through discrete dislocation theory to reduced order generalized and local continuum models.
The predictive character of each construct is considered, along with the notion of uncertainty of modelingphenomena at various scales and for two-scale transitions, either concurrent or hierarchical in nature. In
each case, we list the set of phenomena that each model construct addresses. Challenges to modeling theevolution of the dislocation network are discussed, including the important role of the entropic barrier tocollective dislocation bypass of obstacles. Gaps and future challenges are summarized.

MMM 2018

19

(A3 oral)
From discrete to continuum dislocations and back: a two dimensional
study of microstructure and interaction energies.
*Hengxu Song1, Stefan Sandfeld1
1. TU Bergakademie Freiberg

Continuum dislocation dynamics models, which are proposed to conquer the restrictions of highcomputational cost that discrete dislocation dynamics models generally experience, have made great
progress in the past decades. However, it is inevitable that certain dislocation microstructure informationis ‘lost’ during the averaging/coarse graining process ('upscaling'), for example, dislocation short range
interactions. This information, however, is crucial for a correct prediction of the structure-property
relation in continuum simulations. We will show a new type of multiscale analysis which is based on theenergy density of 2D systems, where we identify the ‘missing’ information during coarse graining forarbitrary averaging resolution. This methodology can be directly generalized to 3D systems of curved
dislocations. Furthermore, based on our database, we will be able to benchmark a number of existing 2D
continuum formulations for dislocation interaction. Last but not least, we will show that our data base also
will be a useful starting point for recovering statistical information of (ensembles of) discrete dislocations
from continuum field data during 'downscaling'.

20

MMM 2018

(A3 oral)
Advances in microstructure prediction: a FFT-based Dislocation
Dynamics approach*Francesca Boioli1, Benoit Devincre1, Riccardo Gatti1, Laurant Dupuy2, Lionel Gélébart2
1. LEM, CNRS-ONERA, Chatillon, 2. SRMA, CEA Saclay

Discrete Dislocation Dynamics (DD) is a well-established simulation technique aimed at reproducing thecollective behavior of dislocations at the mesoscale. Despite the considerable progresses made in last
decades, DD simulations are still unable to precisely reproduce the microstructure of large poly-crystals
and, especially, of irradiated polycrystalline materials. An important step in the development of predictive
simulations of this class of materials is the improvement of the numerical capabilities of DD codes tomodel dislocation properties in large volumes representative of the materials microstructures.
Here we propose a promising strategy based on the coupling between two advanced simulation tools.
First, the Discrete-Continuous Model (DCM) is employed [O. Jamond et al., Int. J. Plast. 80,19(2016)]. This
numerical model based on the Eigenstrain theory, couples an extensive DD simulation code
(microMegas), to an elastic solver dedicated to boundary value problems resolution. The DCM allows forthe rigorous solution of dislocation-surface and -interfaces interactions and has been proven to efficiently
model plasticity in nano- and micro-objects. Nevertheless, its application has been limited to samples offew mm in size. Second, to overcome this difficulty, we employ a solver based on Fast Fourier Transform(FFT) calculation [Bertin et al. Modelling Simul. Mater. Sci. Eng. 23,065009 (2015)]. In particular, we
employ AMITEX_FFTP, a new distributed parallel elastic solver based on FFT calculation. Using thisapproach, the stress state definition in the simulated volumes can be increased from a 64x64x64 grid to a
1024x1024x1024 one, hence allowing the simulation of realistic dislocation density in a multi-grains
periodic volume over significant plastic strains (5-10%). In summary, we aim at improving mechanicalproperties predictions by taking into account both the complexity of multi-crystalline materials
microstructure and the local properties of dislocations.

MMM 2018

21

(A3 oral)
Plasticity and microstructure evolutions at the mesoscale: towards andintegrated framework.
*Laurent Capolungo1, John Graham, Aaron Kohnert, Ricardo Lebensohn, Richard Lesar, Hareesh
Tummala1. Los Alamos National Laboratory

A Fast Fourier Transform (FFT) based hybrid discrete dislocation dynamics(DDD) and cluster dynamics (CD) tool is introduced to study plasticity and microstructure evolution inpolycrystalline media. The overall intent of the work is to introduce a unified modeling platform applicableat the tens of micron scale that allows for the validation and calibration of constitutive models and rate
theory models used at higher length scales. In addition, the proposed framework aims at leveraging
novel/advanced characterization methods. As such the model will be able to be directly comparedagainst in-situ characterization methods (TEM, X-Ray diffraction). With regards to the modelingframework, in addition to its common treatment of plasticity, DDD in combination with FFT methods
allows for the description of dislocations both as discrete and as continuous objects, thereby providing an
ideal linkage with higher order constitutive modeling, which will be demonstrated by taking the example
of grain boundary/dislocation interactions and in bicrystals and polycrystals. Further, an application ofthis framework to the case of twin propagation in hcp metals will be shown. Finally, the coupling between
DDD and CD provides for a treatment of the role of dislocation microstructures on the evolution ofradiation-induced damage as well as for the quantification of plasticity induced by irradiation. This will beillustrated in the case of pure body centered cubic Iron in which a mapping of the dominant deformation
processes as a function of temperature, stress and irradiation condition will be presented.

22

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(A4 invited)
Effective Transport Properties of Polycrystalline Materials
William Beck Andrews1, Min-Ju Choe1, Erik Hanson1, Max Powers1, Hui-Chia Yu1,2, *Katsuyo
Thornton1
1. University of Michigan, Ann Arbor , 2. Michigan State University

Most solid materials have surfaces, interfaces, and grain boundaries that enhance or hinder transport; as aresult, the properties of polycrystalline solids can be vastly different from their intrinsic properties,
especially in nanocrystalline materials. Diffusion in polycrystalline materials plays an important role in awide range of material systems, including those found in batteries and solid oxide fuel cells. Due to thecomputational expense in explicitly considering the grain boundary network, establishing rational design
rules for nanocrystalline materials with desired transport properties remains a challenge. We apply the
Smoothed Boundary Method to evaluate the effective diffusivity of polycrystalline materials with a rangeof grain morphologies. We find that the anisotropy of grain morphologies plays a critical role in the overall
transport behavior, which cannot be quantified using the classical mean field theories. The results areused to obtain an expression for mixed-pathway transport that is capable of universally predicting theeffective diffusivity in complex polycrystalline solids without the use of computationally intensive
simulations. Such an approach enables efficient simulation of transport in larger-scale systems while
accurately capturing the effects of grain morphologies.

MMM 2018

23

(A4 oral)
A new E-VPSC polycrystal formulation: fundamentals*Carlos Tome1, Youngung Jeong2
1. Los Alamos National Laboratory, 2. Department of Materials Science, Changwon National University

Effective medium polycrystal models provide a computationally efficient tool for simulations of elastic andvisco-plastic deformation of aggregates. In particular, visco-plastic self-consistent (VPSC) schemes havebeen widely and successfully applied to predictions of texture, stress-strain, and dislocation density
evolution during large strain deformation. In addition, VPSC schemes have been implemented in finite
element codes as material subroutines to simulate various metal forming operations. However, a limitationof VPSC models is that they only address deviatoric stress and strain and neglect elastic contributions. As
a consequence, they cannot be used to predict internal Cauchy stress evolution (as measured withneutron or X-ray diffraction), or to study constitutive behavior during complex loading scenarios (such as
Bauschinger or spring-back) where the contribution of elasticity needs to be accounted for.
Several formulations have been proposed in the last 20 years that empirically combine visco-plastic andelastic regimes to formulate elasto-visco-plastic (E-VPSC) effective medium models. A disadvantage ofthese formulations is that they increase considerably the computation time, which makes them unwieldy
for using in forming simulations.
Here we present a new approximate E-VPSC formulation that, by treating elasticity as a perturbation to
the VPSC scheme of Lebensohn and Tomé [1], seems to capture the best of both worlds: efficient
numerical processing while providing access to the evolution of Cauchy stress in the grains.
In this presentation we describe such E-VPSC formulation, compare it to previous ones, and provide a
simple application –based on Voce hardening - to predictions of stress strain and internal stress evolution
in steel subjected to tension-compression reversal tests. In a companion paper [2] we present several
applications of this new E-VPSC formulation.

[1] R.A. Lebensohn, C.N. Tomé, “A self-consistent anisotropic approach for the simulation of plastic
deformation and texture development of polycrystals: application to zirconium alloys”, Acta Metallurgica
et Materialia 41(9) (1993) 2611-2624[2] Y. Jeong, C.N. Tomé, “A new E-VPSC formulation: applications”, this conference24

MMM 2018

(A4 oral)
A new E-VPSC polycrystal formulation: applications*Youngung Jeong1, Carlos Tome2
1. Changwon National Univ., 2. Los Alamos Natinal Lab

In this presentation we implement an elasto-visco-plastic (E-VPSC) polycrystal formulation described in acompanion paper [1]. In this formulation the contribution of elasticity is introduced as an eigen strain rate
into the visco-plastic regime of each grain. The VPSC model of Lebensohn and Tomé [2] provides theimplementation platform for the new E-VPSC formulation. The new model is validated by conducting
several applications as follows:
Simple uniaxial tension test to validate the model and to demonstrate the numerical performance incomparison with the VPSC model [2], a separate E-VPSC model [3], and a rate insensitive EPSC model [4].
Large strain cyclic shear of low-carbon steel using Voce hardening law to demonstrate the Bauschinger
effect. The results are compared with the VPSC-RGVB model [5], where the back-stress is linked toevolution of dislocation structures. Relaxation of internal stress in steel subjected to strain holds during
tensile testing.

[1] C.N. Tomé, Y. Jeong, “A new E-VPSC formulation: fundamentals”, this conference[2] R.A. Lebensohn, C.N. Tomé, “A self-consistent anisotropic approach for the simulation of plastic
deformation and texture development of polycrystals: application to zirconium alloys”, Acta Metallurgica
et Materialia 41 (1993) 2611-2624[3] H. Wang, P.D. Wu, C.N. Tomé, Y. Huang, “A finite strain elastic–viscoplastic self-consistent model forpolycrystalline materials”, Journal of the Mechanics and Physics of Solids 58 (2010) 594-612[4] P.A. Turner, C.N. Tomé, "A study of residual stresses in Zircaloy-2 with rod texture", Acta metall mater
42 (1994) 4143-53[5] W. Wen, M. Borodachenkova, C.N. Tomé, G. Vincze, E.F. Rauch, F. Barlat, J.J. Gracio, “Mechanicalbehavior of low carbon steel subjected to strain path changes: experiments and modeling”, Acta
Materialia 111 (2016) 305-314

MMM 2018

25

(A4 oral)
Modeling microstructural material variability with uncertainty
quantification and machine learning techniques*Reese Jones1, Coleman Alleman1, Brad Boyce1, Ari Frankel1, Nathan Heckman1, Mohammad
Khalil1
1. Sandia National Laboratories

Material variability from heterogeneous microstructure, such as grain and pore morphologies, can havesignificant effects on component behavior and creates uncertainty around performance. Current
engineering material models typically do not incorporate microstructural variability explicitly, rather
functional forms are chosen based on intuition and parameters are selected to reflect mean behavior.
Conversely, mesoscale models that capture the microstructural physics, and inherent variability, areimpractical to utilize at the engineering scale. An enhanced design methodology must be developed formaterials with significant variability, such as current additively manufactured (AM) metals, to predict theensemble response.
To address these challenges we have developed a method based on the Embedded Uncertainty
formulation, Sargsyan, Najm, Ghanem (2015) to calibrate distributions of material parameters fromhigh-throughput experimental data. With this method, material variability is directly associated with
commonly-used material parameters using a chosen nominal model. One of the benefits of this approachis that expert knowledge can be extended to interpret the effect of (hidden) microstructure on variablemechanical response. In a complementary effort, we are developing machine learning techniques tohandle the large volume of data from high-throughput methods. The focus of this aspect is on adapting
common machine learning models, such as neural networks, to obey the same exact properties andsymmetries as traditional constitutive models while representing features in the data in a flexible, bias-lessmanner, Tensor Basis Neural Network in Ling, Jones, Templeton (2016). Classical constitutive modelingprovides guidance in selecting appropriate microstructural descriptors as inputs and functionalframeworks for outputs. Examples of application of these techniques to polycrystalline, porous metals,
motivated by current AM materials, will be given.

26

MMM 2018

(A5 invited)
Parameters to consider in the modelling of dislocation boundary
evolution*Grethe Winther Winther1
1. Technical University of Denmark

An essential part of the development of theory and simulations of microstructure evolution is theparameters used as input as well as the output, which should be comparable to experimental
observations. An overview of the experimentally observed dislocation boundary evolution in metals of
medium to high stacking fault energy are presented with emphasis on such parameters. In particular, thespatial distribution of boundaries in terms of boundary planes, spacings and misorientation angles are
considered as a function of crystallographic grain orientation, deformation mode and strain. Based on theexperimental observations relations to crystal plasticity and the Burgers vectors available for boundary
formation are discussed.

MMM 2018

27

(A5 oral)
Data Mining of Indentation Induced Dislocation Microstructures*Dominik Steinberger1, Shyamal Roy1, Dan Mordehai2, Stefan Sandfeld1
1. Chair of Micromechanical Materials Modelling, TU Bergakademie Freiberg, 2. Department of MechanicalEngineering, Technion –Israel Institute of Technology

In general, data mining is the process of discovering patterns in large data sets, with the most common
tasks being, anomaly detection, association rule learning, clustering, classification, regression andsummarization. Within materials science, the most popular application so far is materials discovery, one of
the goals of the well-known Materials Genome Initiative (https://www.mgi.gov).
In order to understand the convoluted interactions of dislocations that define the mechanical propertiesof metals to a large degree, a lot of data has been and is being generated, be it via experiments orsimulations. Due to the complexity of the dislocation microstructure and its discrete nature
aforementioned algorithms can not be applied ad-hoc. Thus, developing methods that enable their use
are highly desirable.
In this presentation we want to outline the key aspects of the data mining framework for dislocation
microstructures that we are currently developing. Among others, we will show examples of how
quantitative comparisons of dislocation data—be it from experimental or computational methods—can be
done. Aforementioned framework will be applied to a dataset consisting of hundreds of realizations of anumerical indentation experiment realized with discrete dislocation dynamics. Rate coefficient beneaththe indenter will be extracted for the dislocation densities commonly found in continuous dislocation
dynamics models. The resulting microstructures will be analyzed for common patterns and theircorrelation with the intially emerging structure to enable predicting the characteristics of the final statefrom the first initial steps.

28

MMM 2018

(A5 oral)
Lattice continua for polycrystal grains: Climb and glide of dislocations,diffusion and grain boundary kinematics.
*sinisa Dj Mesarovic1
1. Washington State University

At high temperatures, the interior of each grain in a polycrystal suffers: (1) dislocation glide, (2) climb, and(3) diffusion of vacancies. Grain boundaries undergo: (4) growth/disappearance, as a result of vacancy
diffusion, and, (5) crystallographic reorientation/mismatch, as a result of dislocations arriving to theboundaries either by glide or by climb. All the above deformation mechanisms are naturally described in
the lattice continuum framework, whereby the lattice represents the material. Climbing edge dislocations
are lattice sink/source which must be reflected in the continuity equation and the transport theorem. Theinteracting kinematics of dislocation glide and climb requires dual definition of crystallographic slip fields:
the true slip and the apparent slip. The transport theorem for grains with lattice growing or disappearing
lattice at different grain boundary faces results in the direct formulation of the boundary condition forvacancy diffusion flux in terms of the boundary velocity (different from the lattice velocity). The field
equations for each grain are derived by means of the principle of virtual power. Additional boundary
conditions result from the relative motion of the adjacent crystal faces: Change of tilt and twist angle andsurface elastic mismatch are derived from the geometry of dislocations arriving to the boundary. The
resulting polycrystal initial/boundary value problem consists of elasticity-plasticity-diffusion field
equations in each crystalline domain with moving boundaries, coupled through the boundary conditions.
Mesarovic, S.Dj. 2018 Physical foundations of mesoscale continua. In Meso-scale models: Frommicro-physics to macro-interpretation. Springer lecture notes. To appear.
Mesarovic, S.Dj. 2017 Dislocation creep: Climb and glide in the lattice continuum. Crystals 7(8), 243.
Mesarovic, S.Dj. 2016 Lattice continuum and diffusional creep. Proc. R. Soc. A 472, 20160039.

MMM 2018

29

(A6 invited)
Design of patchy nanoparticles via the self-assembly of triblock
terpolymers in selective solvents*Eliot Fried1, Nicolás Moreno1
1. Okinawa Institute of Science and Technology

The hierarchical self-assembly of triblock terpolymers in solution is a successful bottom-up methodology
for constructing functional patchy nanoparticles with prescribed topology and shape. Currently, thedesign of such nanoparticles requires an iterative process to identify the experimental phase parameters
needed to produce any target pattern. The broad use of this technology is therefore cumbersome andlimited by an incomplete understanding of the mechanisms underlying patch transition. A detailed
understanding of how thermodynamics and kinetics influence the topology of the assemblies is also
missing. In this work, we present a set of rules for programming desired shapes of the nanoparticles andpredicting the pathways by which they assemble. We investigate systematically the interplay between
entropic and enthalpic parameters governing the self-assembly of ABC triblock copolymers in a selective
solvent for the C block. We use a computational modeling at the mesoscale and thereby encompass thelength and time scales associated with the motion and assembly of the polymer coils, while accurately
approximating the chemically driven interactions. The phase diagram predicted by our model isconsistent with experimentally identified characteristic nanoparticle shapes. We find that the effective
volume fraction of the soluble block determines the size of a particle and the distribution of its patches via
steric interactions. Moreover, we find that the relative fraction of the patch-forming block and its affinity
with the core of a articles dictates the number and shape of its patches. As a major outcome, we constructa morphologically-based library of nanoparticles. That library can be used to hierarchically design
mesoscale aggregates with specific morphologies. Our results provide insight regarding the mechanismsthat determine nanometer scale objects in synthetic and naturally occurring systems.

30

MMM 2018

(A6 oral)
Nanoporous Composites: Giving Polymers Strength and Helping
Metals Move
*Emma Griffiths1, Jana Wilmers2, Swantje Bargmann2, BD Reddy1
1. University of Cape Town, 2. University of Wuppertal

An important benefit of electroactive polymers is their large strain response due to certain stimuli such as
an electric current. However, for sufficient mechanical use in electrochemical systems, they lack thenecessary strength and stiffness. Metals on the other hand have sufficient strength and stiffness but are
unable to function as piezoelectric materials [1]. By creating a metal-polymer composite, however, astronger material with large strain capacity is created that can withstand larger actuation forces.
Specifically, using polymer impregnated nanoporous structures, with their exceptional mechanicalproperties providing a reinforcing base [2,3], creates a composite material with superior actuator
properties by having both sufficient strength and strain capabilities.
This work presents the chemoelectromechanical response of a gold-polymer nanocomposite model. A
representative volume element is modelled using a linear version of the chemoelectromechanical theory
developed by Wilmers et al. [4] and is implemented through a staggered explicit-implicit finite element
simulation in ABAQUS. Additionally the theory is enhanced by introducing an implicit solvent model tocapture the effects of the crowding of ions due to saturation within the material. The micromechanicalresponse of the composite is explored under an imposed electric field and thus providing information to
be used in the optimization of microstructures for sensory and actuation applications.
References[1] Stenner, Shao, Mameka, Weissmüller, J. Piezoelectric Gold: Strong Charge-Load Response in a
Metal-Based Hybrid Nanomaterial, Adv. Funct. Mater. 26, 2016[2] Griffiths, Bargmann, Reddy, Elastic behaviour at the nanoscale of innovative composites of nanoporous
gold and polymer, Extreme Mech. Lett. 17, 2017[3] Soyarslan, Bargmann, Pradas, Weissmüller, J. 3D stochastic bicontinuous microstructures: Generation,
topology and elasticity Acta Mater. 149, 2018
[4] Wilmers, McBride, Bargmann, Interface elasticity effects in polymer-filled nanoporous metals, J. Mech.
Phys. Solids 99, 2017

MMM 2018

31

(A6 oral)
Multiscale modeling of advanced materials for hybridorganic-inorganic solar cells*Alexander E. Kobryn1
1. Nanotechnology Research Center, National Research Council Canada

When it comes to talk about alternatives to non-renewable sources of energy, photovoltaics is one of themost promising choices for converting solar light into electricity with the use of solar cells. At present,
there is wide variety of solar cell devices on the market and this variety will continue to grow because of
the race for better conversion efficiency and the final product sustainability. Within recent years, theseemingly clear boundaries between photovoltaics of inorganic and organic solar cells have started tofade by the advent of metal-halide hybrid organic-inorganic perovskites (OIP). They are very promising
candidates for future photovoltaic applications because they have wide-direct band gap, which can betuned by either changing the organic cation, the metal atom, or the halide. Their power conversion
efficiency now reached over 22%. At the same time, the level of our understanding of the origin of such
performance is still insufficient. We believe that thin film morphology is a key factor determining theperformance of bulk heterojunction organic solar cells, because of its influence on charge separation,
charge transport and recombination losses in donor-acceptor blends. With this respect, we present both
descriptive and predictive multiscale modeling of electronic and structural properties of blends of PCBM
or hybrid OIP of the type CH3NH3PbX3 (X=Cl, Br, I) with P3HT, P3BT or squaraine SQ2 dye sensitizer,
including adsorption on TiO2 clusters. Here, the multiscale nature of modeling means that in a set ofsimple hierarchical approaches we combine different methods for different scales (quantum mechanical,
microscopic, mesoscopic) interpreted independently and that the information obtained at one level istransmitted to the next level as a required input. If necessary, new information is sometime transmitted
back to the previous level for a feedback control. As the result, we arrive at a very reliable methodologythat allows computing the microscopic structure of blends on the nanometer scale and getting insight on
miscibility of its components at various thermodynamic conditions. The calculated nanoscalemorphologies serve as an instrument in rational design of hybrid OIP. They are used in collaboration withexperts who actually make prototypes or devices for practical applications.

32

MMM 2018

(A6 oral)
From cellulose and lignin to kerogen: molecular simulations of ageological process*Roland PELLENQ1, Pierre-Louis Valdenaire1, Christophe Bichara3, Franz Ulm1, Jean-Marc
Leyssale2
1. <MSE>2, MIT- CNRS - AMU, 2. ISM, CNRS - Bordeaux U., 3. CINaM, CNRS - Aix-Marseille U.

The process by which organic matter decomposes deep underground to form petroleum and its
underlying kerogen matrix has so far remained a no man’s land to theoreticians, largely because of thegeological (Millions of years) timescale associated with the process. Using a replica exchange accelerated
molecular dynamics method initially developed in the context of the micro- to milli-second timescale forprotein folding, we simulate the full transformation of cellulose and lignin (the main components of wood)
into kerogen under prevailing geological conditions. We observe in sequence (i) the fragmentation of thecellulose crystal and production of water, (ii) the development of an aliphatic macromolecular phase, (iii)
its aromatization, and (iv) its aggregation into a stiff porous aromatic carbon upon expulsion of the fluid
phase. The composition of the solid residue along the maturation pathway strictly follows what isobserved for natural type III kerogens and for artificially matured samples under closed conditions,providing further evidence of a kinetically controlled, irreversible, decomposition process in which thealiphatic (immature) phase is a metastable intermediate.

MMM 2018

33

(A7 invited)
Mathematical challenges for a mesoscale theory of dislocations*Thomas Hochrainer1
1. TU Graz

During the last two decades, dislocation research saw a revival driven by new experimental techniques forsmall scale testing, high resolution characterization and the rise of large scale discrete dislocation
simulations. This also stimulated intensive research in mesoscale theories of dislocations. Great advanceswere achieved on continuum models of dislocation interactions for strongly simplified straight-dislocation
systems and continuum dislocation dynamics of single slip deformation. However, a transfer of theemployed averaging methods to fully three-dimensional multiple slip deformation remains a challenge forvarious reasons.
In the current talk we shall highlight some challenges appearing on different levels of continuum
dislocation modelling. At the most fundamental level it is yet unclear what continuum measures are suitedfor characterizing 3D networks of dislocations. The alignment tensors successfully used for dislocation
distributions in single slip seem to be of limited value in networks consisting mostly of junctions andreaction products. It is likewise an open question how to describe the kinematics of dislocation networks,
where reactions and junction formation or break-up imply permanent changes in the topology of the
network and the characteristics of dislocations. Closely related is the challenge of capturing the averagedenergetics, kinematics and dissipation of these dislocation interactions.
To overcome the named challenges possibly requires new mathematical tools or at least the adoption ofconcepts from other areas of physics and mathematics which were not yet (fully) exploited in dislocation
theory. We shall discuss several ideas where we might find new impulses and promising tools for further
successes in developing a mesoscale theory of dislocations. Expected key areas include discrete anddifferential topology, gradient flows, rough energy landscapes, and temporal and spatial statistics.

34

MMM 2018

(A7 oral)
Dislocation multiplication in the discrete-continuum transition regime*Katrin Schulz1, Markus Sudmanns1, Kolja Zoller1, Peter Gumbsch1,2
1. Karlsruhe Institute of Technology, 2. Fraunhofer Institute for Mechanics of Materials

The striving for advanced materials with well-defined microstructures has led to an increasing effort
towards a physically based description of the motion of dislocations as the cause of plastic deformation.
Several dislocation based continuum theories have been introduced, but only recently rigorous
techniques have been developed for performing meaningful averages over systems of moving, curved
dislocations, yielding evolution equations for a dislocation density tensor, see [1].
Regarding a self-consistent coarsening of dislocation modeling in order to construct an efficient numerical
implementation, several issues have to be solved including calculation of the stress field of a system of
dislocations, representation of dislocation interactions and reactions as well as boundary conditions.
Accurate solutions have been found for one and two dimensional systems [2, 3]. However, the analysis
and understanding of dislocation networks in three dimensions involves additional challenges such as therepresentation of dislocation multiplication.
In this presentation, we discuss the implications of a homogenization of dislocation interactions andreactions in the discrete-continuum transition regime. Based on the analysis of 3d discrete dislocation
structures, we present a coarse-grained continuum formulation for dislocation multiplication due tocross-slip and glissile reaction events. The formulation is validated by the comparison with discrete
dislocation dynamics simulations and discussed in the context of a continuum limit as lower application
limit of a continuum approach.

[1]Hochrainer, T.; Sandfeld, S.; Zaiser, M.; Gumbsch, P.: Continuum dislocation dynamics: Towards aphysical theory of crystal plasticity. J. Mechan. Phys. Solids. 63 (2014), p. 167–178.
[2] Schmitt, S., Gumbsch, P., Schulz, K.: Internal Stresses in a Homogenized Representation of Dislocation
Microstructures. J. Mechan. Phys. Solids, 84 (2015), p. 528-544.
[3] Schulz, K., Sudmanns, M., Gumbsch, P.: Dislocation-Density based Description of the Deformation of a
Composite Material. Modell. Simul. Mater. Sci. Eng., 25 (2017), 064003.

MMM 2018

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(A7 oral)
The fundamental instability of dislocation transport equations*Michael Zaiser1, Ronghai Wu1
1. FAU University of Erlangen-Nuremberg

We analyze the stability of transport equations which describe dislocation dynamics in terms of fluxes of
dislocation densities defined on scales above the dislocation spacing. We demonstrate that suchtransport equations exhibit a generic instability which emerges as soon as a dislocation flux function,
which in the simplest case is identical with the shear strain rate on a given slip system, has a negative
derivative with respect to the flowing dislocation density. The criterion for instability is equivalent to thecondition that the stress required to sustain plastic flow at a given rate is an increasing function of
dislocation density. We conclude that dislocation patterning of some kind is an almost trivial, andpractically unavoidable, consequence of dislocation transpoort in conjuction with work hardening.
Implications for the theory and interpretation of different types of dislocation patterns, and fordensity-based simulations of plastic flow in general, are discussed.

36

MMM 2018

(A7 oral)
2D continuum theory of dislocations*Istvan Groma1
1. Eotvos Lorand University

During the past 20 years, by a systematic course graining of the equation of motion of straight paralleledge dislocations a 2D continuum theory of dislocation has been developed. It is able to account for
many important feature of the collective properties of dislocations, like size effect, hardening due toprecipitates, dislocation density distribution next to boundaries, and dislocation patterning. The
predictions of the theory are directly compared to discrete dislocation dynamics simulations.

In spite of the success of the theory there are several open issues need to be addressed for further
developing the current theory. In the talk the problems "blocking" the extension of the theory are
discussed:

1. Do we need a continuum theory?
2. Why still in 2D? What we can learn from it?
3. Should we course grain the equations of motion of the dislocations or the energy of the discrete system
to derive the continuum theory?
3. Numerical determination of the dislocation dislocation correlation function. Problems related to the thefinite size effect observed.
4. What to do next to boundaries?
5. Phase field approach. Issues related to the nontrivial mobility function one has to introduce.
6. Slow, power law relaxation of the dislocation system.
7. How to incorporate stochastic effects.

MMM 2018

37

(A7 oral)
Continuum dislocation dynamics for finite deformation mesoscale
plasticity*Anter El-Azab1
1. Purdue University

Many dislocation dynamics simulation models were developed to understand plastic deformation of
metals most of which are based on small deformation kinematics. In this talk, I will present adensity-based dislocation dynamics model within the framework of finite deformation of crystals, focusing
on crystal mechanics and the dislocation transport equations at finite deformation. The geometriccoupling and non-linearity of the dislocation transport on various slip systems will be explained. The
relevant crystal mechanics, thermodynamics, and constitutive closure questions will also be discussed.
Collaboration: Giacomo Po, University of California-Los Angeles; Grethe Winther, Technical University ofDenmark.

38

MMM 2018

(A8 invited)
Pattern formation in doubly curved thin shells*Eleni Katifori1, Desislava Todorova1, Octavio Albarran2, Lucas Goehring2
1. University of Pennsylvania, 2. Max Planck Institute

Pattern formation in thin elastic shells has attracted increasing interest in both fundamental studies andpractical applications. Examples include biological systems and engineering applications, such as the
fabrication of flexible microelectronics. In this talk we explore the mechanical instabilities of an
intrinsically curved thin shell deposited on a liquid surface. Here, the pattern formation is not a direct
result of externally imposed strain, but is due to the geometric incompatibility between a curved, stiff
membrane and an (initially flat) liquid substrate. We observe several types of instabilities, including awrinkle-to-fold transition from periodic sinusoidal solutions to morphologies that combine sinusoidal
wrinkles and folds; a transition from dimples (geometric inversions) to periodic sinusoidal solutions; and atransition from flat bands with zero Gaussian curvature, to dimpled periodic patterns. We show that whe
wrinkling patterns can be described via an effective theory of liquid crystalline smectics at intermediate
length scales. This insight allows better understanding of the wrinkling patterns seen in such systems, withwhich we explain pattern breaking into domains, the properties of domain walls and wrinkle undulation.
We compare our predictions with numerical simulations and experimental observations. We investigate
how the global geometry of the curved shells and their elastic properties control the transitions between
the various morphologies. Last, we discuss various new strategies for creating and controlling patterns inthin elastic shells with natural curvature.

MMM 2018

39

(A8 oral)
Phase-field model for microstructure change in L10 type ordering withlattice distortion*Ryuichiro Oguma1, Long-Qing Chen2, Syo Matsumura3
1. Fukuoka Univ., 2. The Pennsylvania State Univ., 3. Kyushu Univ.

Domain structures consisting of multiple off-phase variants are generally formed in ordering process inbinary alloy systems. The L10 type order is formed in AB type alloys on the basis of an fcc lattice. The
crystal structure is tetragonal. Therefore three orientational variants can be formed depending on whichof <100> directions corresponds to c-axis and two translational variants are possible for each of them; six
distinct crystallographic variants exist in this type of order. The present authors previously developed aphase-field formulation for ordering process of L10 type in binary alloys, taking into account the symmetryof the crystal structure. In the L10 structure the fcc lattice is divided into four simple cubic sublattices. The
atomic occupation probabilities on the sublattices are represented by three independent orderparameters and a composition parameter. If the state of order of atomic arrangement is represented by apoint in the three dimensional Euclidean space spanned by the three order parameters, the six equivalentvariants are defined by the six tips of a regular octahedron centered on the origin for the disordered state.
A mean-field free energy is defined in a form of Landau type expansion with the order parameters and thecomposition parameter. In this presentation we further consider the effect of the lattice distortion. An
elastic energy is introduced with eigen strains according to Khachatrian's method. The interface energy
automatically reflects the direction of c-axis and satisfies symmetries of cubic, tetragonal andorthorhombic structures. Kinetic equations for time-evolution of the order parameters and theconcentration are derived from the Ginzburg-Landau type thermodynamical potential. Formation of
domain structures was simulated by treating the kinetic equations numerically in a three-dimensional cell
scheme. The microstructures obtained are compared with experimental results of TEM observation.

40

MMM 2018

(A8 oral)
Coupling multi-component phase field models for oxide systems tothermodynamic databases –breaking the curse of dimensionality*Inge Bellemans1, Nico Vervliet2, Lieven De Lathauwer2,3, Nele Moelans4, Kim Verbeken1
1. Department of Materials, Textiles and Chemical Engineering, Ghent University , 2. Department of ElectricalEngineering, KU Leuven, 3. Group Science, Engineering and Technology, KU Leuven –Kulak, 4. Department ofMaterials Engineering, KU Leuven

An important focus in creating more realistic phase field models is the incorporation of thethermodynamic driving forces in multicomponent multiphase-field models by coupling to thermodynamic
databases. This coupling, which aims at retrieving Gibbs energies and chemical potentials, becomesincreasingly intricate as the number of components/chemical elements in the system increases. This iscaused by the fact that the number of elements in a tensor increases exponentially with the number ofdimensions, and so do the computational and memory requirements. The exponential dependency (andthe problems that are caused by it) is called the curse of dimensionality.
Alternatives need to be investigated for the storage and handling of the thermodynamic data required forthe phase field simulations. A possible solution for this might be the use of a canonical polyadic
decomposition on the tensors containing the thermodynamic data. In this way, the huge tensors areapproximated well by compact multilinear models or decompositions. Tensor decompositions are moreversatile tools than the linear models resulting from traditional matrix approaches.
This solution promises to be suitable for this challenge and has been applied to a quaternary and quinary
metal-oxide system in a multicomponent multiphase-field model which incorporates facetted anddendritic growth and can also treat the boundaries as an open boundary where the melt is locally inequilibrium with the atmosphere at a certain partial oxygen pressure. The results and improvements incomputational requirements are compared for various methods to couple to a thermodynamic database.

MMM 2018

41

(A8 oral)
A Sharp Phase Field Method*Alphonse FINEL1, Yasunori YAMADA2, Yann LE BOUAR1, Benoît DABAS1, Benoît APPOLAIRE1,
Tetsuo MOHRI2
1. Laboratoire d'Etude des Microstructures, CNRS, ONERA, 2. Institute for Materials Research, Tohoku University

Phase field modelling offers an extremely general framework to predict microstructural evolutions incomplex systems. However, its computational implementation requires a discretisation scheme with a grid
spacing small enough to preserve the diffuse character of the theory. We present a new formulation inwhich the interfaces are resolved with essentially one grid point with no pinning on the grid and an
accurate rotational invariance, allowing to multiply the accessible linear dimensions by an order ofmagnitude or, conversely, to reduce the computational time by almost three orders of magnitude. We
show that this Sharp Phase Field Method (S-PFM) reproduces interfacial kinetic properties with a very
high accuracy. Then, we apply the model to a situation where conserved and non-conserved fields arecoupled. Finally, to couple S-PFM to elastic fields, we propose a new elastic solver that efficiently treatsstrong elastic heterogeneities and that is mathematically stable.

42

MMM 2018

(A9 oral)Group-theoretical construction for constitutive equation of the firststrain gradient elasticity*Ryuichi Tarumi1, Shunsuke Kobayashi2, Yoji Shibutani2
1. Graduate School of Engineering Science, Osaka Univ., 2. Dept. of Mechanical Engineering, Osaka Univ.

Development of the theory of continuum mechanics is one of the central issues for quantitative
understanding of multiscale phenomena. The first strain gradient elasticity (FSGE) is an attractive
framework as it includes non-locality in the constitutive equation. Application of the theory for multiscalematerial modeling is however still limited due to the large number of independent strain gradient elastic
constants. This study aims to overcome the difficulty by using the group theory. We conduct theirreducible decomposition for the 5th and 6th-rank strain gradient elastic constants, M and A, undergeneral linear group GL(3). The decomposition is based on the Schur-Weyl duality principle and Youngsymmetrizer technique in Frobenius algebra. For the isotropic and centro-symmetric case, the 6th-rank
tensor is decomposed into three sub-tensors, A = A{6}+A{4,2}+A{3,2,1}, whereas the 5th-rank tensor M
vanishes identically. Here, the one-dimensional sub-tensor A{6} is called the primitive symmetry class andsatisfies the generalized Cauchy solid condition. The corresponding stress equilibrium equation yields a
simplified 4th-order partial differential equation that is different from Aifantis’ Laplacian-type gradient{3,2,1}

theory. Another one-dimensional sub-tensor A

denotes the elastic null-Lagrangian which is absent in

the classical elasticity. The remaining three-dimensional sub-tensor A{4,2} has two characteristic lengthscales and one of which has an imaginary value. We also revealed that the combined sub-tensors, A{4,2}+A{3,2,1}, with a proper kinematic constrain condition, reduces the well-known couple stress elasticity.

Implementation of the Cauchy-type FSGE for NURBS-based isogeometric analysis (IGA) demonstrates thatstress field around a point defect is regularized due to the non-locality included in A{6}.

MMM 2018

43

(A9 oral)
Physically based strain gradient plasticity model for length scaledependent yield strength*Peter Gudmundson1
1. Department of Solid Mechanics, KTH Royal Institute of Technology

Many experimental studies have shown a plastic strengthening effect for structural length scales
approaching microstructural dimensions. Both increases in initial yield strength and strain hardening havebeen observed. Over the last 30 years different strain gradient plasticity (SGP) theories have beendeveloped in order to capture these length scale dependences. However, up to now no generally
accepted theory has emerged. In the present paper, focus is directed into a physically based SGP model
for initiation of plastic deformation. The plastic behavior is governed by a dissipative part that primarily
controls the hardening at moderate plastic strains and an energetic part that is of importance for the
initiation of plastic flow. It is shown that a model based on the self-energies of dislocations can betranslated into an internal free energy in terms of plastic strain gradients. In this way a physical connection
is made between the SGP framework and dislocation mechanics. A microstructural length scale can thenbe defined as the Burgers vector divided by the strain for initiation of plastic deformation. When structurallength scales approach this microstructural length scale, strengthening effects result. It the Taylor modelis used for the dissipative part, the same microstructural length scale appears. The so developed
three-dimensional SGP model is specialized to the simple load cases of tensile tension with a passivation
layer that prohibits plastic deformation on surfaces as well as pure bending with free and fixed boundary
conditions for plastic strain. Simulations for varying thicknesses are compared to experimental
observations reported in the literature. It is shown that the model in a good way can capture the lengthscale dependences. Suggestions for improvement of the dislocation theory based model for the internal
free energy are discussed.

44

MMM 2018

(A9 oral)
FTMP-based Seamless Description of Deformation-Fracture
Transitions*Tadashi Hasebe1
1. Kobe Univ.

This study attempts to tackle one of the ultimate problems in mechanics of materials based on FTMP(Field Theory of Multiscale Plasticity), i.e., modeling deformation-fracture transitions. Three typical
fracture modes are targeted: (1) ductile fracture, (2) creep rupture and (3) fatigue crack initiation, whose
evolutions are respectively triggered or promoted by deformation-induced substructures. For (1), criticalconditions governing the outsets of macro/micro instabilities are extensively discussed, while (2) isconcerned with localized recovery-induced rupture in packet models for lath martensite structures, inconnection with the interaction fields. The mode (3), on the other hand, is successfully reproduced via
evolving PSB ladder structure and attendant vacancy formation/diffusion processes. A noteworthy output
here is that the use of the corresponding duality diagram representations enables us to “visualize”
these complex processes, manifested as dynamic energy conversion between fluctuating elastic strain

MMM 201845

(A9 oral)
On the crucial role played by instantaneous and hidden multifieldfeaturesof lattice dynamics in their nonlocal pseudocontinuum modeling*Miguel Charlotte1
1. University of Toulouse, Institute Clement Ader, CNRS -- UMR 5312 INSA/UPS/ISAE-SupAero/Mines Albi

In the past, a very large number of investigations has been interested in building various continuum
descriptions of periodical mass-spring lattices which are capable of capturing at least somemicroscopically fast and microscopically localized phenomena that cannot be handled correctly by
the classical continuum theories relying on a single displacement field. Very few of these works
have however tried to illuminate some of the less well-known aspect of the concept of “action-at-a
distance” involved in periodical mass-spring lattices, namely the “instantaneous propagation of
disturbances”,
and its nonlocal effects in finite material systems. These instantaneous features, that aredeveloped in the present work, have notably been discussed formerly partly by Lord Rayleigh and
T.H. Havelock for other dispersive media and more recently evoked by M. Charlotte and L. Truskinovsky.
Such features have although been ignored or minimized up to now in most derivations ofpseudocontinuum hamiltonian models of elasticity. In order to account for these features, the alternative
viewpoint developed here uses on an implicit but somewhat standard multi-displacement
field description of the periodical mass-spring lattice motions and their (spatially nonlocal)
pseudocontinuum
wave mechanic representations which allow to deal in the same framework with bothquasi-continuum (atomically diffuse) and anti-continuum (atomically localized) phenomena in finite
material systems. In particular, the focus is put on the lattice motions of a simple finite particle
chain with nearest neighbor interactions owing to its simplicity, complete analyticity, and physicalclearness. The current analysis also explores an alternative standpoint, including both bulk- andboundary-dependent multi-field or multi-modal continuum descriptions, and that may constitute a
new direction in which the continuum modeling theory could be fundamentally generalized. Thus,
the outcome of the present multi-field approach is merely another non-classical quasi-continuum
elastodynamics with both non-trivial nonlocal inertia and elasticity whose complexity depends naturally
on the non-trivial interplay between the atomic bulk interactions and the boundary conditions.

46

MMM 2018

(A10 oral)
Interdiffusion and atomic mobilities in fcc Ag-Mg, Ag-Mn andAg-Mg-Mn alloy*Qianhui Min1, Yuling Liu1, Yong Du1, Huixin Liu1, Li Chen1, Biao Hu3, Changfa Du2, Zhoushun
Zheng2
1. Powder Metallurgy Research Institute,Central South University, 2. School of mathematics and statistics,Central
South University, 3. School of Material Science and Engineering, Anhui University Of Science And Technology

On the basis of Ag/Ag-Mg, Ag/Ag-Mn, Ag-Mg/Ag-Mn, and Ag/Ag-Mg-Mn diffusion couples, theinterdiffusion coefficients in face-centered cubic (fcc) phase of the Ag-Mg, Ag-Mn, and Ag-Mg-Mn alloys
were messured at the temperature range between 873 and 1173 K by using semi-infinite diffusion
couples together with the Sauere-Freise method. A reliable method was applied to evaluate the errors of
the identified interdiffusivities in consideration of the propagation of errors. Based on available
thermodynamic information, the atomic mobilities for the fcc Ag-Mg, Ag-Mn,and Ag-Mg-Mn syetems are
obtained by using the DICTRA software package, and the computed results agree in general withexperimental data.

MMM 2018

47

(A10 oral)
Stacking and Multilayered Nature of Martensite in Copper Based
Shape Memory Alloys*Osman ADIGUZEL1
1. Firat University

Shape memory effect is characterized by the recoverability of two certain shapes of material at different
temperatures. These materials are often called smart materials due to the functionality and their capacityof responding to changes in the environment. Shape memory is initiated by cooling and stressing material
and processed by heating and cooling.
Shape memory effect is a result of successive thermal induced and stress induced martensitic
transformations. Thermal induced martensitic transformation occurs as twinned martensite on cooling as
martensite variants with cooperative movement of atoms by means of lattice invariant shears on {110} type planes of austenite matrix. Twinned structures turn into detwinned martensite by means of stressinduced martensitic transformation on stressing in martensitic state. Lattice invariant shears occurs in<110 > -type directions on the {110}-type basal planes.
Copper based alloys exhibit this property in metastable β-phase region, which has bcc-based structuresat high temperature parent phase field. Twinning and lattice invariant shear is not uniform in these alloys
and multilayered martensite with complex stacking structures, like 9R or 18R martensites occur by meansof thermal induced martensitic transformation. All of these martensite phases are long-period stacking
ordered structures that is the underlying lattice is formed by stacks of close-packed planes.
In the present contribution, x-ray diffraction and transmission electron microscopy studies were carriedout on two copper based CuZnAl and CuAlMn alloys. X-ray diffraction profiles and electron diffraction
patterns reveal that both alloys exhibit super lattice reflections inherited from parent phase due to thedisplacive character of martensitic transformation.

48

MMM 2018

(A10 oral)
Mesoscale understanding of ionic conduction in yttria stabilized
zirconia*Abhijit Chatterjee1
1. Dept. of Chemical Engineering, Indian Institute of Technology Bombay

Yttria-stabilized zirconia (YSZ) is widely used as an electrolyte in solid oxide fuel cells (SOFCs). Much of
past research on ionic conductivity in YSZ has focused on understanding diffusion process atomistically,e.g., migration barriers for O2- ion movement and vacancy trapping behavior, in an attempt to explain thebulk O2- transport. How the mesoscale structure of YSZ, i.e., structures at length scales lying between theatomic/micro-scale and the macroscale, influences ionic conduction remains poorly understood. An
improved understanding of mesoscale factors that affect ionic conduction in YSZ can potentially benefit
our efforts to search for new SOFC electrolyte materials.
Two mesoscale aspects will be covered in this talk. The first part of the talk relates to the percolation
network structure formed in YSZ. It is well known that Y3+ ions in the YSZ structure block the movementof O2- ions. Other parts of YSZ that locally do not contain the Y3+ ions form a fast ion conducting
percolation network. The network topology is determined by the underlying cation arrangement. Insights
into the percolation network composition, topology and O2- ion conduction rates are provided using
kinetic Monte Carlo simulations. A relationship between the ionic conductivity and the topological detailsof the network is derived.
The second part relates to free energy of finding O2--vacancy (O2--vac) pairs in a local environment, which
again is a mesoscale property of the material. Higher probability of O2--vac pairs in some regions of YSZ
can result in greater number of transitions in those regions. O2- ion movement in bulk YSZ is studied using
multiple independent short molecular dynamics (MD) trajectories. Analysis of the MD trajectories yields
free energy of O2--vac pairs in 42 different local cation (Y3+/Zr4+) environments, as well as effective O2hopping rates and Arrhenius parameters. Based on the free energies we conclude that it is possible thationic movement is hindered in some environments not just because of high migration barriers or vacancy
trapping as believed earlier, but also because O2--vac pairs are destabilized by these environments.
Increasing the temperature and/or decreasing the dopant composition stabilizes O2--vac pairs in theseenvironments, which in turn affects the YSZ conductivity.

MMM 2018

49

(A10 oral)
Accelerating stochastic simulations with path integrals*Steve Fitzgerald1
1. University of Leeds

Monte Carlo-type simulations can extend accessible timescales c.f. atomistic models by many orders ofmagnitude. However, their efficiency can be significantly reduced when some of the species being
simulated have very low migration barriers. This can be addressed by coarse-graining the low-barrier
migration into pseudo-free continuum diffusion and, and evolving the system by drawing timesteps andhop lengths from a first passage time distribution [1,2], but this neglects the effects of spatial variation in
the potential landscape in which the particles move, i.e. drift. Existing extensions require every step of arandom walk to be computed, or a Fokker-Planck equation to be solved numerically [3], thus increasing
the computational cost.

In this work I will describe an alternative approach, beginning from the Langevin equation for a particle
moving in a general smooth potential V(x). Assuming Gaussian white noise, the stochastic equation of
motion can be recast as a path integral, which can be analysed using methods borrowed from quantum
field theory. Closed-form solutions for the Green function / propagator P(x,t|0,0) in a general potential are
possible when the noise strength (temperature) is less than the energy scale characterising the barriers
the particle has to overcome (i.e. the same range of validity as the Arrhenius rate function exp(-E/kT). )Potential applications to material simulations will be discussed.

[1] Opplestrup, Tomas, et al. "First-passage Monte Carlo algorithm: diffusion without all the hops."

Physical review letters 97.23 (2006): 230602.

[2] Muller, Mervin E. "Some continuous Monte Carlo methods for the Dirichlet problem." The Annals of

Mathematical Statistics(1956): 569-589.

[3] Mauro, Ava J., et al. "A first-passage kinetic Monte Carlo method for reaction–drift–diffusion
processes." Journal of Computational Physics 259 (2014): 536-567.

50

MMM 2018

(P1-01)
The Coadsorption Effect of Cl- and H2O on the Various Defect Al2O3
Film Surface*Chuan-Hui Zhang1, Bao Chen1, Peng Shi1
1. University of Science and Technology BeijingFirst-principles calculations have been performed on the perfect surface, point-defect surface, step-defectsurface, layer-defect surface of Al2O3 film with water molecules and chloride ions. The coadsorption
mechanism has effect on the reaction and erosion of the surface. The adsorption energies (Eads), stable
adsorbed sites, binding of film, charge transfer, reactants and products, activation energies and transition
states are calculated and discussed. The results evidence that for the perfect Al2O3 surface, the criticalmonolayer of Cl- is 3/7, the Eads decrease in three steps, each Eads step only relate to the adsorbed site
and the morphology. For point-defect surface, substitution point defects are more sensitive than vacancy
point defects for reaction and erosion. The species of products depend on the energy barrier andorientation of water. For step-defect surface, Al1 step-defect and Al3 step-defect surfaces prefer to obtain
Al-H2O compounds, while O2 step-defect surface prefers to form Al-Cl products. There is no obvious
linear relationship between the number of products and the number of steps. For layer-defect surface,when low concentrations of Cl ions reach the surface, they prefer to erode the Al layer-defect surface withproducing Al-Cl compounds, while they prefer to interact with H2O upon the O layer-defect surface.

MMM 2018

51

(P1-02)
Incorporation of double cross-slip in continuum dislocation dynamics*Xingjian Zhou1, Yichao Zhu1
1. Faculty of vehicle Engineering and Mechanics, Dalian University of Technology

It is widely agreed that the cross-slip mechanism of screw dislocations plays a key role as crystalline
materials deform plastically. In discrete dislocation dynamics (DDD) models, the onset of cross-slip is
widely formulated as a random event. However, a proper formulation of cross-slip in models of continuum
dislocation dynamics (CDD), where dislocation microstructures are described as a density distribution, is
still missing. In this presentation, we present a CDD framework incorporating cross-slip mechanism. The
discrete-to-continuum (D2C) transition is carried out by translating the probability of a discrete event intoa collective frequency. This work is conducted under the framework of dislocation density potentialfunction (DDPFs), where the mean-field stress can be evaluated with related ease. Numerical examples
will be shown to demonstrate the role of cross-slip in the determination of material’s stress-strain
relation.

52

MMM 2018

(P1-03)
Effect of Defect on Structural and Optical Properties in
Methyl-Ammonium Lead Iodide(MAPI) Perovskite via First Principles
Calculations*Abdullah Al ASAD1, Keiichi MITANI1, Atsushi ISHIKAWA1, Kenji TSURUTA1
1. Dept. of Electrical and Electronic Engineering, Okayama University

Hybrid organic/inorganic perovskites(HOIPs) have drawn significant research interests due to theincomparable rapid rise in energy conversion efficiency seen in photovoltaic devices based on CH3NH3PbI
3. Current research attempts in this field have concentrated on searching for similar perovskites with

better properties, especially stability under a humid condition and/or irradiation [1]. In this work,
employing first-principles calculations based on the density functional theory, we have investigatedeffects of vacancy on the optimized structures, bandgap, total and partial density of state, effective on-siteand bond charge, by comparing with relevant experimental and/or theoretical data [2]. These analyses
reveal that a Pb vacancy induces an asymmetric distortion of the lattice, which leads to a local volume
expansion at low temperature [3]. The degenerate states at the conduction band minimum(CMB) are split
each other due to the defect and it promotes broadening of the light absorption spectra. Possible effects
of interstitial impurity, such as a water molecule, will also discussed in the presentation.
[1] A. K. Chauhan and P. Kumar, J. Phys. D: Appl. Phys. 50 (2017) 325105.
[2] Y. Wang et al., Phys. Chem. Chem. Phys. 16 (2014)1424.
[3] A. Walsh et al., Angewandte Chemie 2 (2015) 1791.

MMM 201853

(P1-04)
Isogeometric modeling and large-scale computation for stress fieldaround lattice defects*Shunsuke Kobayashi1, Ryuichi Tarumi2, Atsushi Suzuki3, Masao Ogino4, Yoji Shibutani1
1. Graduate School of Engineering, Osaka Univ., 2. Graduate School of Engineering Science, Osaka Univ., 3.
Cybermedia Center, Osaka Univ., 4. Information Technology Center, Nagoya Univ.

It is well known that classical elasticity yields singular stress field around the core of lattice defects suchas point defects and dislocations. Regularization of the stress singularity is therefore primarily importantto understand mechanical behavior of defects from a continuum viewpoint. First strain gradient elasticity(FSGE) generalizes the classical elasticity by introducing the characteristic length l in the constitutive
equation. Recent theoretical investigations revealed that FSGE removes the stress singularity at the coreof dislocations and disclinations. Hence, the non-local elasticity theory is suitable for multiscale modelingof defects in solid. In the present study, we aim to develop (i) isogeometric models for lattice defects and(ii) their numerical implementation for large-scale computation within the framework of FSGE. At first, wederive the weak form stress equilibrium equation for Cauchy-type FSGE. The integrodifferential
Euler-Lagrange equation is then solved numerically using isogeometric analysis (IGA), i.e., Galerkinmethod with the non-uniform rational B-spline (NURBS) basis functions. Consequently, the boundary
value problem is cast into a system of linear algebraic equations whose degree of freedom exceeds 20
million. The large-scale computation was conducted on a supercomputer OCTOPUS which is installed in
the cyber media center of Osaka Univ. Point defects were modeled using the first-order approximation for
the force dipole and implemented into the weak form equation through the body force. The resulting
stress fields showed singularity free distribution and they converged to classical solution with increasing
in distance from the core. Dislocation model was constructed referring to the extended finite element
method (XFEM). This model successfully reproduces the kink deformation from the pairs of equallyarrayed edge dislocations.

54

MMM 2018

(P1-05)
A local/nonlocal plasticity model for upscaling microstructural effects*John Mitchell1
1. Sandia National Laboratories

Despite its importance, why is mesoscale plasticity advancing so slowly? The
fundamental equations of equilibrium used for mesoscale modeling have notchanged in centuries. In this talk, I will present an innovative idea for
combining nonlocal/local models to upscale effects of microstructures onplastic deformations. A local von Mises plasticity model is evolved using anonlocal and peridynamics [1] inspired yield condition [2]. The application area isthe analysis ofadditively manufactured metal parts.
Additive manufacturing produces heterogeneous and poorly understood material
microstructures. Practical macroscale models that reflect the nature of AM
microstructures do not currently exist: length scale effects, grain shapemorphologies, grain orientations, intragrain defects, grain boundary
heterogeneities, aggregrate textures. For the purpose of engineering
design and qualification, it is essential that we develop andadvance models for AM built parts.
[1] S.A. Silling, Reformulation of elasticity theory for discontinuities and long-range forces,
Journal of the Mechanics and Physics of Solids, 48(1), (2000), 175-209.
[2] J.A. Mitchell, A nonlocal, ordinary, state-based plasticity model for peridynamics,
Sandia National Laboratories, tech report SAND2011-3166, May, 2011.

MMM 201855

(P1-06)
First-principles investigation of atomic hydrogen adsorption anddiffusion on/into Mo-doped Nb (100) surface*Zhong-min Wang1, Yang Wu1, Qingrong Yao1, Yan Zhong1, Chaohao Hu1, Huaiying Zhou1
1. Guilin University of Electronic Technology

Based on sequential study of the surface model, surface-model slab, H-adsorption sites, and H-diffusion
coefficient (D), we investigate the most likely process of atomic hydrogen adsorption and diffusion on/into
Mo-doped Nb (100) surface/subsurface (in the Nb12Mo4 case) via first-principles. Our results reveal thatthe (100) surface is the most stable Mo-doped Nb surface with the smallest surface energy (2.75 J/m2).

Hole sites (HSs) in the Mo-doped Nb (100) surface are H-adsorption-favorable mainly due to their large
adsorption energy (4.27 eV), and the H-diffusion path should preferentially be HS→TIS (tetrahedral
interstitial site) over HS→OIS (octahedral interstitial site) because of the correspondingly lower
H-diffusion energy barrier. With respect to a pure Nb (100) surface, the Mo-doped Nb (100) surface has asmaller energy barrier along the HS→TIS pathway (0.31 eV) and larger H-diffusion coefficient (5.65 ×10-10m2s-1).

56

MMM 2018

(P1-07)
First-principles Study on Electronic Properties of Hybrid MABX3
perovskites (MA= CH3NH3+; B= Pb, Sn, Ge; X= I, Br, Cl)*Narasak Pandech1,2, Thanundon Kongnok1, Sirichok Jungthawan1, Sukit Limpijumnong1, Walter
R.L Lambrecht2
1. School of Physics and NANOTEC-SUT Center of Excellence on Advanced Functional Nanomaterials, Suranaree
University of Technology, NakhonRatchasima 30000, Thailand, 2. Department of Physics, Case Western Reserve
University, Cleveland, Ohio 44106-7079, USA.

Recently, hybrid MABX3perovskites (MA= CH3NH3+; B= Pb, Sn, Ge; X= I, Br, Cl) have revolutionized
emerging photovoltaic technologies with the development of highly efficient solar cells, and haveattracted significant fundamental research interest. Despite the extremely fast progress in device
fabrication, the materials electronic properties, which determine the photovoltaic performance, are notyet fully understood. Here, we performed calculations of these materials, using a van der Waals-corrected
density functional theory (DFT) method using the Perdew–Burke–Ernzerhof (PBE) and
Heyd–Scuseria–Ernzerhof (HSE) hybrid functionals. Subsequently, we use the GW-approximation to
calculate the bands as quasiparticle excitations including also the spin-orbit coupling. The orientationand position of the MA-cation is found to significantly affect the electronic properties around the bandedges of the Pb and Sn based compounds. Because Ge based halides already prefer a rhombohedrally
distorted perovskite structure with off-centered Ge even for a simple inorganic cation (e.g. Cs), thethree-fold symmetry of the MA ion is compatible with a preferred orientation of the molecules along therhombohedral axis. The interplay between its dipole and the Ge off-centering leads to interesting
ferro-electric behavior.

MMM 2018

57

(P2-01)
Multiscale model of solid state amorphization during processing ofpharmaceutical materials
Chunyu Li1, Yifei Zeng1, Lorena Alzate-Vargas1, Pilsun Yoo1, Rachel Frocino2, Jeff Brum2, Peilin
Liao1, Marisol Koslowski1, *Alejandro Strachan1
1. Purdue University, 2. GlaxoSmithKline, Analytical Sciences and Development

Processing of active pharmaceutical ingredients and excipients to reduce and control particle size involve
milling and micronization which result in severe plastic deformation and fracture. The increase in freeenergy of the crystal during deformation can result in polymorphic transformations and amorphization
and affect the physical properties of the product, including bioavailability.
In order to predict how materials properties and processing conditions affect plastic deformation andphase transitions in pharmaceutical materials we developed a multiscale model that combines electronic
structure using density functional theory, large-scale molecular dynamics simulations and a phase fieldmodeling. At the finer scale, we use DFT to predict elastic constants of the crystals and amorphoussystems of interest and validate the force fields used with MD. MD simulations provide insight into theprocess of amorphization and enables the characterization of the difference in enthalpy between the
crystal and amorphous phase and their interfacial energy, critical to describe the nucleation and growth of
the amorphous phase. Finally, the materials properties from DFT and MD calculations are used to inform aphase field model that describes, self-consistently, plastic deformation, including the nucleation of crystal
defects informed by dislocation dynamics, with phase transformations.
Using the multiscale model, we investigate the effect of deformation, shear, impact and particle surface
roughness on the evolution of the crystallite size and the nucleation and growth of an amorphous phase inmolecular crystals of interest for pharmaceutical applications.

58

MMM 2018

(P2-02)
FTMP-based Modeling and Simulations of HCP Mg Single Crystal*Takahiro Kitano1, Tadashi Hasebe1
1. Dept. of Mechanical Engineering, Kobe Univ.

Extended usages of Mg and Mg alloys have been widely promoted to date, attempting to make the bestuse of their attractive properties, such as light weight, high specific strength, superior recyclability andexcellent bio-compatibility. Modeling attempts of Mg and Mg alloys, however, often meet difficulty in thetreatments of the complex plasticity, mainly stem from their highly anisotropic slip systems and intricate
activities of twinning. The present study aims at enhancing the feasibility of the recently-proposed modelfor deformation twinning based on FTMP (Field Theory of Multiscale Plasticity) toward extended
applications to HCP Mg single crystal. The major updates of the model are two fold: one is the extension
to efficient 3D simulations by introducing special computational schemes, while the other is to apply
modified lattice rotation based on the rotational part of the incompatibility tensor. The twinning modelwith the modified lattice rotation is introduced in the hardening law in the FTMP-based crystalline
plasticity framework, and is further implemented into a finite element code accommodated with the above
new scheme. Deformation analyses are performed based on 2D/3D FEM for pure single crystal
magnesium with HCP (hexagonal close-packed) structure, and critical comparisons are made withexperimental data obtained under plain-strain compression in multiple orientations by Kelly and Hosford,
particularly, with those exhibiting the unique stress-strain responses induced by deformation twinning,i.e., orientations A, E and F. Natural expression of twin-induced lattice rotation is attempted byintroducing a modification based on the pure rotation part of the incompatibility tensor, spontaneously
evolved during deformation. The modification is shown to allow autonomic transitions between slip andtwinning modes, e.g., from twin-dominant stress response to that carried by slip, as in the orientations E
and F by Kelly-Hosford.

MMM 2018

59

(P2-03)
Nonlinear elasticity on Riemannian manifold and its application to
general surface development*Yuto Horikawa1, Ryuichi Tarumi2, Yoji Shibutani1
1. Dept. of Mechanical Engineering, Osaka Univ., 2. Grad. Sch. of Engineering Science, Osaka Univ.

This study aims to develop a theoretical framework to construct a three-dimensional curved surface frompieces of an elastic sheet which is embedded in two-dimensional Euclidean space. Our formulation isbased on the standard nonlinear elasticity within the framework of differential geometry. We first
introduce the Riemannian manifolds which equip the metrics, g[0] and g[t], for reference and current
configurations. The strain energy density is defined as a quadratic form of Green strain tensor under theassumption that elastic medium is isotropic in the reference configuration g[0]. Then, the surface
development problem ends up with a variational problem such that to find an embedding mapping which
minimizes the strain energy functional. We solve the variational problem numerically using theisogeometric analysis (IGA). To this end, we first derive a weak form equilibrium equation from the firstvariation of the functional. The embedding mapping is approximated by a linear combination of
non-uniform rational B-spline (NURBS) functions with the coefficients ξ. Consequently, the equilibrium
equation yields a system of nonlinear algebraic equations for ξand which is solved iteratively around a
linearized solution by the Newton method. It should be noted here that present method consider in-plane
deformation of the elastic sheet exclusively. It implies that two-dimensional isometric deformation, suchas out-of-plane bending deformation, produces no strain energy in the medium. In this regards, present
method is a fair generalization of conventional surface development method which is frequently used in
the paper craft such as origami. Numerical analysis for a one-dimensional system demonstrates that
present method converges to an exact solution within a sufficient accuracy.

60

MMM 2018

(P2-04)
The challenge of achieving quantitative phase field models for
microstructure evolution in irradiated solids - the case of voids*Anter El-Azab1
1. Purdue University

We present a thermodynamics-consistent formalism of a phase field model of void growth in irradiatedsolids, along with its sharp interface counterpart. Asymptotic matching of the two models was performed
to yield all phase field model parameters in terms of real materials properties. In this presentation, theresults of a first quantitative simulations of void growth driven by irradiation using our phase fieldapproach will be reported. This work was performed in collaboration with Srujan Rokkam, Thomas
Hochrainer, and Karim Ahmed.

MMM 2018

61

(P2-05)
A Molecular-Dynamics Study of Surface Tension: From Alloy Droplets
to Bubbles in Molten Alloy*Xiangming Ma1, Hongtao Liang1, Yang Yang1
1. East China Normal Univercity

We present a substantial atomistic simulation study of the equilibrium surface tension for liquid-statebinary alloy systems. Four types of spherical surface/interfaces are investigated: liquid Pb droplet
embedded in bulk liquid Al, liquid Al droplet in bulk liquid Pb, a bubble in liquid state Pb-Bi eutectic (LBE)
and a LBE droplet. The surface tension for these non-planar surfaces are calculated based on thecalculation of the local pressure tensor in spherical coordinates. Several thermodynamics property(droplet size, temperature, capillary pressure, mutual miscibilities) dependencies of surface tension are
obtained. Our results provide useful input data for the mesoscale simulations of the selective laser
melting, advanced welding and bubble formation in nuclear coolants.

62

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Symposium B(B1 oral)
Thermal stability of carbon-vacancy complexes in iron alloys andsteels*Milan J Konstantinovic1, Lorenzo Malerba1
1. Belgian nuclear institute, SCK.CEN

Many industrially important alloys and steels are intentionally processed and explored, as metastablemicrostructures comprising a supersaturation of crystal defects in various forms of aggregation. Even
though it is well understood that this microstructural complexity governs the material performance, thedefects composition, thermal stability, how they are distributed and in which concentrations are not yet
resolved experimentally. Hardened ferritic steels are important example where the lattice defects playdeterministic role in their deformation behavior..
While it is long-known that carbon atoms dispersed in the matrix of iron and steels have an influence on
the type of defects produced, it is only recently that atomistic simulations have clearly and quantitatively
revealed that carbon atoms bind very strongly to vacancies, giving rise to the formation of a whole faunaof carbon-vacancy (mCnV) complexes. These complexes may act as traps for other defects, whichsuggests that hardening in iron and its alloys will be the consequence of the interaction of dislocations
with complex defects formed by not only point defects but also carbon atoms. In this context, theformation of mCnV clusters is the triggering mechanisms for the growth of other complex defects.
In this study, the focus is given on the examination of relaxation processes which occur in the internal
friction spectra of a variety of iron alloys and steels with different carbon concentrations, andout-of-equilibrium vacancy concentrations, achieved either by fast cooling (quenching) or plastic
deformation. The relaxation peaks are analyzed on the basis of the Debye relaxation model and theiractivation energies and integrated intensities are determined. By comparing the results with theoretical
calculation, positron annihilation spectroscopy results, and magnetic after effect measurements, theobserved relaxation processes are assigned to the dissolution of carbon-vacancy (mCnV) clusters [1].
Dissolution energies and relaxation strengths (concentrations) of various mCnV clusters are found to be in
the correct energy range according to density functional calculations.
[1] M. J. Konstantinović and L. Malerba, Phys. Rev. Mat. 1, 053602 (2017).

MMM 2018

63

(B1 oral)
Computational study of phosphorous migration to grain-boundary inalpha-iron*Ken-ichi Ebihara1, Tomoaki Suzudo1
1. Center for Computational Science & e-Systems, Japan Atomic Energy Agency

Phosphorus(P) causes Grain-boundary(GB) embrittlement that is considered as one factor of
ductile-brittle transition in reactor pressure vessel steels. While the prediction of GB P segregation isdesired, the detailed process of P segregation to GB is still unclear. We studied the process of P
segregation to GB by simulating the migration of a mixed interstitial dumbbell, an octahedral interstitial P
atom and a P-vacancy complex using a molecular dynamics method. As a result, it was found that the P
atom of them becomes a substitutional atom before they reach the GB. The self-interstitial atom (SIA) or
vacancy of them was absorbed by the GB. This phenomenon was also confirmed partially by
first-principles calculations. In addition, it was found that the P atom left behind approaches the GB bysuccessively interacting with an SIA or a vacancy coming to its nearest neighbor site. We think that theconventional understanding of GB P segregation should be revised.

64

MMM 2018

(B1 oral)
Properties of interstitials in concentrated Fe-Cr alloys from firstprinciples*Marcin Roland Zemla1, Jan Stanislaw Wrobel1, Marek Muzyk1, Tomasz Wejrzanowski1, Duc
Nguyen-Manh2, Sergei L. Dudarev2, Luca Messina3, Par Olsson3, Christophe Domain4
1. Faculty of Materials Science and Engineering, Warsaw University of Technology, Woloska 141, 02-507 Warsaw, 2.
CCFE, Culham Centre for Fusion Energy, Abingdon, Oxon OX14 3DB, 3. KTH Royal Institute of Technology, Nuclear
Engineering, 106 91 Stockholm, 4. Département Matériaux et Mécanique des Composants, EDF-R&D, Les Renardiè
res, F-77250 Moret sur Loing

Point defects, for example self-interstitials and vacancies, play an important part in controlling propertiesof materials and their kinetic evolution. As a consequence, proper understanding and modelling ofmaterials properties require precise knowledge of point defect behaviour and their characteristics, inparticular their formation and migration energies. In magnetic alloys such as FeCr, the investigation ofpoint defects is complicated, because their properties strongly depend on various parameters such as Cr
composition, alloy short-range ordering and the defect local environment [1-3].
In this work, we have investigated properties of dumbbell-type defects in FeCr with a large dataset ofdensity functional theory calculations, for Cr concentrations ranging from low to approx. 35%. Analysis
shows that properties of self-interstitial and interstitial (IA) defects in concentrated bcc FeCr alloys
fluctuate significantly as functions of the above parameters. These fluctuations are observed to a lesser orgreater extent for the lattice parameter, chemical potentials, bulk moduli, relaxation volumes, magnetic
moments, and migration energies of dumbbells. We conclude that the formation energy of dumbbells
depends on the Cr content and also on the number of Cr atoms in the local environment of a defect. For
each chemical composition the average migration energies of Cr IA defects (Cr-Cr, Cr-Fe dumbbells)
moving to the first nearest-neighbour (1NN) position, are significantly smaller than those of Fe-Fe
exchanging with the 1NN Fe atom. The relaxation volumes of dumbbells increase in the ordered structureand decrease in the disordered one with the increase of number of Cr atoms in the local environment of adefect. The trends exhibited by the mean values of the various properties have been observed andanalysed, and the most stable directions of Fe-Fe, Fe-Cr and Cr-Cr dumbbells have been determined.
Moreover, the energy of elastic interaction between such defects are quantitatively estimated using thesuitably parametrized dipole tensors of the defects.
[1] D. Costa, G. Adjanor, C.S. Becquart, et al., J. Nucl. Mater. 452 (2014) 425–433[2] D. Nguyen-Manh, M.Y. Lavrentiev, S.L. Dudarev, C. R. Physique 9 (2008) 379-388[3] D. Nguyen-Manh, M.Y. Lavrentiev, M. Muzyk et al., J Mater Sci (2012) 47: 7385

MMM 201865

(B1 oral)
Defect production in cascade overlap with defect clusters in iron andtungsten*Jesper Byggmastar1, Fredric Granberg1, Andrea E Sand1, Kai Nordlund1
1. Department of Physics, University of Helsinki

During the lifetime of a typical reactor, the radiation damage in the wall materials will reach high damage
doses. Collision cascades initiated by subsequent irradiation will therefore become increasingly likely tooverlap with pre-existing atomistic damage, such as dislocation loops, voids, and other defect structures.
We have therefore carried out a large number of molecular dynamics simulations using well-establishedinteratomic potentials, to build a database for the defect production in collision cascades overlappingwith pre-existing defect clusters in both iron and tungsten. We primarily study cascades overlapping withdislocation loops of different size. Cascade overlap on other defect clusters leads to similar results. We
find that cascades overlapping with pre-existing interstitial clusters result in a significant reduction in thenumber of new point defects. The overlap effect on vacancy clusters is weaker. By running cascades on
different-sized clusters, with different cascade energies and at different temperatures, we obtain adatabase that can be used to fit a simple analytical model for the defect production bias due to theoverlap effects. The data and the fitted analytical model can be transferred to larger-scale simulation
methods, such as Monte Carlo techniques, to model the radiation damage accumulation at longer lengthand time scales.

66

MMM 2018

(B1 oral)
Multiscale modelling of radiation damage evolution in Fe andFe-based alloys*Fredric Granberg1, Haixuan Xu2, Kai Nordlund1
1. Department of Physics, University of Helsinki, 2. Department of Materials Science and Engineering, University ofTennessee

In order to predict the behaviour of materials used in places where radiation is present, many aspects
must be considered. The macroscopic effects like swelling and cracking, are a product of defect evolution
starting at atomistic level. The energetic ion will, on the atomic level, transmit energy to lattice atoms,
which will start collision cascades in the material. To be able to predict what the irradiation will do to thematerial, we must start on this level. The problem arises from that there is no single method that canaccurately simulate everything from single atoms or electrons up to macroscopic parts that builds up theconstruction. To tackle this problem, a combination of two or more methods can be used consecutively,
to be able to extend the time and/or length scale of the simulation. In this study, we focus on extending
the time scale, to be able to predict the evolution of irradiation induced defects.
To study the irradiation effect in Fe and Fe-based alloys we utilize Molecular Dynamics (MD), to simulate
the primary damage production on an atomistic level. The simulations are carried out by giving a recoil
energy to a random atom in the simulation box. These simulations are very accurate to study the
movement of all atoms, but are limited to nanoscale simulation boxes and times on the pico- or
nanosecond time scale. The length scale is sufficient to accurately simulate the defect production,
however the time scale is not sufficient for a long term evolution of the system. This long term evolution isneeded to obtain comparable results with experiments and to compare with materials in use in thesekinds of environments. To remedy the time scale problem, we utilize Kinetic Monte Carlo (KMC), to obtain
longer time scales. A special version of KMC, Self-Evolving Atomistic KMC (SEAKMC), is used as nopredefined and pre-calculated barriers are needed. This on-the-fly method is perfect for cases where thedefects produced can have many different complicated structures. In these cases, it would be impossible
to predict their structure beforehand and calculate all the barriers. A combination of MD and SEAKMC is
used to both accurately predict the produced primary damage (with MD) as well as its evolution (withSEAKMC), before a second cascade will hit the same region, as in experimental cases where intensive
irradiation is present.

MMM 2018

67

(B2 invited)
Atomic scale calculations of nuclear fuel properties to sustain
multiscale modeling of fuel behavior*Emeric Bourasseau1, Marjorie Bertolus1, Michel Freyss1, Gérald Jomard1, Ibrahim Cheik Njifon1,
Martin-Stéphane Talla Noutack1, Cyrille Takoukam Takoundjou1
1. CEA/DEN/DEC, Centre CEA de Cadarache

Until recently, simulations of the fuel behavior during normal or accidental operation in nuclear reactor
were based on rather empirical laws built from numerous experimental observations. Nowadays,
simulation codes try to render the underlying physics, and are used to understand the measurements
performed during irradiations or post-irradiation experiments. Thus, the fuel modelling and the laws used
to predict its behavior must be improved by fundamental researches and feed by results from separate
effects experiments. In support to these experiments, multi-scale modelling is an important tool, which
aim is to understand the phenomena occurring at the microscopic scale to improve the modelling of thematerial at the macroscopic scale.
In this presentation, we will show our recent works concerning atomic scale simulations used to improve
the knowledge of nuclear fuel properties before and during irradiation. Three points will be emphasized.
First, we will present ab initio calculations of point defects in MOX fuels, and how they can be used todetermine the elementary mechanisms of atomic diffusion in the irradiated fuel. Then, we will show a
study of the energetic and structural properties of grain boundaries in UO2 using classical molecular
dynamics. In particular, an interesting correlation observed between cleavage energy and disorientation
angle will be shown. And finally, we will present the results of the determination of thermodynamic
properties of (U,Pu)O2 through molecular Monte Carlo method. This method appears to be particularly
relevant to explore the various cationic configurations available in mixed oxide.
For each examples, we will try to underline the link with experimental works, and to show how our results
are included in simulations at higher scales.

68

MMM 2018

(B2 oral)
Influence of vibrational entropy on the concentrations of uranium
vacancies in UO2*Jean-Paul Crocombette1, Fabien Bruneval1, Aurélien Soulié1, Mihai-Cosmin Marinica1, SamuelMurphy2
1. CEA Saclay, 2. Lancaster University

The wide deviations of the composition of UO2 around its nominal composition are accommodated bypoint defects or clusters of oxygen interstitials. The formation energies of these defects have been thesubject of many studies DFT calculations. In the high temperature understoichiometric regime, atomistic
calculations and experiment agree on the fact that oxygen vacancies are the dominant defect. At the
opposite for the overstoichiometric material the formation energies predict uranium vacancies todominate over oxygen interstitials. This contradicts experimental facts which show that oxygen
interstitials, either isolated or clustered are accommodating the overstoichiometry in UO2+x. This so-called“uranium vacancy problem” has been noticed for many years [1] and has resisted the various
calculations improvements, either methodological (e.g. use of DFT+U) or numerical (e.g. calculations
made on bigger boxes).
Considering the vibrational entropy of the defects enables to solve this problem. We have combined abinitio formation energies of point defects and oxygen clusters in UO2 [2] with harmonic entropies
calculated with a many body empirical potential with charge equilibration [3]. Including the vibrational
contribution to the free energy of defects de-stabilizes the uranium vacancies compared to the oxygen
defects. The latter are then predicted to be dominant at high temperatures or large overstoichiometry inagreement with experiments. However our calculations exhibit a composition and temperature domain
where uranium vacancies may in fact be the majority defect in UO2+x. Some experimental observations(dilatometry, positron annihilation spectroscopy and uranium diffusion measurements) tend to confirm
this prediction.
[1] J.P. Crocombette, et al., Phys. Rev. B 64, 104107 (2001).
[2] F. Bruneval, et al., Phys. Rev. Materials 2, 023801 (2018).
[3] A. Soulié, et al., Acta Mater. 150, 248 (2018).

MMM 2018

69

(B2 oral)
Modelling of defect and rare gas transport properties in UO2 from theatomic to the grain scale*Marjorie Bertolus1, Michel Freyss1, Emerson Vathonne1, Emeric Bourasseau1, Serge Maillard1,
Gérald Jomard1
1. CEA, DEN, Département d'Etude des Combustibles, Centre de Cadarache, 13108 Saint-Paul-lez-Durance

One of the challenges for the next generation of nuclear reactors is to increase significantly the efficiency
in designing and qualifying innovative fuels. One way of doing this is to develop a more physically-based
description of nuclear fuels to enhance the predictive capability of fuel behaviour simulation. This should
enable a better selection of promising fuel systems and a reduction of the effort necessary for the designof new fuels. Basic research approaches combining multiscale modelling and separate effect experiments
can bring significant insight into key phenomena involved in the evolution of nuclear fuels during theirreactor life.
We will describe the multiscale modelling approach developed from the atomic to the mesoscopic scale
to investigate the transport properties of defects and rare gases in nuclear fuels. We will show examples of
the results obtained at atomic scale on uranium dioxide concerning the data necessary for the mesoscalemodels and for the interpretation of separate experiments on fuels. We will also present the application ofmesoscale models, in particular cluster dynamics.

70

MMM 2018

(B2 oral)
Intragranular bubble impact on nuclear fuel thermomechanicalproperties
Fabienne Ribeiro1, Mehdi Colbert1, *Jack Arayro1, Guy Tréglia2
1. Institut de Radioprotection et de Sûreté Nucléaire/PSN-RES/SEMIA/LPTM, France, 2. Centre Interdisciplinaire deNanoscience de Marseille, CNRS, France

UO2 is used as a standard fuel in pressurized water reactors. During fission reactions bubbles of xenon
are generated. The presence of these bubbles modifies the thermo-mechanical properties of the fuel. The
need to characterize these effects led to an extensive work both from experimental and theoretical pointsof view. Our contribution belongs to the later type.
First, the variation of the thermomechanical properties of UO2 versus porosity is studied throughatomistic simulations with semi-empirical potentials. A good agreement is found between the elastic
properties calculated in the present simulations, and those coming from micro-mechanical modelling andexperimental ones. Concerning thermal properties, an analytical model taking into account thenanoporosities is derived. This study emphasizes the importance of bubble surface effects of intragranular
bubbles on the thermomechanical behavior of the matrix.
Second, to clarify this effect, we study simplified systems of xenon on UO2 surfaces. We first determine
the surface relative stability according to their orientations and then to their polarities, by combining
thermostatistical relaxation and analytic formulations within a simple electrostatic model. The main result
is that, whereas the (111) surface appears stable with only minor reorganization, the polar (100) one isonly stabilized through drastic rearrangement of the surface region. Xenon adsorption on these relaxedsurfaces is then realized through Grand Canonical Monte Carlo simulation.
Finally, the pressure inside the Xenon and in the UO2 matrix is investigated. For the Xenon, we show thatwhatever its cristallographic structure, the pressure increases with the density, but not with thetemperature for a fixed density. In the latter case, we present stress profiles through the UO2 matrix
before and after xenon adsorption. The next step will be to introduce these results in a micromechanicalmodel in order to derive a thermomechanical behavior law for the porous UO2.

MMM 201871

(B3 oral)
Elastic fields and interaction between self-interstitial atom defects in
bcc metals.
*Sergei L Dudarev1, Pui-Wai Ma1
1. UK Atomic Energy Authority

Strongly anisotropic self-interstitial defect configurations form spontaneously in body-centred cubic
metals, like sodium or tungsten, if an extra atom is inserted in the crystal lattice and the resulting structure
is relaxed into the lowest energy configuration. The equilibrium structure and modes of Brownian motionof individual SIA defects in body-centred transition metals are now well established. Yet, there is still no
regular approach to modelling evolution of ensembles of such defects that would include the treatment ofelastic interaction between them. The difficulty appears fundamental, illustrating the lack of a suitable
mathematical formalism, linking the discrete atomistic representation of nano-scale defects with
continuum elasticity. We derive an analytical expression for the dipole tensor of a dislocation loop, valid in
the isotropic and anisotropic elasticity approximations, and explore it in the limit of infinitely small loop
size. We discover that the prediction for the dipole tensor of a point defect that does not agree withnumerical calculations even for defects in tungsten, a material that is well described by isotropic elasticity
theory. We then derive an analytical formula for the dipole tensor of a defect using a two-parameter
tensor form, which shows that in addition to a pure prismatic dislocation loop character, the elastic fieldof a SIA defect contains a significant dilatation component. We evaluate the energy of interaction between
SIA defects, and between a SIA defect and a dilatation centre, e.g. a vacancy cluster. To illustrateapplications of the new formalism, we compute the energy of interaction between SIA defects ordered asa periodic simple cubic super-lattice, encountered in a DFT calculation. Surprisingly, we find that theenergy minimum of such a periodic configuration corresponds to the orientation of the directional unit
vector of the defect collinear with a 111 direction.
This work has been carried out within the framework of the EUROfusion Consortium and has received
funding from the Euratom research and training programme 2014-2018 under grant agreements No
633053 and No 755039. Also, it has been part-funded by the RCUK Energy Programme (Grant Number
EP/P012450/1).

72

MMM 2018

(B3 oral)
Molecular dynamics simulation study of the interaction of Re withinterstitial defects in tungsten bulk
Yangchun Chen1, Jingzhong Fang1, Ning Gao2, Hong-Bo Zhou3, Wangyu Hu4, Fei Gao4,5, *Huiqiu
Deng1,4
1. School of Physics and Electronics, Hunan University, 2. Institute of Modern Physics, Chinese Academy of Sciences,
3. Department of Physics, Beihang University, 4. College of Materials Science and Engineering, Hunan University, 5.
Department of Nuclear Engineering and Radiological Science, University of Michigan

Tungsten (W) and W-based alloys have been considered as promising candidates for plasma-facing
materials (PFMs) in future fusion reactors. However, the neutrons generated in fusion reactions result in
not only the cascade damage but also the transmutation products (mainly Re and Os) in W bulk. The
irradiation defects and transmutation products will have a serious effect on the service behaviors of W
PFMs under the condition of fusion irradiation. With molecular dynamics simulations and W-Re potentials
developed recently in our group, we have systematically investigated the interaction between Re
atom/cluster and interstitial defects in W bulk. It is found that: (1) the self-interstitial atoms (SIAs) andinterstitial Re atoms are energetically favorable to align along <111> direction in the form of W-W and
Re-W dumbbell/crowdion, respectively; (2) the SIA clusters and 1/2<111> interstitial dislocation loops
can undergo fast migration along <111> direction and their migration barriers (< 0.1 eV) are nearly
independent of defect size; (3) Re atoms inhibit the migration and growth of SIA clusters, due to theirstrong attractions; (4) newly formed Re-Re dumbbells and Re-W mixed dumbbell clusters are thermallystable and immobile, which can serve as the trapping center for subsequent Re-W mixed dumbbell,leading to the growth of Re clusters in W. The present work is helpful for understanding the interaction of
Re with interstitial defects and dislocation loops and the initial nucleation mechanism of Re atoms in bulk
W, and the results can also be used to supply more accurate inputs for larger-scale simulations such as
object kinetic Monte Carlo simulations.

MMM 201873

(B3 oral)
Hydrogen super-saturated layers in plasma loaded tungsten: a global
model combining Density Functional Theory data, Thermodynamic
and Kinetic models*Yves Ferro1, Etienne Hodille1
1. Aix-Marseille University

In this work, we combined Density Functional Theory data, a Thermodynamic and a kinetic model to build
a global model enabling to determine the total concentration of hydrogen implanted in the sub-surface oftungsten exposed to a hydrogen flux. This is achieved given a flux of hydrogen, a temperature of
implantation, and the energy of the incoming hydrogen ions. This model is built step by step; anequilibrium with a molecular hydrogen gas phase is first considered and the resulting solubility iscompared with experimental results. A kinetic model is subsequently used to determine a chemical
potential for hydrogen in the sub-surface of tungsten. Combining both these models, two regimes arethen established in which hydrogen is trapped at interstitial sites or in vacancies. The existence of these
two regimes are driven by the temperature of implantation; above a temperature of implantation is theinterstitial regime, below is the vacancy regime in which super-saturated layers are formed. A simple
analytical and easy to use expression is work-out for the transition temperature, which allows to plot a
diagram of existence of the super-saturated layer depending on the implantation temperature, theincident energy and the flux of the hydrogen ions.

74

MMM 2018

(B3 oral)
Mobility of small vacancy and interstitial prismatic dislocation loops in
BCC tungsten*Jan Fikar1, Roman Gröger1, Robin Schäublin2
1. Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Brno, 2. Laboratory of Metal Physics
and Technology, Department of Materials, ETH Zürich

Vacancies and self interstitials produced in collision cascades often form clusters, which can further
collapse into prismatic dislocation loops. In BCC metals dislocation loops with Burgers vectors 1/2<111>
and <100> are observed and they can be either vacancy or interstitial type. These dislocation loops
usually move easily in the direction of the Burgers vector. The mobility of prismatic dislocation loops isstudied by molecular dynamics. The small dislocation loops under 100 defects seem to behave more likepoint defect clusters and the mobility of vacancy loops is substantially lower than the one of interstitialloops. Similar effect is observed in formation energies, where the small vacancy loops have higher
formation energies than the corresponding interstitial loops. At larger sizes the dislocation loops seem tobehave as perfect dislocations and there is no difference between vacancy and interstitials loops
mobilities and formation energies.

MMM 201875

(B4 invited)
Kinetics of Precipitation in Fe-Cr and Fe-Cr-C alloys under Irradiation*Frederic Soisson1
1. CEA Saclay

In ferritic steels, the precipitation of the Cr-rich α’phase leads to a degradation of mechanicalproperties. Below 400°C, the precipitation is usually too slow to be a real concern, but it can besignificantly accelerated by irradiation, due to an increase in point defect concentrations. Irradiation also
produces a ballistic mixing between Fe and Cr atoms, and therefore prevent the nucleation ofα’precipitates or the dissolution of existing ones.
We present a multiscale approach which models these contradictory effects by taking into account thedifferent mechanisms of atomic transport taking place under irradiation: the creation of vacancies andself-interstitials and the ballistic mixing within displacement cascades, the thermally activated diffusion ofpoint defects, and their elimination at sinks or by mutual recombination. The method combines DFT
calculations (which provide the fundamental information on thermodynamics and point defects
properties), atomistic Kinetic Monte Carlo Simulations (to model the diffusion of point defects and theα’precipitation), and Cluster Dynamics (to model the evolution of the sink density under irradiation).
The simulations show that the acceleration of the α’precipitation occurs under irradiation at low ormoderate dose rates, but that ballistic dissolution becomes dominant under ion irradiation at high dose
rates -- due to a large density of sinks that limits the point defect supersaturation.
The results of the simulations are compared with recent experimental studies. The same multiscale
approach is used to model the diffusion of carbon atoms in Fe-Cr alloys and their strong interaction withpoint defects. We will consider their possible effect on Fe and Cr diffusion, and on the kinetics ofα’precipitation under irradiation.

76

MMM 2018

(B4 oral)
The complex problem of the experimental validation of atomistic andmicrostructural evolution models*Lorenzo Malerba1
1. CIEMAT, Energy, Environment and Technology Research Centre

The development of physical models for the description of the behaviour of materials under certain
conditions goes hands-in-hands with the use of advanced experimental characterization techniques. The
experiments provide data and reveal phenomena that the models are expected to predict and explain.
The exchange of information between models and experiments is therefore continuous and goes in two
directions: models may predict results that need to be verified experimentally and experiments observe
processes that the models are expected to reproduce and explain in terms of physical laws andmechanisms. This continuous interplay is often denoted as "experimental validation" of models. While thisdefinition is somewhat restrictive, the validation of models is a discipline in itself that requires first of all
the performance of experiments that may be long and expensive, and need to be carefully designed. In
addition, the comparison between the results of the models, in particular computer simulations, and theobservations from characterization techniques is not always straightforward, because both theexperimental and the simulation outcomes need to be properly treated.
In this presentation these problems will be discussed taking the example of the comparison between
models that describe the microstructural evolution of iron alloys under irradiation and the results of
microstructural characterization techniques such as positron annihilation, small angle neutron scattering,
atom probe and transmission electron microscopy.

MMM 2018

77

(B4 oral)
Physically based prediction of radiation hardening: application tosteels and model alloys*Ghiath Monnet1
1. EDF - R&D, MMC

Based on a multiscale modeling framework, it is shown that radiation hardening can be rationalized inseveral industrial materials and model alloys. Microstructure features of the as-received materials are firstconsidered for the assessment of the yield stress prior to irradiation. The later can be decomposed into
two contributions: short-range stresses induced by local obstacles (such as forest dislocations,precipitates, etc.) and long-range stresses resulting from sub-grain dislocation walls and grain boundaries.
Introducing radiation defects as local obstacles, the same methodology can be employed to predict theyield stress of the irradiated materials. The considered radiation defects are solute clusters (SCs) andDislocations Loops (DLs). SCs are assumed to be spherical coherent precipitates that can be sheared bydislocations with a given shear resistance, computed from atomistic simulations. This resistance is used in
Dislocations Dynamics (DD) simulations to compute hardening induced by SCs. The sizes and densities
implemented in DD simulations are those given by experiment (TEM, APT, SANS etc.). The contribution of
DLs is computed from DD simulations. Upon interaction with mobile dislocations, small DLs are absorbed
leading to strong pinning and high hardening level, while large DLs experience classicaldislocation-dislocation interactions (junction, annihilation, etc.). The yield stress of the irradiatedmaterials is predicted using the constitutive equations of radiation hardening induced by SCs and DLs.
Several FeCr, FeMnNi model alloys and some RPV steels are considered for the validation of the model. It
is shown that local obstacles (present before irradiation) may have a significant effect on radiation effects
on the mechanical properties: radiation hardening is not independent of the initial yield stress. With
almost no adjustable parameters, the predicted increase in the yield stress is found in close agreementwith experiment.

78

MMM 2018

(B4 oral)
Oxygen diffusion in bcc Fe under the influence of foreign atoms andvacancies*Xiaoshuang Wang1, Matthias Posselt1, Jürgen Faßbender1
1. Helmholtz-Zentrum Dresden-Rossendorf (HZDR)

A multiscale approach including Density Functional Theory (DFT) and Atomistic Kinetic Monte Carlo(AKMC) simulations is applied to investigate the diffusion of oxygen in bcc Fe under the influence of
substitutional foreign atoms or solutes (Al, Si, P, S, Ti, Cr, Mn, Ni, Y, Mo, W) and vacancies. The solutes canbe assumed to be immobile since their diffusion coefficient is much smaller than that of oxygen. On theother hand, the vacancy mobility must be considered in the calculations because it is comparable to thatof oxygen. The most stable state of oxygen in pure bcc Fe is the octahedral interstitial configuration.
Recently, jumps of oxygen in pure bcc Fe, between first-, second-, and third-neighbor octahedral
interstitial sites were investigated by DFT. It was found that the first-neighbor jump is most relevant. The
second-neighbor jump consists of two consecutive first-neighbor jumps whereas the barrier of thethird-neighbor jump is too high to be significant for the diffusion process. In this work DFT is used todetermine the modified migration barriers in the presence of solutes. It is found that Si, P, Ni, Mo and W
have some effect on the migration barriers of oxygen and their interaction with O is mainly repulsive. Al, Cr
and Mn have a significant influence on the barriers and they exhibit strong attractive interactions with O.
The most important modification of the barriers is found for S, Ti, and Y where deep attractive states exist.
The barriers for oxygen jumps near a vacancy and barriers for vacancy jumps in the environment ofoxygen are also calculated by DFT. Based on the migration barriers obtained by DFT, AKMC simulations
on a rigid lattice are employed to determine the diffusion coefficient of oxygen in a dilute iron alloy
containing different substitutional foreign atoms. It is found that Si, P, Ni, Mo, and W have almost no
influence on the diffusivity of O. The presence of Al, Cr, Mn, S, Ti, and Y causes a significant reduction of
the mobility of oxygen. Another version of the AKMC code is applied to investigate the mutual influence ofoxygen and vacancy diffusion as well as the migration of the oxygen-vacancy pair.

MMM 2018

79

(B5 oral)
He impurities in boron carbide : structure, kinetics, and Raman
signatures*Guido Roma1, Kevin Gillet1, Anton Schneider1, Jean-Paul Crocombette1
1. CEA, Univ. Paris-Saclay

Helium impurities, produced by nuclear reactions in the neutron absorber boron carbide, are a matter ofconcern because they form bubbles and eventually affect the mechanical properties of the absorbing
elements in fast breeder sodium reactors. In order to shed light on the mechanisms of bubble formation
we have investigated, using first principles methods, the insertion of He in boron carbide (withstoichiometry B4C) and the possible migration mechanisms. Whereas in absence of preexisting damage, at
equilibrium, He is expected to occupy interstitial sites and diffuse via an interstitial mechanism [1], weshow that, under irradiation where a supersaturation of vacancies is expected, He atoms can be trappedat vacancies and diffusion might be slowed down.
How to probe such kind of impurities? Raman spectroscopy is increasingly used to quantify damage in
irradiated materials, including boron carbide. In order to help the interpretation of such experiments wecalculated from first principles the first order Raman spectrum of B4C containing defects ; to circumventthe size effect we devised an inclusion procedure allowing us to obtain the dynamical matrix and theRaman tensor of a supercell containing a defect at a dilution lower than 0.1 atomic %. The results showthat interstitial He impurities do not significantly perturb the spectrum, while substitutional ones do.
[1] A. Schneider, G. Roma, J.-P. Crocombette, V. Motte, D. Gosset, J. Nucl. Mater. 496, 157 (2017).

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Irradiation damage in nuclear graphite at the atomic scale*Alain Chartier1, Laurent Van Brutzel1, Justin Pageot2
1. DEN, Service de la Corrosion et du Comportement des Materiaux dans leur Environment, CEA Saclay, 2. DEN,
Service d’Etudes des Materiaux Irradies, CEA Saclay

Radiation response and microstructure evolution of nuclear graphite are investigated by combination ofboth Molecular Dynamics (MD) simulations and experimental observations with mainly High Resolution
Transmission Electron Microscopy images (HRTEM) and X-ray diffraction patterns (XRD). Radiation
response of single crystal graphite is examined using MD with point defects accumulation method. Results
reveal that graphite structure undergoes three stages before amorphisation: (i) an increase of the numberof point defects; (ii) a wrinkling of graphene layers pinned by small amorphous pockets; and (iii) a full
amorphisation of the structure via percolation of the small amorphous pockets [1]. Each stage can be
related to the swelling along the c-axis and the shrinking in the basal plane. In particular rippling
contributes significantly to the strain. One filler particle - composed of almost aligned crystallites
separated by Mrozowski cracks –is also generated to provide insights on a more realistic microstructure of
an-irradiated nuclear graphite.
Subsequently, simulated XRD pattern and HRTEM images have been generated from the MD simulations
and are compared with experimental observations [2]. Simulated HRTEM images show many features
observed in experimental images in both virgin and irradiated nuclear graphite. Some of these features
can be linked unequivocally to defined atomistic configurations. Basal grain boundaries (GBs), Mrozowski
cracks, graphene sheets and their folding belong to this category. Conversely, some patterns in simulated
HRTEM cannot be related to a unique atomistic configuration and might eventually give rise to misleadinginterpretation. This is evidenced for edge dislocations in virgin nuclear graphite as well as for residues ofgraphene layers in highly damaged graphite.
These findings confirm that univocal identification of atomic scales structures in graphite from HRTEM
images only is uneasy.

[1] A. Chartier, L. Van Brutzel, B. Pannier, and Ph. Baranek, Carbon 91 (2015) 395.
[2] A. Chartier, L. Van Brutzel, and J. Pageot, Carbon 133 (2018) 224.

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(B5 oral)
Using computational modeling to understand radiation damage
tolerance in complex oxides both from the bottom-up and the
top-down*Blas Pedro Uberuaga1
1. Los Alamos National Laboratory

Meeting the ever-increasing demand for energy is a key challenge for the 21st century. Nuclear energy is aproven and green energy source that will be a key component of the world’s energy profile. However,maximizing the efficiency of nuclear energy systems requires materials that have significantly increased
tolerance against radiation damage. Computational modeling has an important role in understanding anddiscovering new materials for next-generation nuclear energy systems.
In this talk, we will describe research efforts that apply computational modeling to understand the
response of materials to radiation damage. We will focus on a class of complex oxides, pyrochlores, thathave been proposed for nuclear waste encapsulation. Pyrochlores, with the chemical formula A2B2O7,
are related to the simpler fluorite structure, with the added complication of having two cation species andoxygen structural vacancies. Past work by numerous groups has shown that the radiation tolerance of
these materials is sensitive to the nature of the A and B cations and, in particular, their propensity todisorder. However, these observations are empirical at best and there is still a lack of understanding on
the factors that govern the radiation response of these materials.
We have tackled this problem from two different perspectives. First, using accelerated molecular
dynamics, we have studied how cation disorder, often created during radiation damage, impacts defectkinetics and thus the transport mechanisms that dictate damage recovery. This bottom-up approach has
revealed that a percolation transition occurs as disorder is introduced that leads to higher defectmobilities, which in turn promotes self-healing of the damage. On the other hand, we have used materials
informatics to analyze the role of pyrochlore chemistry on radiation tolerance. In this case, divorced fromthe complexities of making true predictions of performance, we instead use machine learning to take a
top-down perspective and discover heuristic relationships between the material composition and thesusceptibility of the material to amorphization. While neither study provides a complete understanding ofradiation damage in these materials, together they provide a more complete picture of the factors that
dictate their response to irradiation.

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Development of defect mechanics-based multi-scale simulation
techniques for reliability study of high performance electronic devices
in radiation environments*Yukeun Eugene Pak1, Youjung Seo2, Dhaneshwar Mishra1, Keonwook Kang3, Jihyun Kim4
1. Advanced Institutes Convergence Technology, 2. Seoul National University, 3. Yonsei University, 4. Korea University

GaN-based electronic devices are very attractive for space applications since their radiation hardness
characteristics are superior to the Si-based devices. For space missions requiring very conservative design
margins, the presence of fabrication-induced defects as well as the radiation-induced defects limit theutilization of space-borne electronics. Therefore, prediction models to understand the defect generation
mechanisms and the radiation effects on GaN devices are needed to properly engineer the reliability ofthese devices for the radiation environment.
To this end, we developed the quantitative linear elasticity models that can predict the stresses andpiezoelectric fields induced in multilayer quantum wells in the presence of lattice defects such as
dislocations. These electroelastic field calculations are coupled with the quantum mechanical formulation
to predict the electronic and optical behavior of GaN-based devices such as LEDs andhigh-electronic-mobility transistors(HEMTs). To characterize the radiation effects, molecular dynamics(MD) simulations were performed to study the formation mechanisms of point defects caused by theenergy transfer from irradiated particles. The required parameters for MD simulations were obtained by
the Monte Carlo (MC) simulation code, GEANT4, and the results are compared with the irradiation test
results.

MMM 2018

83

(B6 invited)
Ion Irradiation as a Surrogate for Reactor Irradiation: The Expected
and the Surprises*Gary S Was1, Zhijie Jiao1, David Woodley1, Stephen Taller1, Gerrit Vancoevering1, Anthony
Monterrosa1, Elizabeth Getto4, Brian Wirth2, Arthur Motta3
1. University of Michigan, 2. University of Tennessee, 3. Pennsylvania State University, 4. United Stated Naval
AcademyReactor core materials in both fast reactors and LWRs granted life extension must withstand irradiation tohigh doses at high temperature. Ferritic-martensitic (F-M) alloys are attractive candidates for structuralcomponents of fast and thermal reactors, and high chromium and high nickel-containing austenitic steels
are potential replacement alloys for LWR core materials. Both require high dpa, for which ion irradiation is
ideally suited. To simulate the reactor radiation environment, self-ion irradiation and He injection are
conducted simultaneously into both F-M and austenitic alloys. Reactor irradiations of the same alloys
have been, or are being conducted in the BOR-60 fast reactor to assess the capability of ion irradiation toemulate the evolution of reactor generated microstructures and mechanical properties. Computationalmodels for defect cluster evolution are being developed and benchmarked against experimental data toultimately provide predictive capability for the response of both microstructure (loops, voids, precipitates,etc.), and mechanical properties (hardening, ductility, slip behavior) to irradiation. To date, agreement isquite promising with many outcomes ocurring as expected. However, there are a number of observations
or results that are unexpected. Results will be presented on the microstructure and mechanical property
evolution in ion and reactor irradiation integrated with computational modelling in an effort to understandthe extent to which ion irradiation can be used as a surrogate for reactor irradiation, and to illuminate
processes that are not well understood.

84

MMM 2018

(B6 oral)
Dose Effect on the Irradiation Induced Loop Density and Burgers
Vector in Ion-Irradiated Ferritic/Martensitic Steel HT9 Through In-Situ
TEM
Ce Zheng1, Stuart Maloy2, *Djamel Kaoumi1
1. North Carolina State University, 2. Los Alamos National Laboratory

TEM samples of F/M steel HT9 were irradiated to 20 dpa at 420°C, 440°C and 470°C in a TEM with 1 MeV
Kr ions so that the microstructure evolution could be followed in situ and characterized as a function ofdose. Dynamic observations of irradiation induced defect formation and evolution were done at thedifferent temperatures. The irradiation-induced loops were characterized in terms of their burgers vector,
size and density as a function of dose and similar observations and trends were found at the threetemperatures: (i) both a/2 <111> and a <100> loops are observed; (ii) in the early stage of irradiation,
the density of irradiation induced loops increases with dose (0-4 dpa) and then decreases at higher doses(above 4 dpa), (iii) the dislocation line density shows an inverse trend to the loop density with increasing
dose: in the early stages of irradiation the pre-existing dislocation lines are lost by climb to the surfaces
while at higher doses (above 4 dpa), the build-up of new dislocation networks is observed along with theloss of the radiation-induced dislocation loops to dislocation networks; (iv) at higher doses, the decreaseof number of loops affects more the a/2 <111> loop population; the possible loss mechanisms of the a/2<111> loops are discussed. Also, the ratio of a <100> to a/2 <111> loops is found to be similar to casesof bulk irradiation of the same alloy using 5 MeV Fe2+ions to similar doses of 20 dpa at similar
temperatures.

MMM 201885

(B6 oral)
Novel Deformation Mechanism of Helium Irradiated Copper*WEIZHONG HAN1
1. Xi'an Jiaotong University

The workability and ductility of metals usually degrade with exposure to irradiation, hence the phrase“radiation damage”. Here, we found that Helium (He) radiation can actually enhance theroom-temperature deformability of submicron-sized copper. In particular, Cu single crystals with diameterof 100 nm to 300 nm and containing numerous pressurized sub-10 nm He bubbles, become stronger,
more stable in plastic flow and ductile in tension, compared to fully dense samples of the same
dimensions that tend to display plastic instability (strain bursts). The sub-10 nm He bubbles are seen to
be dislocation sources as well as shearable obstacles, which promote dislocation storage and reduce
dislocation mean free path, thus contributing to more homogeneous and stable plasticity. Failure happens
abruptly only after significant bubble coalescence. Furthermore, we discover that the helium bubble not
only can coalesce with adjacent bubbles, but also can split into several nanoscale bubbles under tension.
Alignment of the splittings along a slip line can create a bubble-free-channel, which appears softer,
promotes shear localization, and accelerates the failure in shearing-off mode. Detailed analyses unveilthat the unexpected bubble fragmentation is mediated by the combination of dislocation cutting andinternal surface diffusion, which is an alternative micro-damage mechanism of helium irradiated copper
besides the bubble coalescence. These results shed light on plasticity and damage developments inmetals. Ref. PRL 117 (2016) 515501, Nano Lett.16 (2016)4118 and Nano Lett. 17 (2017) 3725.

86

MMM 2018

(B6 oral)
Isotope effect on quantum diffusion of interstitial hydrogen in
face-centered cubic metals*Hajime Kimizuka1, Shigenobu Ogata1,2, Motoyuki Shiga3
1. Osaka University, 2. Kyoto University, 3. Japan Atomic Energy Agency

Safe control of tritium is one of the technological challenges in the development of next-generation
deuterium-tritium fusion reactors where tritium is used as nuclear fuel. From a scientific viewpoint,
measuring the rate of diffusion of hydrogen isotopes in metals is important for understanding the isotope
effect on the kinetics of hydrogen transport and hydrogen-induced deterioration of materials. However, aconsensus on the physical mechanisms and numerical values of diffusivities of hydrogen isotopes inmetals is still lacking. In this study, we clarified the site preference and diffusion rate of interstitialhydrogen and tritium in several face-centered cubic (fcc) metals, such as palladium, aluminum, andcopper, by performing ab initio path-integral molecular dynamics (PIMD) modeling in the framework ofdensity functional theory. This was necessary as the hydrogen atom has sufficiently low mass that itexhibits significant nuclear quantum delocalization and zero-point motion even at ambient temperature.
The nuclear quantum effect on the activation free energies for hydrogen migration was characterizedaccording to the PIMD-based free energy profiles obtained from the thermodynamic integration of thecentroid force along the migration path at 75-1200 K. We found that the nuclear quantum effects
significantly affected the activation barrier for hydrogen migration and the difference between theenergies of the hydrogen atom at the octahedral and tetrahedral interstitial sites even at ambient
temperature. Consequently, we revealed the role of quantum fluctuations on the reversed isotope
dependence for hydrogen diffusion in certain fcc metals.

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87

(B7 invited)
Thermal properties of fluorite-type metal dioxides: CeO2, ThO2, UO2,
NpO2, PuO2 and AmO2
*Masato Kato1
1. Japan Atomic Energy Agency

Actinide dioxide having a fluorite structure is one of prospective candidates as a fuel of advanced
reactors. Thermal property of nuclear fuels is essential data to evaluate fuel performance. Therefore, great
efforts to measure properties and understand their mechanism have been made so far. It is well-known
that thermal properties of actinide oxides changes significantly depending on temperature, and theirmechanism is complicated. Author’s research group has studied on thermal properties of CeO2, UO2,
PuO2 and their solid solution. In this work, basic properties of various fluorite-type dioxides weresummarized, and temperature dependence of thermal properties were evaluated.
In fluorite-type oxides of CeO2, ThO2, UO2, NpO2, PuO2 and AmO2, mechanical property, Debye
temperature, Grunisen constant, thermal expansion, oxygen potential, heat capacity Cp and thermal
conductivity were reviewed, and the data were compared. Heat capacity at volume constant Cv andthermal expansion term Cd were considered from their data in heat capacity evaluation. The calculation
result of (Cv+Cd) was in good agreement with experimental data in CeO2 and ThO2, but the calculated
data of other actinide dioxides underestimated the experimental data by 10-20 J/molK. The extra Cp from
(Cv+Cd) was considered as a Schottky term Csch which related to 5f electrons. In fact, 5 f electrons do notexist in CeO2 and ThO2. Experimental data of UO2, NpO2, PuO2 and AmO2 were described with thecalculated (Cv+Cd+Csch). The extra Cp was described assuming two level energy model.
Thermal conductivity of the dioxides was evaluated by phonon conduction mechanism using Slack’s
equation. Phonon conduction of pure oxide can be represented by 1/(BT). Thermal conductivity of CeO2
and ThO2 was represented by Slack’s equation very well. But, other conduction mechanism is needed toevaluate thermal conductivity of UO2, NpO2, PuO2 and AmO2.
It is reported that excited term at high temperatures which is contributed with Frenckel defect, electron
and so on exists in heat capacity and thermal conductivity. The experimental data at high temperatures
are lacked to evaluate the excited terms. In addition of experimental data in high temperature region,
computational approach is expected to understand deeply mechanism in property change.

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MMM 2018

(B7 oral)
SCIANTIX: A new inert gas behaviour module ready for use*Davide Pizzocri1, Tommaso Barani1, Lelio Luzzi1
1. Politecnico di Milano

Being able to bridge lower length-scale calculations with the engineering-scale simulations of fuelperformance codes requires the development of dedicated intermediate-scale codes. In this work, we
present SCIANTIX, a recently developed code which aims to fill this gap. SCIANTIX is a 0D stand-alone
computer code under development at Politecnico di Milano since 2016. It is designed to be included as amodule in existing fuel performance codes (e.g., TRANSURANUS). It contains models describing inert gas
behaviour at the scale of a fuel grain, represented as a point (i.e., 0D). Since it is 0D, no spatial
discretization is required, and all variables are treated as average in space. Nevertheless, the models
available in SCIANTIX (covering intra- and inter-granular inert gas behaviour, and high burnup structureformation and evolution as well) are physics-based and not correlation-based. The main characteristic of
SCIANTIX is thus the simplicity of the models (i.e., low computational burden, in line with therequirements of fuel performance codes), paired with the possibility to inform them with parameters fromlower length-scale calculations. SCIANTIX is validated against hundreds of experimental data describing
inert gas behaviour at the scale of fuel grains. The validation is supported by uncertainty analyses on themain model parameters. Moreover, sensitivity analyses are performed to prioritize further research
activities. Showcases of validation, uncertainty and sensitivity analyses are presented in this work (e.g.,concerning gas concentrations, evolution of intra- and inter-granular bubble populations, swelling, highburnup structure formation). As for the numerical treatment of the model equations, SCIANTIX isdeveloped with full numerical consistency and entirely verified with the method of manufactured
solutions (verification of different numerical solvers is also showcased in this work). The open source
release of the code is planned soon.

MMM 201889

(B7 oral)
Modeling swelling in U3Si2 nuclear fuel using a multi-scalecomputational approach*Larry Aagesen1, Karim Ahmed2, Benjamin Beeler1, Daniel Schwen1, Yongfeng Zhang1
1. Idaho National Laboratory, 2. Texas A&M University

U3Si2 is a leading candidate for application as an accident-tolerant fuel for commercial nuclear power
generation. Due to U3Si2’s high thermal conductivity relative to conventional UO2 fuel, it is believed tohave a wider margin to melting/relocation in accident scenarios. However, based on limited experimental
data, it is believed that U3Si2 undergoes much more significant swelling. To predict the swelling of U3Si2,
a multi-scale computational approach has been developed. A phase-field model of fission gas bubble wasdeveloped, which tracks vacancies on the U lattice sites and Xe atoms as defect species. The proportional
swelling of the fuel was assumed to be equal to the volume fraction of the bubble phase. The phase-fieldmodel was parameterized using density functional theory and molecular dynamics calculations. The
incorporation of the swelling predictions to engineering-scale fuel performance models will be discussed.

90

MMM 2018

(B7 invited)
Why Multiscale Modeling of Nuclear Fuel is Absolutely Essential and
Why it is so Challenging*Michael R Tonks1, Yongfeng Zhang2, David Andersson3, Simon Phillpot1, Christopher Stanek3
1. University of Florida, 2. Idaho National Laboratory, 3. Los Alamos National Laboratory

The performance of reactor fuel is historically determined using codes that rely on empirical fits toexperimental data. However, as reactor operation becomes more variable and as we consider changes tothe fuel and cladding system to increase accident tolerance, these empirical models become insufficient
to meet our needs. The US Nuclear Engineering Advanced Modeling and Simulation (NEAMS) program isbeginning to implement an alternative approach to fuel performance materials models that relies on
representing the evolving state of the fuel microstructure using state variables that impact the material
properties. However, developing the required models for this approach using only experimental data
would be extremely difficult, time consuming, and expensive. Hierarchical multiscale modeling andsimulation, ranging from density functional theory to mesoscale simulation approaches, provides a
powerful means to obtain additional data to inform the development of these new materials models.
However, applying this multiscale approach to reactor fuel has a number of challenges. In thispresentation I will summarize this multiscale approach being taken in the US and discuss a number of theissues that make it difficult.

MMM 2018

91

(B8 oral)
Kinetic Monte Carlo study of tungsten fuzz formation under low energy
helium irradiation*Zhangcan Yang1, Yingzhao He1, Quansong Peng1
1. School of Energy and Power Engineering, Huazhong University of Science and Technology

Tungsten has been considered as the primary candidate for the plasma-facing materials of fusion reactors.
However, the formation of fuzz-like nanostructures on tungsten surfaces under low-energy helium plasma
irradiation has raised big concerns since these fuzzy structures have been shown to be detrimental to theperformance of the materials and the stability of plasma. Unfortunately, the mechanism of fuzz formationis still under debate. Here we present an Object Kinetic Monte Carlo study that links the nucleation andgrowth of helium bubbles with the evolution of surface morphology. The key difference of our modelcompared with a previous model by Lasa’s[1] is that the motion of the punched dislocation loops andthe diffusion of surface ad-atoms are modeled in detailed in our model. As a result, our model is able toproduce real fuzz-like structures rather than the glassy structures in Ref [1]. We first study the sub-surface
helium clustering behavior in tungsten as a function of temperature, helium implantation rate, andconcentration of pre-existing defects [2]. The key parameters that affect helium clustering behavior havebeen identified. We then investigate the possible processes that may contribute to the surface evolution
and quantitatively analyze their relative contribution. We find that the motion of dislocation loops and thesurface diffusion are the two key processes to the surface evolution. Bubble rupture only affects the initial
surface roughness, but plays a negligible role in the growth of fuzz structures.
Reference:[1] A. Lasa et al 2014, EPL, 105(2), 25002.
[2] Z. Yang et al 2017, Fusion Science and Technology, 71(1), 60.

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MMM 2018

(B8 oral)
Constrained thermodynamic model for multi-component alloys underirradiation: A matrix formulation from first-principles Hamiltonian*Duc Nguyen-Manh1, J.S Wrobel2, A. Fernandez_Caballero 3, S.L. Dudarev1
1. Materials Science and Scientific Computing Department, Culham Centre for Fusion Energy, UKAEA, 2. Faculty ofMaterials Science and Engineering, Warsaw University of Technology, 3. School of Mechanical Aerospace and Civil
Engineering, University of Manchester

Understanding the behaviour of multi-component alloys under irradiation is one of the great challenges
for the development of materials for nuclear applications. Recent experimental investigations revealed
that micro-structural evolution of multi-component metallic alloys formed by transmulation under neutron
irradiation can be very complicated since they may undergo spinodal decomposition andradiation-induced precipitation due to the strong coupling between defects and local chemical
environment. Very recently, it was shown that concentrated solute solution alloys including high-entropy
alloys (HEAs) may exhibit significantly improved performance under irradiation that depends strongly on
the number of alloying elements and local alloy composition.
In this work, a revisited constrained thermodynamic model, initially proposed by Georges Martin, hasbeen developed to model multi-component alloys under irradiation. The model is based on ab initio
calculations in combination with a cluster-expansion Hamiltonian generalized for systems containing
vacancy (V) and interstitial (I) defects. It is found that this formalism can be mathematically represented interms of a matrix formulation for any N-component system via cluster correlation functions, which in turncan be deduced consistently from Monte-Carlo simulations. Analytical expressions for local short-rangeorder parameters for alloy components and configurational entropies as functions of temperature andcomposition have been derived explicitly from this matrix representation. In the first nearest-neighbour
approximation, the new approach reproduces the ABVI Ising model for a binary system as well as thethermodynamic limit of the Cluster Variation Method. We apply this formalism to anomalous precipitationin W(Re,Os,Ta) alloys under neutron irradiation as well as in low activation bcc and high-radiation
resistance fcc HEAs and their derivatives.

MMM 2018

93

(B8 oral)
Kinetic Monte-Carlo Simulations of Radiation Damage in W(Re,Os)
Alloys*Matthew James Lloyd1,2, David Armstrong1, Enrique Martinez Saez3, Duc Nguyen-Manh2
1. Department of Materials, University of Oxford, 2. Culham Centre for Fusion Energy, 3. Los Alamos National
Laboratory

The realisation of fusion energy is dependant on the development of high performance materials that canwithstand the extreme conditions that they will be subjected to. The plasma facing components of afusion reactor will experience a combination of high thermal loading (~10MWm-2) and an intense flux ofboth 14MeV neutrons and high energy He/H ions (~5-20dpa/yr), meaning they will have to operate atvery high temperatures (~1300 K). Tungsten is currently the leading candidate material due to its high-1 -1

melting temperature (~3695K); good thermal conductivity (~150Wm K ) and resistance to sputtering(Eth=200eV). But at such a high neutron flux, transmutation of W in a fusion reactor is significant,resulting in several at.% Re, Os and Ta over the lifetime of the reactor. Precipitation of Re and Os has been
observed well below their solubility limit in W, resulting in embrittlement, hardening, and a reduction inthermal conductivity.
The research presented examines irradiated W-(Re,Os,Ta) alloy systems, using a combination of DFTparametrised, multicomponent atomistic kinetic Monte-Carlo (AKMC) modelling, and high resolution
nanoscale characterisation techniques such as atom probe tomography (APT). The role of interstitialdefects in W-Re and W-Os is thought to play an important role in precipitation, because of the low
rotation energy of the W-Re/W-Os mixed dumbbell. This enables the 3D transport of solute atoms, thatwould otherwise be confined to the <111> direction. We present a multicomponent kinetic Monte Carlomodel, incorporating both vacancy and interstitial defects, and solute concentration dependantinteractions. The predictions made by our model are supported by APT data of W-Re and W-Os alloys
irradiated at high temperature, using W ions.

94

MMM 2018

(B8 oral)
Modeling Re-precipitate hardening in neutron irradiated W and W-Re
alloys: from point defects to macroscopic hardening
Chen-Hsi Huang1, Leili Gharaee2, Paul Erhart2, *Jaime Marian1
1. University of California Los Angeles, 2. Chalmers University

High-temperature, high-dose, neutron irradiation of W results in the formation of Re-rich clusters atconcentrations one order of magnitude lower than the thermodynamic solubility limit. These clusters may
eventually transform into brittle W-Re intermetallic phases, leading to severe embrittlement and loss ofthermal conductivity. Standard theories of radiation-enhanced diffusion and precipitation cannot explain
the formation of these precipitates and so understanding the mechanism by which nonequilibrium
clusters form under irradiation is crucial to predict material degradation and devise mitigation strategies.
In this work, we integrate neutronics, primary damage calculations, molecular dynamics results, Re
transmutation calculations, and stochastic cluster dynamics simulations to study neutron damage insingle-crystal tungsten to mimic divertor materials. We study the material response under experimental
conditions at the JOYO fast reactor and the High Flux Isotope Reactor, for which measurements of cluster
densities and hardening levels exist. We then provide calculations under expected DEMO fusion
conditions. Several key mechanisms involving Re atoms and defect clusters are found to govern theaccumulation of irradiation damage in each case. We use established correlations to translate damage
accumulation into hardening increases and compare our results to the experimental measurements. We
find hardening increases in excess of 5000 MPa in all cases, which calls into question the performance of
these materials under service conditions in fusion reactors.

MMM 2018

95

(B8 invited)
In-situ TEM of Formation Processes of Defects in Tungsten under
Irradiation: Comparison between Electron and Self-ion Irradiations*Kazuto Arakawa1
1. Shimane University

Nuclear-fission and fusion materials are degraded primarily due to the accumulation ofradiation-produced lattice defects, such as point defects (self-interstitial-atoms (SIAs) and vacancies) andpoint-defect clusters (dislocation loops and cavities). In order to precisely predict the lifetimes of nuclear
materials, accurate understanding of the origins of the defect accumulation—generation of defects andtheir subsequent dynamics—is crucial.
In-situ transmission electron microscopy (TEM) is a powerful technique for probing defect dynamics, in
response to external stimuli such as irradiation under heating or cooling. As the irradiation sources for thein-situ TEM, electrons and ions are available. In the electron irradiation, only point defects are generated
as the primary damage via knock-on displacement. In contrast, in the ion irradiation, point-defect clusters
are also generated as the primary damage, which is called “collision cascade”, like neutron irradiation.
In this presentation, we focus on the formation process of dislocation loops in tungsten under irradiation.
Firstly we show our results on dynamic properties of SIAs [1] and SIA dislocation loops, which have been
mainly obtained with high-voltage electron microscopes in Osaka University and Nagoya University inJapan. And, we provide our results on the formation processes of dislocation loops under self-ion
irradiation, which have been obtained with an ion-accelerators combined microscope in the
JANNuS-Orsay facility in France. Through the comparison between these results, we try to extract theeffects of collision cascade on the formation processes of dislocation loops.

References[1] T. Amino, K. Arakawa, and H. Mori, Scientific Reports 6 (2016) 26099.

96

MMM 2018

(B9 oral)
Ab Initio Modeling of Self-Interstitial and Vacancy Migration in
Zirconium*Emmanuel Clouet1, Céline Varvenne2
1. CEA Saclay, 2. CNRS - Aix-Marseille University

Irradiation in hexagonal close-packed zirconium leads to the creation of point-defects, both vacancies
and self-interstitials. Migration and clustering of these point-defects control the microstructure evolution
under irradiation. In particular, the faster diffusion of self-interstitial in the basal planes than along the caxis is often assumed to explain the self-organization of the microstructure observed in irradiated Zr as
well as the breakaway growth visible for high irradiation dose. As no direct experimental measurement
characterizing a possible diffusion anisotropy is possible, at least for the self-interstitial, ab-initio
calculations appear as a suitable alternative.
We model with ab initio calculations all possible configurations of the self interstitial and calculat with
NEB the different migration barriers between these configurations [1]. The attempt frequencies
corresponding to the different migration events are deduced from phonon calculations thanks to theharmonic approximation and the transition state theory. We thus fully characterize the migration of boththe self-interstitial and the vacancy. The obtained ab initio data is validated by modeling internal friction
in irradiated zirconium and comparing the low temperature peaks with the ones observed in experiments[2]. Once this validation step performed, diffusion coefficients are calculated and diffusion anisotropy ischaracterized.
[1] C. Varvenne, F. Bruneval, M.-C. Marinica and E. Clouet, Phys. Rev. B 88, 134102 (2013).
[2] R. Pichon, E. Bisogni and P. Moser, Radiation Effects 20, p. 159 (1973).

MMM 2018

97

(B9 oral)
Atomistic modelling of point defect clusters in zirconium and impact
on the microstructure evolution and crystal growth under irradiation
Benjamin CHRISTIAEN3, Christophe DOMAIN3,2, *Ludovic THUINET1,2, Antoine AMBARD3,2,
Alexandre LEGRIS1,2
1. UMET, UMR CNRS 8207, Lille university, F-59655 Villeneuve d’Ascq, 2. EM2VM, EDF-CNRS, 3. EDF R&D, MMC,Les Renardieres, F-77810 Moret-sur-Loing

Zirconium alloys are used to manufacture fuel cladding as well as fuel assemblies of pressurized water
nuclear reactors. Under irradiation, they show a dimensional change commonly called growth.
Experimental observations have shown that above a threshold dose, these alloys are subject toaccelerated growth called "breakaway". It has been well established that the irradiation formation of and<c> dislocation loops is directly responsible for the growth of irradiated zirconium alloys and that theappearance of <c> loops is correlated with this growth acceleration. However, the nucleation
mechanisms of the <c> loops are still poorly understood. In order to improve our understanding,atomic-scale calculations based on the density functional theory (DFT) and empirical potentials are used
to determine the properties of clusters of point defects (vacancies as well as self-interstitials) in terms offormation energy, binding energy and eigenstrain, of prime importance to assess their influence on the
deformation of the material. In particular, DFT simulations of dislocation loops in large supercells allow torevisit their energies used as input parameters of mesoscale models (Object KMC), resulting in predictionof growth and its acceleration. Moreover, pyramids of stacking faults are studied in more detail and theircharacterization sheds light on their plausible role in the nucleation sequence of <c> loop, which allows
to propose an original scenario for their appearance.

98

MMM 2018

(B9 oral)
Modeling of dislocation climb assisted glide in crystal plasticity models*Alankar Alankar1, Vikram Phalke1, Ritesh Dadhich1, Ashish Mishra1
1. IIT Bombay

We present crystal plasticity modeling of climb assisted glide in fcc, bcc and hcp metals. Dislocation
density is divided into edge and screw characters. Dislocation density evolution takes care of non-schmid
effect and the effect of screw dislocation core assymetry in hcp metals. A chemical stress component due
to non-equilibrium vacancy concentration acts on edge dislcoations and is called drag-stress. Thischemical stress component depends strongly on crystallographic orientation as well if the vacancy
concentration is low. Evolution of chemical stress and thus dislocation climb rate is discussed as afunction of dislocation density, strain, crystallographic orientation and temperature. Model is calibrated
against data from literature. The model is able to predict mechanical behavior under conditions of creep.
Also, depending on whether creep occurs or not, the model is able predict crystallographic texture during
deformation. Such model has been used for modeling of irradiation in Zr and zircaloys.

MMM 2018

99

(B9 oral)
The role of oxide grain boundaries in the oxidation of zirconium alloy
fuel cladding*Maria S Yankova1, Felicity Baxter1,2, Alistair Garner1, Philipp Frankel1, Christopher P Race1
1. Materials Performance Centre, School of Materials, University of Manchester, 2. Corrosion and Materials Science,
Wood. PLC

Oxidation of the Zr alloy fuel cladding in light water reactors is one of the key degradation mechanisms,
limiting the amount of fuel burned. The structural and electronic properties of monoclinic and tetragonal
zirconia grain boundaries strongly affect the transport of species through the oxide layer. Improving theunderstanding of oxygen transport through the oxide grain boundaries, and its interaction with dopant
point defects, is an important factor in achieving a more mechanistic knowledge and better control of thecorrosion process. We are exploiting a combination of density functional theory (DFT) simulation,scanning precession electron diffraction in the transmission electron microscope (SPED-TEM) and novel
Python-based texture analysis to obtain an improved mechanistic understanding of the oxide
microstructure. We have investigated the effect of oxygen defects and key alloying elements such as Sn
and Nb on the structural and electronic properties of representative oxide grain boundaries using DFT.
We have investigated dopant-oxygen vacancy binding which can have a significant effect on oxygen
conductivty. We have characterised the grain boundary misorientation distribution in non-irradiated andirradiated oxides with estimates on the relative boundary energetics. We have further used the results of
our DFT calculations to test a range of empirical potentials for the Zr-O system to establish their suitability
for computational modelling of zirconia at microstructural length-scales.

100

MMM 2018

(B9 oral)
Advances in X-ray Diffraction Line Profile Analysis of Dislocation Loops
in Zr - Insights from Atomistic Modelling.
*Chris P Race1, Rory Hulse1, Tamas Ungar1
1. University of Manchester

Irradiation-induced growth (IIG) of Zr-alloy nuclear fuel cladding can limit the service life of nuclear fuel. It
involves a macroscopic shape change driven by the formation and growth of populations of dislocation
loops.
Efforts to create Zr-alloys that are resistant to IIG thus rely on accurate determination of the size
distribution and number density of dislocation loops in irradiated candidate alloys. X-ray diffraction (XRD)
can, in principle, provide this information via an analysis of changes to the diffraction peak shapes. Suchmethods are well developed in the study of plastic deformation, but the different character of the defects
formed under irradiation complicates the analysis. An improved understanding of the effect of dislocation
loops on the diffraction peak shapes is therefore required.
We have constructed atomistic models of controlled defect populations in Zr and generated theoretical
XRD profiles. We have compared these with experimental profiles and analysed changes in lineshape interms of contributions from the strain fields of individual defects. In particular, we are able to explain theappearance of features in the experimental profiles that are peculiar to irradiated material. We show thatthese new features contain information about the character of the dislocation loops in irradiated material.

MMM 2018

101

(B10 oral)
Sink strengths of point defects near tilt grain boundaries: A phase fieldmodel*Pengchuang Liu1, Songlin Zheng2, Pengcheng Zhang1, Sanqiang Shi3, Kaiguo Chen2
1. Institute of Materials, China academy of engineering physics, 2. Institute of Fluid Physics, China academy ofengineering physics, 3. Department of Mechanical Engineering, The Hong Kong Polytechnic University

Dislocations and grain boundaries are perfect sinks for point defects (PDs), the ability of sinks to absorb
point defects is a critical factor for the irradiation damage behaviors of materials. Phase field (PF) method
was used to describe low angle tilt grain boundaries which composed of edge dislocations. Evolution
processes of tilt grain boundaries and PDs were coupled in a same PF model. We considered the climbing
mechanism rather than fixed dislocations. Sink strengths of grain boundaries were computed based on
the obtained steady-state average PDs concentrations. We studied the saturation properties of tilt grainboundaries and obtained the relations between sink strengths and dislocation densities. PDs generation
rate represents the irradiation intensity, it is also undemanding to take into account in this PF model.
Elastic interactions of point defects and dislocations have great impacts on the sink properties of tilt grainboundaries. We compared the effects of different eigenstrains and applied stresses and got theconnections between sink strengths and climbing speeds of dislocations. A physical phase-field model
was built, which covered material processes to study sink properties of tilt grain boundaries without
additional assumptions. We compared the calculated results with the theoretical and existing ones,
clarified the inherent differences between them, results are favorable for designing extremely radiation
tolerant materials.

102

MMM 2018

(B10 oral)
Modelling swelling and growth under irradiation using the phase field
method*Daniel Schwen1, Andea M. Jokisaari1
1. Dept. of Fuels Modeling and Simulation, Idaho National Laboratory

In extreme environments with high point defect production rates, some materials can experience
macroscopic dimensional changes due to the mechanisms of swelling and growth. Both processes are
related to preferential absorption of different point defect species at different sinks. In the case of swelling
this entails the creation of voids which act as vacancy sinks, while interstitials are absorbed at dislocations
or grain boundaries where new lattice sites are created, leading to a total volume increase of the sample.
Growth occurs if anisotropic interstitial and vacancy fluxes with differing spatial orientations lead to latticesite creation and removal along non-parallel lattice planes, leading to a volume conserving shape change.
Prediction of swelling and growth behavior requires an understanding of the defect transport and reaction
processes at the mesoscale. The phase field method has been established as a standard modelingtechnique on these length scales.
In this work we develop a modified set of phase field equations that allow for the local and anisotropic
creation and destruction of lattice sites at sinks using a tensor field describing the local lattice sitechanges. We apply this method to study the effects of microstructure on swelling and growth behavior inpolycrystalline materials, in the presence of applied mechanical loading, and microstructural features
such as precipitates.

MMM 2018

103

(B10 oral)
Phase-field modelling of dislocation loop evolution under irradiation :application to radiation induced segregation prediction near thedislocation cores*Gabriel Franck BOUOBDA MOLADJE1, Ludovic THUINET1, Alexandre LEGRIS1, Charlotte
BECQUART1, Maylise NASTAR2, Frédéric SOISSON2
1. CNRS, université de Lille, 2. CEA, Université Paris-Saclay

Understanding and predicting the microstructural evolution of metallic alloys constituting the reactor
vessels are crucial issues for safety in the nuclear industry. Under irradiation, point defects (PD) arecreated and diffuse towards microstructural defects such as dislocations. These microstructural defects
will evolve according to their ability to absorb PD, also known as sink strength. Dislocation climb is one
consequence of PD absorption and leads to growth or shrinkage of dislocation loops. Phase field (PF)models have already been proposed to describe dislocation climb when only vacancies are considered,
which couple non-conservative dislocation motion and vacancy diffusion through an absorption term. To
simulate growth/shrinkage of dislocation loops under irradiation in metallic alloys where vacancies but
also self-interstitials are created, a new formulation of these models is required, and this will constitutethe first part of the presented work. In particular, it will be shown that such a formulation is not astraigthforward generalization of the existing models. Thanks to this new PF model, well-known
phenomena observed under irradiation, such as radiation induced segregation, will be studied near thedislocation cores. The particular influence of climb and elastic interactions between PDs and dislocations,generally ignored in RIS calculations, will be discussed in detail.

104

MMM 2018

(B10 oral)
Theoretical derivation of the ABVI model from cluster expansion
Hamiltonian*Antonio Fernandez Caballero1,2, Jan Wrobel3, Paul Mummery1, Duc Nguyen-Manh2
1. School of Mechanical, Aerospace and Civil Engineering, University of Manchester, 2. Department of Materials
Science and Scientific Computing, CCFE, United Kingdom Atomic Energy Authority, Abingdon OX14 3DB, UK, 3.
Faculty of Materials Science and Engineering, Warsaw University of Technology, UI. Wołoska 141, 02-507 Warsaw,
PolandIn nuclear fusion research, degradation of material microstructure and nanostructure by radiation damage
is of interest both from experimentally and modelling perspectives. Modelling from first principles can beused to rationalize mechanisms behind degradation phenomena caused by the simplest type of damage
produced by radiation: vacancies and interstitials and their clusters. Recent studies found that density
functional theory and kinetic monte carlo can be used to investigate radiation induced segregation andprecipitation[1] incorporating both vacancy and interstitial into Ising-like ABVI model Hamiltonian[2]. The
kinetic processes of solute transport both by vacancies and interstitials can be captured by a Hamiltonianparametrised by bond energies and derived from DFT.
In this work, we treat multi-body defect-solute or solute-solute interactions as cluster entities from thecluster expanded Hamiltonian of the alloy system containing point defects. Binding energies for elements
within defect cluster are expressed in terms of effective interactions parameters found from the cluster
expansion of the Hamiltonian of the system containing vacancies and interstitials. Local effects fromdefect surroundings are incorporated by offseting the binding enery by the cluster expanded referenceenergy depending on chemical compositions of the surroundings[3,4]. This model can be used toinvestigate the kinetic simulations of segregation and precipitation of binary alloys under irradiation.

[1] G. Martin, 1980, Phys. Rev. B 21, 2122
[2] C. Huang and J. Marian, 2016, J. Phys.: Condens. Matter 28 425201[3] J.S. Wrobel, D. Nguyen-Manh, K.J. Kurzydlowski, S.L. Dudarev, 2017, Journal of Physics: Condensed

Matter, 29 145403.
[3] A. Fernandez-Caballero, J.S. Wrobel, P.M. Mummery, D. Nguyen-Manh, 2017, J. Phase Equilibrium andDiffusion, 38 391-403.

MMM 2018

105

(P2-06)
Effects of rhenium on the mechanical behavior of irradiated tungsten:a molecular dynamics study using neural-network potential*Ryo KOBAYASHI1
1. Nagoya Institute of Technology (NITech)

Tungsten (W) is considered as a promising candidate for the plasma facing wall material at divertor in
nuclear fusion reactors. In order to assess the lifetime of the divertor for safe operation of the nuclear
fusion reactors, mechanical response to the irradiation of high energy neutrons and helium atoms should
be precisely understood from atomistic scale. Under the irradiation of high energy neutrons, not only
defects are formed but also transmutation occurs, which will produce a few percent of rhenium (Re)concentration within several-year operation of nuclear fusion reactor. Thus the effect of solute Re atoms
on the mechanical behavior of host material, W, should be taken into account, such as recovering rate
after cascading damage, H/D/T retention, He bubble formation, or dislcation mobility. In this study, we
create a neural-network (NN) potential for quarternary system, W-Re-H-He, combined to an EAM potential
for W-Re binary system. It is known that machine-learning (ML) potentials such as NN can well reproduce
DFT energies of wide variety of atomic configurations, but it requires big data to learn a lot of freeparameters and it is usually much slower than the classical potentials such as EAM. By combining the NNwith EAM potentials, we can construct a potential with smaller number of referece data and make it much
faster than fully NN potential. The properties of the potential and the effect of Re on the recovering rate
after cascading damage, He bubble formation/growth rate, and dislocation punching will be discussed.

106

MMM 2018

(P2-07)
Diffusion behavior and temperature dependence of hydrogen intungsten and molybdenum under isotropic strain state*QuanFu Han1, Yue-Lin Liu2, Ying Zhang1, Guang-Hong Lu1
1. Beihang University, 2. Yantai University

The interaction between hydrogen and metal exhibits a great scientific and technological importance sothat it has a direct impact on the design and operation of metallic materials. Here, we have carried out
first-principles calculations to investigate interstitial hydrogen diffusion behaviors in tungsten andmolybdenum by considering double effects of temperature and strain. The temperature and strain effects
are reflected by the vibration Helmholtz free energy in the quasi-harmonic approximation and isotropic
loading, respectively. The hydrogen diffusion is analyzed through two nearest neighbor tetrahedral sites.
At a ground state (0-K) condition, the hydrogen diffusion activation energy can increase and decrease
notably with rising compressive and tensile strain, respectively. While at each compressive/tensile strain
case, the hydrogen diffusion activation energy depends distinctly on the temperature and increases withrising temperature. This is mainly originated from the contribution of vibration Helmholtz free energy
induced by the larger vibration frequency of hydrogen in two metals. With the increasing temperature, thehydrogen diffusivity is shown to be dependent on the compressive/tensile strain alteration. The present
study demonstrates a remarkable influence of “temperature and strain” on the hydrogen diffusion
behaviors in tungsten and molybdenum.

MMM 2018

107

(P2-08)

Diffusion of Point Defects on Tungsten Surface
1

1

2

1

1

Jiannan Hao , *Shuo Jin , Haixuan Xu , Xiaolin Shu , Guanghong Lu

1. School of Physics and Nuclear Energy Engineering, Beihang University, 2. Department of Material Science andEngineering, The University of Tennessee, Knoxville

Hydrogen (H) / helium (He) retention in tungsten (W) and can significantly reduce the thermal
conductivity and sputtering threshold of W, and at the same time, continuous bombardment with 14.1
MeV neutron can introduce Frenkel defects (composed of self-interstitial atoms (SIAs) and vacancies),
which lead to a high concentration of W impurity. The surface deformation and blisters are observed
experimentally in W, in which the surface morphology is relevant to the plasma fluence and surface
directions [1-2]. The near-surface atomistic configuration is changed via diffusion of frenkel defects which
is driven by heat and/or bubble loop punching process. Therefore, revealing the evolution mechanism ofpoint defects on the W surfaces under H/He irradiation is crucial for W application in future fusion
reactors.
We employ the first-principles and molecular dynamics simulation to calculate the energy barriers of
single adatom (AD) and vacancy (VA) diffusion on the W (100), (110) and (111) surface. The diffusion
paths of defects is determined by the dimmer method built in the SEAKMC code [3]. At least threediffusion paths with high energy barriers are found for both AD and VA on the (100) surface. However,
only one path with low energy barrier of both AD and VA on the (110) surface are dominated throughout
the diffusion process. Moreover, the diffusion of AD and VA on the (111) surface is investigated, in which
the formation energies of different surface defects are biased. Nonetheless, the symmetry of the (111)
surface will break spontaneously at the certain temperature and concentration of impurity through thethermodynamic calculation, and the morphology is in good agreement with the SEM images [1]. The work
will help to understand the early stage of surface morphology evolution under irradiations or in theannealing process.
Keywords: tungsten, point defects, diffusion, surface
References[1] H. Y. Xu, Y. B. Zhang, Y. Yuan, B. Q. Fu, A. Godfrey, G. De Temmerman, W. Liu, and X. Huang, J. Nucl.
Mater. 443, 452 (2013).
[2] F. W. Meyer, H. Hijazi, M. E. Bannister, K. A. Unocic, L. M. Garrison, and C. M. Parish, Phys. Scr. T167,
14019 (2016).
[3] H. Xu, Y. N. Osetsky, and R. E. Stoller, J. Phys. Condens. Matter 24, 375402 (2012).

108

MMM 2018

(P2-09)
Influence of anisotropic strain and temperature on hydrogendissolution in tungsten*Ying zhang1, Quan Fu Han1, Yue lin Liu2, Guang Hong Lu1
1. Beihang University, 2. Yantai University

Tungsten (W) is an alternative candidate for a plasma facing material in the future fusion reactor due to its
high melting point, high thermal conductivity and low plasma sputtering yield. The solubility of hydrogenin tungsten is a basic and crucial factor to influence the formation of hydrogen bubbles. In this work, wehave investigated the effects of anisotropic strain and temperature on the dissolution of H in tungsten via
the first-principles calculation in combination with thermodynamic models. The temperature and strain
effects are reflected by the vibration Helmholtz free energy in the quasi-harmonic approximation anduniaxial/biaxial strain loading, respectively. It is found that the solubility of hydrogen can be enhanced byboth compressive and tensile anisotropic strain, independent with of the sign of strain. This is different
from the influence of isotropic strain, where the solubility of hydrogen in tungsten responds to theisotropic strain monotonically. Besides, the difference of the dissolution energy between hydrogendissolves in tetrahedral and octahedral site constantly changes in the anisotropic strain range of -4%–4%.
Further, under same anisotropic strain condition, the solution energy of hydrogen in tungsten increases
with the increasing of the temperature from 300–1800 K, which can be mainly contributed to thevibration Helmholtz free energy. Our finding suggests that the local anisotropic strain and temperature
can significantly influence the dissolution of hydrogen in tungsten, which may play a key role on hydrogen
bubble formation.

MMM 2018

109

(P2-10)
Screw dislocation-interstitial solute coevolution in W-O alloys using
atomistically-informed kinetic Monte Carlo simulations*Yue Zhao1, Lucile Dezerald3, Jaime Marian1,2
1. Dept. of Materials Science and Engineering, University of California, Los Angeles, 2. Dept. of MechanicalEngineering, University of California, Los Angeles, 3. Institut Jean Lamour, University of Lorraine

The interaction of interstitial impurities with in-grown dislocations in metals can lead to various processes,
including solute solution hardening and/or dynamic strain aging. The coevolution of solutes anddislocations occurs on length and time scales that are very challenging to resolve experimentally. Theinteraction of interstitial solutes with dislocation segments is highly local, however, and models must be
capable of resolving the fine details of the interaction if we are to gain any understanding from theprocess. Here we develop a kinetic Monte Carlo model of dislocation motion in the presence of diffusing
solutes. We focus on the W-O solid solution, such that the subject of our study is screw dislocations, as
they control plastic flow at low temperatures in body-centered cubic metals and alloys. Solute diffusion isaffected by dislocation strain fields, which we study via the elastic dipole tensor using electronic structure
calculations. As well, we calculate binding energies of O atoms to screw dislocation cores, and discuss thejoint structures formed, the implications of the calculated energetics, and show the effect on thedislocation velocity of solute diffusion at several temperatures.

110

MMM 2018

(P2-11)
Kinetic Monte-Carlo Simulations of Radiation Damage in W(Re,Os)
Alloys*Matthew James Lloyd1,2, David Armstrong1, Enrique Martinez Saez3, Duc Nguyen-Manh2
1. Department of Materials, University of Oxford, 2. Culham Centre for Fusion Energy, 3. Los Alamos National
Laboratory

The realisation of fusion energy is dependant on the development of high performance materials that canwithstand the extreme conditions that they will be subjected to. The plasma facing components of afusion reactor will experience a combination of high thermal loading (~10MWm-2) and an intense flux ofboth 14MeV neutrons and high energy He/H ions (~5-20dpa/yr), meaning they will have to operate atvery high temperatures (~1300 K). Tungsten is currently the leading candidate material due to its high-1 -1

melting temperature (~3695K); good thermal conductivity (~150Wm K ) and resistance to sputtering(Eth=200eV). But at such a high neutron flux, transmutation of W in a fusion reactor is significant,resulting in several at.% Re, Os and Ta over the lifetime of the reactor. Precipitation of Re and Os has been
observed well below their solubility limit in W, resulting in embrittlement, hardening, and a reduction inthermal conductivity.
The research presented examines irradiated W-(Re,Os,Ta) alloy systems, using a combination of DFTparametrised, multicomponent atomistic kinetic Monte-Carlo (AKMC) modelling, and high resolution
nanoscale characterisation techniques such as atom probe tomography (APT). The role of interstitialdefects in W-Re and W-Os is thought to play an important role in precipitation, because of the low
rotation energy of the W-Re/W-Os mixed dumbbell. This enables the 3D transport of solute atoms, thatwould otherwise be confined to the <111> direction. We present a multicomponent kinetic Monte Carlomodel, incorporating both vacancy and interstitial defects, and solute concentration dependantinteractions. The predictions made by our model are supported by APT data of W-Re and W-Os alloys
irradiated at high temperature, using W ions.

MMM 2018

111

(P2-13)
Dynamics of Magnetism in Neutron Irradiated Iron-Chromium Steels*Jacob B. J. Chapman1, Pui-Wai Ma1, Sergei L. Dudarev1
1. Culham Centre for Fusion Energy (CCFE)

Ferritic-martensitic (F/M) steels, with a high chromium content, are critical materials for magnetic
confinement fusion technology due to high temperature stability and corrosion resistance, facilitating thethermal efficiency necessary for fusion power plant operation. To predict the evolution of the steel
microstructure and mechanical properties when subjected to sustained high doses of radiation atelevated temperatures, we must understand how radiation damage affects magnetic properties of thematerials, which strongly influence phase stability and chromium solubility.
We investigate the effect of neutron irradiation on the magnetic properties of F/M steel alloys, relating
experimentally observed [1] concentrations, radii and number densities of Cr precipitates within a Fe-Cr
matrix to the dynamics and degradation/enhancement of magnetism across a broad range oftemperatures. The Curie temperature is shown to vary, strongly dependent upon microstructure,
increasing significantly with Cr precipitation observed at high doses and ageing times. These large-scale
non-collinear calculations are performed using a method implementing magnetic cluster expansion(MCE), parameterised using a database of DFT-generated observables [2], with spin-dynamics (SD)incorporating both transverse and longitudinal spin fluctuations [3]. SD simulations provide valuable
insight into the ageing of alloys under realistic magnetic confinement conditions to inform material
choices and construction strategy for fusion power plant design.
This work has been carried out within the framework of the EUROfusion Consortium and has received
funding from the Euratom research and training programme 2014-2018 under grant agreement No
633053. The views and opinions expressed herein do not necessarily reflect those of the European
Commission.
[1] E.R.Reese et al., JNM 500, 192 (2018); [2] M.Y.Lavrentiev et al., JAP 109, 07E123 (201); [3] P.-W.Ma

et al., PRB 81, 184202 (2017)112

MMM 2018

(P2-14)
Atomistic insights into the grain boundaries interaction withradiation-induced point defects in bcc Fe-Cr alloys*Marcin Roland Zemla1, Jan Stanislaw Wrobel1, Tomasz Wejrzanowski1
1. Faculty of Materials Science and Engineering, Warsaw University of Technology, Woloska 141, 02-507 Warsaw

Grain boundaries (GBs) are an immanent components of crystal structure of the structural materials, suchas e.g. Fe-Cr steels. Moreover, they have considerable influence on the materials properties, especially on
the mechanical one. As a consequence of that, is necessary to investigate effect of radiation-induced
defects on the GBs in order to a deeper understanding of the radiation damage.
In current study, we investigated characteristics of bcc-Fe and Fe-Cr based tilt GBs interacting with point
defects such as vacancy, self-interstitial (SIA, dumbbell-type), and interstitial atoms (He impurities).
Several tilt GBs with the rotation axis along [100] and [110] directions were modelled with He impurities,
vacancy, and SIA in Fe-Cr. Molecular dynamics (MD) simulations using the interatomic Fe-Cr-He
embedded atom model potential were conducted, for twelve GBs, in order to investigate GBs energies, He
segregation energies, and the weakening effect of He impurity for several Cr and He concentrations.
Furthermore, spin-polarized density functional theory (DFT) calculations focused at two GBs, Σ3(111)and Σ5(210), allowed to deeper insights into GBs properties. For example, the DFT results show that thepresence of He significantly influences the magnetic properties of the system in the relatively distantneighbourhood [1]. The fluctuation of magnetic moments, chemical potentials, formation and migrationenergies of point defects were studied as a function of distance from GB’s plane. Representative
structures of GBs, with Cr content ranging 6-10%, generated using DFT-based Monte Carlo simulations [2]were used to analyse how parameters, such as alloy short-range ordering or local environment, effects on
defects properties.
[1]. M. R. Zemla, J. S. Wrobel, T. Wejrzanowski, D. Nguyen-Manh, K. J. Kurzydlowski, Nuclear Instruments
and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 393:118 (2017).
[2]. J.S. Wrobel, D. Nguyen-Manh, M.Y. Lavrentiev, M. Muzyk, S.L. Dudarev, Physical Review B 91 (024108)
2015.

MMM 2018

113

(P2-15)
Production and Process of Cascade Development in Irradiated Pure α-Zr from Molecular Dynamics Simulations*Rongjian Pan1, Lu Wu1, Xiaoyong Wu1, Aitao Tang2, Bang Wen1, Wen He1, Y.R. Ovcharenko3,
D.O. Kharchenko3
1. The First Sub-Institute, Nuclear Power Institute of China, 2. College of Materials Science and Engineering,
Chongqing University, 3. Institute of Applied Physics, National Academy of Science of Ukraine

The formation and development of cascade in pure α-Zr crystals at sample temperatures T=300K and T=500K, energy of 2, 6 and 10 keV and initially driving directions and of primary knock atoms (PKA) areinvestigated by molecular dynamics. The simulation results show that when crystals temperature is T
=300K and 500K and initial direction of motion for PKA is the cascades volume is greater than for the
initial direction at same all other parameters. The largest size of cascade is found after 0.4 ps, which
regardless of crystal temperature, energy and initial direction of primary knock-on atom (PKA). The
formation of crowdions is caused by channeling during cascade development.

114

MMM 2018

(P2-17)Microstructure evolution of cascade annealing in irradiated pure α-Zr
from molecular dynamics simulations*Bang Wen1, Rongjian Pan1, Wei Zhang1, Lu Wu1, Xiaoyong Wu1, Aitao Tang2, Wen He1, Y.R.
Ovcharenko3, D.O. Kharchenko3
1. The First Sub-Institute, Nuclear Power Institute of China, 2. College of Materials Science and Engineering,
Chongqing University, 3. Institute of Applied Physics, National Academy of Science of Ukraine

The microstructure evolution of cascade annealing in α-Zr crystals at T=300K, 400K, and 500 K forenergy of 2, 6 and 10 keV with initially driving directions <0001> and <01-10> of primary knocked atoms(PKA) are investigated by using molecular dynamics simulations. The results show that the relaxation time(τr) of cascades can play a role of an effective parameter for describing radiation damages duringmolecular dynamics simulations. of cascade with the larger surface area but with the same volume ismuch smaller, which promotes faster recombination of defects during cascade annealing. Energy is a
crucial factor in the formation of cascade displacements of atoms and appearing of defects comparedwith temperature and direction of movement for initially knocking atoms.

MMM 2018

115

Symposium C(C1 invited)
Kinetic Monte Carlo model of screw dislocation-solute coevolution in
W-Re alloys
Yue Zhao1, Lucile Dezerald3, *Jaime Marian1,2
1. Dept. of Materials Science and Engineering, University of California Los Angeles, 2. Dept. of Mechanical andAerospace Engineering, University of California Los Angeles, 3. Institut Jean Lamour, University of Lorraine

Dislocation-solute interactions are at the heart of many important processes in materials science andmetallurgy, such as solid solution strengthening, dynamic strain ageing, pipe diffusion, etc. In
body-centered cubic (bcc) metals, plastic flow at low-to-intermediate homologous temperatures iscontrolled by screw dislocation glide. In this temperature range, both solute diffusion and dislocation
motion are thermally activated processes sensitive to stress, and the overall plastic behavior can be
reduced to the study of a single screw dislocation interacting with solutes. This interaction is complex,
and can result in material softening and/or hardening depending on temperature and soluteconcentration. Here, we solve this coupled transport problem for W-Re alloys in three dimensions using
kinetic Monte Carlo simulations. The interaction between Re solutes and dislocation segments is captured
via the elastic dipole tensor, parameterized with electronic structure calculations, while dislocation
segment-segment interactions are described using nonsingular elasticity theory. We find that there are
two clearly defined regimes as a function of Re concentration. For low values, the softening effect on
kink-pair nucleation energy overcomes kink-solute collisions, leading to an overall reduction in alloy
strength compared to pure W. The situation is reversed at higher concentrations, resulting in overall
hardening. Our results are in reasonable agreement with several experimental measurements, and point
to the intrinsic nature of the softening/hardening transition in W-Re alloys. We also report on preliminary
simulations of dislocation-impurity interactions in the W-Cu system, where the basic mechanisms behind
dynamic strain aging in substitutional bcc solid solutions starts to manifest itself in a specific temperature
range.

116

MMM 2018

(C1 oral)
Thermally activated solute-drag strengthening by interstitial impurities
in BCC Cr*Christian Brandl1
1. Karlsruhe Institute of Technology

The application of body centered cubic (BCC) refractory metals is usually limited by the low temperature
brittleness, which is intrinsically linked to the limited screw dislocation mobility. We present a combined
ab-initio and molecular dynamics study on the role of impurity interstitials on the dislocation mobility andthe implications on the brittle-ductile transition.
The interaction forces between dislocation and impurities are computed for the kink-pair nucleation andkink-drift to predict strengthening contribution additionally to the kink-pair nucleation limited mobility
below the Knee temperature. Continuum solute-drag models informed by atomistic simulations withsemi-empirical potentials and with the chemically accurate ab-initio simulation are both used to predictexperimental temperature regime for solute-drag strengthening. The role of dislocation core contribution
compared to the elastic interaction is discussed and compared to recent nanoindentation experiments ofhigh purity Cr with temperature-dependent hardness, activation volume and activation energies.

MMM 2018

117

(C1 oral)
Generalized yield criterion in BCC metals from first principles*Antoine Kraych1, Lucile Dezerald2, Emmanuel Clouet3, Lisa Ventelon3, François Willaime3, David
Rodney1
1. Institut Lumière Matière, Université Lyon 1, 2. Institut Jean Lamour, Université de Lorraine, 3. DEN-Départementdes Matériaux pour le Nucléaire, CEA, Université Paris-Saclay

At low-temperature, the plastic deformation of tungsten and other body-centered cubic (BCC) metals is
anisotropic, and does not follow the Schmid law applicable to most other metals. This feature arises fromthe behavior of 1/2<111> screw dislocations, which control the plasticity of BCC materials. Under theapplication of a simple shear stress, the dislocation trajectory undergoes microscopic deviations that are
directly linked to the so-called twinning/antitwinning asymmetry (Dezerald et al. 2016). However, othercomponents of the applied stress tensor, the non-glide stresses, are also known to influence thedislocation mobility (Duesbery & Vitek 1998). In this work we use first principles calculations to explore
the influence of non-glide stresses on the mobility of screw dislocations in BCC tungsten and determine ageneralized yield criterion.
DFT calculations and nudged elastic band method are used in order to determine the Peierls potential ofscrew dislocations under stress. Different stress tensors are applied to the simulation cells, allowing tocalculate the sensibility of the Peierls potential to non-glide stresses, and obtain the dependence of thedislocation Peierls stress on these stresses. These calculations are used to adjust a yield criterion,
predicting the response of a single crystal to a tensile test, and the corresponding activated glide systems.
Implications regarding non-Schmid slip on weakly stressed systems will be discussed.

118

MMM 2018

(C1 oral)
Dislocation Motion in High Entropy Alloys
Luchan Zhang1, *Yang Xiang2, Jian Han3, David Srolovitz3
1. National University of Singapore, 2. Hong Kong University of Science and Technology , 3. University of Pennsylvania

We propose a stochastic model for dislocation motion in high entropy alloys (HEAs) that focuses on theatomic scale composition disorder. Our model is based upon how atomic-scale variations in composition
effect dislocation motion through the variability of the core, rather than long range stress fields. Ourmodel is constructed within the relatively simple Peierls-Nabarro (PN) framework, where the site
occupancy disorder produces random variations in the amplitude of the interplanar potential. This in turnleads to stochastic variations in the dislocation core width, Peierls stress, and Peierls energy. Since truly
random alloys are not thermodynamically stable except in the extreme high temperature limit, we also
consider the effects of short range spatial correlations in the randomness. We then introduce ahomogenization procedure that allows for simple incorporation of correlation. We then extend our
homogenized PN results to the entire slip plane to predict the effect of randomness on the strength of thematerial.

MMM 2018

119

(C1 oral)
A random walk model of screw dislocation cross-slip in face-centeredcubic solid solution alloys*Wolfram Georg Noehring1,2, William Arthur Curtin2
1. Department of Microsystems Engineering, University of Freiburg, 2. Institute of Mechanical Engineering, École
Polytechnique Fédérale de Lausanne

The energy barrier for cross-slip of screw dislocations in FCC solid solution alloys is controlled by local
fluctuations in the solute distribution [1]. Here, a random-walk-like model of cross-slip in solid solution
alloys is presented. Cross-slip is treated as a discrete process, where on each step a one Burgers vector
long dislocation segment moves from the glide to the cross-slip plane. Each step causes (i) a random
energy change due to the random change in solute-dislocation and solute-solute binding energies, and(ii) a deterministic energy change due to constriction formation and stress effects. The random walkmodel allows to calculate the distribution of cross-slip activation energies for long (several 100 to 1000
Burgers vector long) dislocations, which is relevant for deformation of real materials, but not easily
accessible by direct atomistic calculations. At zero stress, thermally activated cross-slip of long
dislocations is unlikely because high activation energies become more frequent with increasing length.
However, at moderate stresses (few MPa), these barriers disappear; the remaining barriers are typicallywell below the average barrier that one would expect if considering only average alloying effects (averagechange in stacking fault energy, elastic constants, etc.). Moreover, cross-slip becomes a weakest-link
problem, meaning that the activation energy distribution for a long dislocation under stress can be
estimated from a reference distribution for a short (40 Burgers vectors) dislocation at zero stress, whose
determination is computationally inexpensive and needs to be done only once.
[1] Nöhring, W.G.; Curtin, W.A. Acta Materialia 2017, 128, 135–148.

120

MMM 2018

(C1 oral)
Modeling the climb-assisted glide of edge dislocations through arandom distribution of nanosized vacancy clusters*Marie Landeiro Dos Reis1, Laurent Proville1, Maxime Sauzay2, Mihai Cosmin Marinica1,
Normand Mousseau3
1. SRMP-CEA Saclay, 2. SRMA-CEA Saclay, 3. Département de Physique, Université de Montréal

A multi-scale model is developed to simulate the climb-assisted glide of edge dislocations anchored by arandom distribution of nanosized vacancy clusters. For a shear stress much smaller than the critical stressbeyond which dislocations cross the obstacles by simple glide, we found that dislocations remainanchored for a waiting time sufficient to allow the diffusion-controlled absorption of vacancies. Then thedislocations climb perpendicularly to their glide planes up to circumvent the obstacles and subsequently
glide until they encounter a different anchoring configuration. Atomic-scale simulations allowed us tocharacterize the interactions between an edge dislocation and nano-voids as a function of their sizes andshapes. Our atomic-scale data served to calibrate an elastic-line model which we used to evaluate theglide distance of a dislocation with realistic dimensions through a random distribution of obstacles. To
complete our scheme, a standard diffusion-based model for the climb velocity of edge dislocations wasused to determine the deformation rate expected through the climb-assisted glide. Our predictions madefor the archetypical case of Al are in good agreement with experiments of different types, i.e. tensile
deformation tests and steady-creep tests, although no parameter was adjusted in the theory to recoverexperimental data.

MMM 2018

121

(C2 invited)
Finite deformation Mesoscale Field Dislocation Mechanics
Rajat Arora1, Xiaohan Zhang2, *Amit Acharya1
1. Carnegie Mellon University, 2. Stanford University

We will describe a model of dislocation mechanics based (crystal) plasticity of unrestricted geometric andmaterial nonlinearity that, when exercised on a sufficiently fine scale, can rigorously predict fields ofarbitrary dislocation distributions in finite bodies of arbitrary anisotropy, and when exercised at larger
scales of resolution adequate for meso/macro scale structural response, suitably adapting establishedmacroscale phenonemology related to kinetics of plastic flow, makes predictions up to finite strains of size
and rate-dependent mechanical behavior, texture, and mesoscale dislocation microstructure evolution inpolycrystalline aggregates and single crystals. The phenonemology used to go to the mesoscale can besystematically improved as the need arises, as can the geometric fields involved along with their governing
equations.
The framework will be demonstrated by results on size-effects, effects of boundary constraints on plastic
flow, volume change due to dislocations, polygonization fields, evolution of lattice rotations, normal
stress-effects-in-shear dependent plastic flow instabilities, all up to large strains (sometimes 100%) andwith a focus on effects not predictable within linear dislocation statics or dynamics, or geometrically linear
or nonlinear phenomenological plasticity theories.

122

MMM 2018

(C2 oral)
Anisotropic and non-symmetric continuum dislocation dynamics*Thomas Hochrainer1
1. TU Graz

Continuum dislocation dynamics (CDD) is a recently developed crystal plasticity theory based on the
kinematics and kinetics of dislocations as moving flexible lines in crystals. In CDD the dislocation state
may be characterized in varying levels of details by considering alignment tensors of different order [1].
Constitutive laws for the lowest level CDD theory yield microscopic stresses which determine the averagedislocation velocity. Such a constitutive law was recently derived for the lowest order CDD variant [2] froman energy functional.
In the lowest order theory, the average dislocation velocity is assumed to be independent of dislocation
character and orientation. However, edge and screw dislocations may have different mobilities, which
requires the consideration of anisotropic dislocation velocities. Moreover, two recent papers on
continuum modelling of simplified systems of straight parallel edge dislocations found a new microscopic
stress contribution which is related to a possible asymmetry between the average velocities of edge
dislocations of opposite sign [3,4].
In the current talk we show how the kinematic equations may consider asymmetric and anisotropic
dislocation velocities and how these modified evolution equations yield new microscopic stresscontributions in the driving force for thermodynamically consistent CDD theory. The influence of selected
terms is highlighted in small example problems.
References[1] T. Hochrainer, 2015. Philos. Mag., 95(12), 1321–1367.
[2] T. Hochrainer, 2016. J. Mech. Phys. Solids 88, 12–22.
[3] I. Groma, M. Zaiser and P. D. Ispánovity, 2016. Phys. Rev. B, 93(21) 214110.
[4] P.-L. Valdenaire, Y. Le Bouar, B. Appolaire and A. Finel, 2016. Phys. Rev. B, 93(21) 214111.

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123

(C2 oral)
Numerical simulation of model problems in Plasticity based on Field
Dislocation Mechanics*Leo Morin1,2, Renald Brenner3, Pierre Suquet2
1. PIMM, Arts et Métiers-ParisTech, CNAM, CNRS, UMR 8006, 151 bd de l’Hopital, 75013 Paris, 2. Laboratoire deMécanique et d’Acoustique, Aix-Marseille Univ, CNRS UMR 7031, Centrale Marseille, 4 impasse Nikola Tesla, CS
40006, 13453 Marseille Cedex 13, 3. Sorbonne Université, CNRS, UMR 7190, Institut Jean Le Rond d'Alembert,
75005 Paris

The aim of this work is to investigate the numerical implementation of the Field Dislocation
Mechanics (FDM) theory for the simulation of dislocation-mediated plasticity. First, a revisited
elastoplastic formulation of the FDM theory is derived which permits to express the set of
equations under the form of a static problem, corresponding to the determination of the local
stress field for a given dislocation density distribution, completed by an evolution problem,
corresponding to the transport of the dislocation density. The static problem is classicallysolved using FFT-based techniques (Brenner et al., 2014), while an efficient numerical scheme
based on high resolution Godunov-type solvers is implemented to solve the evolution problem.
Model problems of dislocation-mediated plasticity are finally considered in a simplified 2D case.
First, uncoupled problems with constant velocity are considered, which permits to reproduce
annihilation of dislocations and expansion of dislocation loops. Then, coupled problems withseveral constitutive laws for the dislocation velocity are considered. Various mechanical behaviors
such as perfect plasticity and linear kinematic hardening are reproduced by the theory.

124

MMM 2018

(C2 oral)
Direct computation of the stress field due to geometrically necessary
dislocation densities*Yichao Zhu1, Yang Xiang2
1. Dalian University of Technology, 2. The Hong Kong University of Science and Technology

Computation of the stress field due to geometrically necessary dislocations (GNDs) is widely believed tobe typical and straightforward, if compared to other challenging issues in modelling with continuum
descriptions of dislocation microstrutures. A common treatment is to adopt the Kroener's formulation
which relates the dislocation density field to plastic distortion (or the eigenstrain). Then the internal stressfield can be computed by using the corresponding Green's formulae subject to particular loading
conditions, e.g. periodic boundary conditions or in R3. For arbitrary loading conditions, complementary
finite element (FE) analysis is needed in analogy with Van Der Giessen and Needleman's superposition
method for discrete dislocation dynamics. In this talk, however, we will show that the stress field due to
GNDs subject to arbitrary loading conditions can be resolved even more directly. With the introduction of
a set of dislocation density potential functions (DDPFs), one simply needs to do one step of FE analysis,
while the time for eigenstrain calculation is fully saved. Other advantages of adopting DDPFs, e.g., its
capability of accommodating the anisotropic dislocation motion (e.g. glide v.s. climb) will also be
addressed in the proposed presentation.

MMM 2018

125

(C2 oral)
Meshfree Analysis for Kink Band Formation in Mg-based LPSO Phase
Based on Crystal Plasticity Cosserat Model Considering Disclination
Density*Yuichi Kimura1, Yuichi Tadano2, Kazuyuki Shizawa3
1. Grad. School of Science and Technology, Keio Univ., 2. Dept. of Mechanical Engineering, Saga Univ., 3. Dept. of
Mechanical Engineering, Keio Univ.

Mg alloys with a long period stacking ordered structure (LPSO) phase has attracted attention as anext-generation structural material due to its high strength and low specific weight. The strength of thisalloy is mainly attributed to the kink deformation in LPSO phase. Since the boundary between the kinkand matrix has a large misorientation similar to that of a grain boundary, dislocation glides are prevented
at the kink boundary. The clarification of detailed behavior of kink formation through numerical simulation
is highly expected. Whereas the mechanism of kink band formation has conventionally been explained by
the motion and accumulation of dislocations, it is recently attempted to express its process from the pointof view of a rotational crystal defect called the disclination. The authors have developed a crystal plasticity
Cosserat model classified into couple stress theory with a microscopic rotational degree of freedom todescribe the disclination, and carried out FE analyses on a single crystal of LPSO phase. However, theFEM using C0 continuous shape function is not applicable for the Cosserat model because such function
cannot ensure the continuity of higher-order gradient of displacement field peculiar to the Cosserat
model and the analysis is limited to the infinitesimal deformation.
In this study, a meshfree method is introduced into our numerical scheme instead of the FEM and it isshown that this method can express the continuous higher-order gradient of displacement field and canbe adapted to boundary conditions for microscopic rotation originating in the disclination. Also, a new
boundary value analysis suitable for the crystal plasticity couple stress model is presented in the categoryof finite deformation theory. Then, a meshfree analysis based on the present model considering
disclination density is conducted to reproduce the kink formation based on disclination behaviors, and it
is shown that this model can predict the kink deformation through a quadrupole structure of disclination
and array structures of GN dislocation formed around the kink band. Furthermore, the size effect on the
deformation response and the mesh independence of kink band width of the present model areinvestigated and the influence of the boundary condition for microscopic rotation on the kink deformation
is discussed.

126

MMM 2018

(C2 oral)
Dynamic recrystallization model for Mg/LPSO alloys couplingphase-field model and dislocation-based crystal plasticity model*Sho KUJIRAI1, Kazuyuki SHIZAWA2
1. Grad. School of Science and Technology, Keio Univ., 2. Dept. of Mechanical Engineering, Keio Univ.

Magnesium alloy with LPSO (Long-Period Stacking Ordered Structure) phase has been developed andattracted much attention for a next-generation structural material because of its outstanding mechanicalproperties, i.e., high yield strength, light specific weight and flame retardance. This alloy is mainlycomposed of α-Mg and LPSO phases. The strengthening mechanism of this alloy is attributed to a kinkband formulation in LPSO phase and a grain refinement of α-Mg phase in the vicinity of LPSO phase. The
deformation kink, which is one of plastic buckling in laminated materials, is formed by dislocation glide inbasal slip systems. The grain refinement of α-Mg phase occurs through the dynamic recrystallization in a
warm plastic work. The dynamic recrystallization is a self-organization phenomenon that recrystallized
nuclei grow in the deformation process of α-Mg phase. Mechanical properties of structural metals aremainly determined by microstructures generated in the deformation process. Our research group recently
developed a dynamic recrystallization model for FCC crystals by coupling the multi-phase-field (MPF)
model with the dislocation-based crystal plasticity model as a strain gradient theory. In this report, we
extend the above multiphysics model to that for HCP crystals so that we can predict the strengthening
behavior of Mg/LPSO alloys. It is known that additional elements prevent the nucleus growth in α-Mgphase. Such effect is called the pinning effect and it is introduced into the present model. The grainboundary segregation of additional elements is also considered. We conduct multiphysics FE analyses for
Mg/LPSO alloys and then reproduce the dynamic recrystallization along the deformation kink bands
numerically. From the results obtained in this study, it is revealed that Mg/LPSO alloys are strengthenedby the deformation kink in the LPSO phase, the grain refinement in the α-Mg phase and the interaction
between those effects in the alloys.

MMM 2018

127

(C3 invited)
Quantifying the effect of hydrogen on dislocation dynamics inmicrocrystals: A three-dimensional discrete dislocation dynamics
study
Yejun Gu1, *Jaafar A. El-Awady1
1. Johns Hopkins University

We present a new framework to quantify the effect of hydrogen on dislocation plasticity using large scale
three-dimensional (3D) discrete dislocation dynamics (DDD) simulations. In this model, the first orderelastic interaction energy associated with the hydrogen-induced volume change is accounted for. The
three-dimensional stress tensor induced by hydrogen concentration, which is in equilibrium with respect
to the dislocation stress field, is derived using the Eshelby inclusion model, while the hydrogen bulk
diffusion is treated as a continuum process. This newly developed framework is utilized to quantify theeffect of hydrogen on the different aspects of dislocation-mediated plasticity in Ni single crystal
microcrystals. The combined effect of hydrogen concentration, crystal size, and dislocation density isquantified. Finally, the effects of pipe diffusion is also rationalized from comparisons with experimental
results.

128

MMM 2018

(C3 oral)
Discrete dislocation plasticity modelling of hydrogen dislocationinteractions in micro-cantilevers*Haiyang Yu1, Alan Cocks2, Ed Tarleton1
1. Dept. of Materials, Univ. of Oxford, 2. Dept. of Engineering, Univ. of Oxford

One of the most widely accepted theories for hydrogen embrittlement is the hydrogen enhanced localized
plasticity (HELP) mechanism which claims hydrogen enhances dislocation activity locally. Experimental
evidence for this mechanism has been reported across different scales. Simulations however are mainly
limited to the atomistic scale, and continuum scale. Direct simulation of hydrogen dislocation interactions
in micromechanical tests is rarely simulated, due to the fact that this scale is beyond the computational
capacity of most atomistic calculations and beyond the resolution of most continuum approaches.
Discrete dislocation plasticity (DDP) is an ideal technique to bridge the gap in multiscale modelling ofhydrogen embrittlement. Recently, a framework which incorporates hydrogen effects in a discrete
dislocation simulation was derived by Gu and El-Awady [1] enabling direct investigation of hydrogen
dislocation interactions at the microscale. We have implemented this formulation in a new code called

EasyDD, a GPU accelerated version of DDLab coupled with FEM, and performed virtual micromechanicaltests with mixed boundary conditions. We have simulated the behaviour of hydrogen charged
micro-cantilevers and included the hydrogen induced tractions on the free surfaces. Hydrogen isobserved to shield dislocation mutual interactions and to enhance dislocation generation, thecombination of these effects leads to global softening and local stress concentrations promoting failure;consistent with the HELP mechanism. To the best of our knowledge, this work is the first to revealhydrogen dislocation interactions at the discrete dislocation scale in finite volumes, acting as an essential
bridge in multiscale modelling.

[1] Gu, Y., & El-Awady, J. A. (2018). Journal of the Mechanics and Physics of Solids, 112, 491–507.

MMM 2018

129

(C3 oral)
Dislocation climb models from atomistic scheme to dislocation
dynamics*Yang Xiang1, Xiaohua Niu2
1. Department of Mathematics, Hong Kong University of Science and Technology, 2. Department of Mathematics,
Jimei University

We develop a mesoscopic dislocation dynamics model for vacancy-assisted dislocation climb by
upscalings from a stochastic model on the atomistic scale. Our models incorporate microscopic
mechanisms of (i) bulk diffusion of vacancies, (ii) vacancy exchange dynamics between bulk anddislocation core, (iii) vacancy pipe diffusion along the dislocation core, and (iv) vacancy
attachment-detachment kinetics at jogs leading to the motion of jogs. Our mesoscopic model consists ofthe vacancy bulk diffusion equation and a dislocation climb velocity formula. The effects of thesemicroscopic mechanisms are incorporated by a Robin boundary condition near the dislocations for thebulk diffusion equation and a new contribution in the dislocation climb velocity due to vacancy pipe
diffusion driven by the stress variation along the dislocation. Our climb formulation is able toquantitatively describe the self-climb of prismatic loops at low temperatures when the bulk diffusion is
negligible. Simulations performed show evolution, translation, coalescence of prismatic loops by
self-climb that agree with the experimental observations.

130

MMM 2018

(C3 oral)
Investigation of the Hall-Petch Effect with DD Simulation*Maoyuan Jiang1,2, Benoit Devincre2, Ghiath Monnet1
1. EDF - R&D, Les Renardieres, 2. Laboratoire d'Etude des Microstructures, CNRS/ONERA

Dislocation Dynamics (DD) simulation is used to identify the elementary mechanisms controlling theHall-Petch (HP) effect on the dislocation scale. The influence of grain size is explored by considering
simple periodic polycrystalline aggregates made of grains with cube, plate or needle shapes. We showthat the HP effect is globally well reproduced with DD simulations. The HP constant is found to be afunction of the grain orientations and shapes. A model is proposed to quantify the influence of the grainmorphology. The simulated HP effect is justified by the existence of a backstress inside the grains thatemerges from strain incompatibility between grains in a deformed polycrystal. As theoretically expected,
Geometrically Necessary Dislocations (GND) are found to accumulate at grain boundaries toaccommodate the distortion field discontinuities. By virtue of the Nye’s tensor, the above property canbe directly tested and quantified in DD simulations. For the simplest simulation geometry, the calculated
backstress is compared with theoretical predictions. Those results suggest a new strategy for theprediction of the grain size effect in crystal plasticity models.

MMM 2018

131

(C4 invited)
Hydrogen consequences on cyclic behaviour of <001> nickel single
crystals : a multi-scale approach.
*xavier Feaugas1, Guillaume Hachet1, Arnaud Metsue1, abdelali Oudriss1
1. université de la rochelle

A multi-scale study on the influence of hydrogen on cyclically strained <001> oriented nickel single
crystals is conducted in order to understand the impact of the solute on the microstructure and thefatigue properties of nickel single crystal. At macroscopic scale, uniaxial cyclic tests are performed toevaluate the impact of pre-charged hydrogen on the work hardening of the material. Then, at microscopic
scale, a mechanical database is provided and associated with the dislocation features, which are exploredby transmission electronic microscopy (TEM) observations and correlated with the mechanical behaviourof nickel single crystal with hydrogen. In addition, at atomistic scale, molecular dynamics simulations arecarried out to quantify the influence of hydrogen on the dipole sizes and the dislocation organisations,which are partially connected to the elastic properties of nickel. Therefore, we have finally performed acomprehensive study on the impact of hydrogen on the elastic properties of nickel single crystal using atheoretical formalism based on Density Functional Theory and uniaxial tensile tests on <001> oriented
nickel single crystals with different concentrations of hydrogen.
This multi-scale approach allows to question the main effect of hydrogen on cyclic plasticity mechanisms.
The impact of hydrogen on cyclic stress-strain curves highlights a softening behaviour for <001> multi
slips orientation in a similar way as previously related for single slip orientation. This result is firstly
discussed in term of dislocation organisations, dipole sizes and distribution but also internal stresses
states. Hydrogen impacts the density and the distribution of dislocations and consequently, modifies theinternal stress state. Moreover, according to TEM observations, hydrogen ingress promotes the formationof vacancy clusters, in agreement with the super-abundant vacancies (SAV) model. The mainconsequence of this result is a decrease of the elastic properties of the material, which depends mainly on
the formation of the defect induced by the incorporation of hydrogen than a direct effect of the soluteitself.

132

MMM 2018

(C4 oral)
Atomistic and continuum approaches to analyse precipitation
hardening in metallic alloys
Gustavo Esteban-Manzanares1,2,3, Rodrigo Santos1,2, Anxin Ma1, Ioannis Papadimitriou1, EnriqueMartínez3, Laurent Capolungo3, Javier Segurado1,2, *Javier LLorca1,2
1. IMDEA Materials Institute, 2. Polytechnic University of Madrid, 3. Los Alamos National Laboratory

Precipitation hardening is one of the most efficient mechanisms to increase the yield strength of metallic
alloys but accurate quantitative models for this phenomenon are still lacking. Two different approaches,
based on atomistic simulations and discrete dislocation dynamics, are presented to address this problem.
Atomistic simulations, in combination with the transition state theory, were used to determine theinteraction between Guinier-Preston zones and dislocations in an Al-Cu alloy and between Mg17Al12
precipitates and basal dislocations in an Mg-Al alloy. The rate at which dislocations sheared theprecipitates (determined by means of molecular dynamics) was controlled by the activation Gibbs freeenergy, in agreement with the postulates of the transition state theory. However, harmonic TST does nothold for this interaction. In addition, the activation enthalpy energy and the activation volume were
determined and an estimation of the initial shear flow stress as a function of temperature was carried out
from the thermodynamic data provided by the atomistic simulations.
In the case of large precipitates that cannot be sheared by dislocations (such as q’precipitates in Al-Cu
alloy), the dislocations overcome the precipitates by the formation of an Orowan loop. The mechanisms of
dislocation/precipitate interaction were studied by means of discrete dislocation dynamics using thediscrete-continuous method in combination with a fast Fourier transform solver to compute themechanical fields. Simulations took into account the effect of precipitate shape, orientation and volume
fraction as well the elastic mismatch between the matrix and the precipitate, the stress-freetransformation strain around the precipitate and the dislocation character as well as dislocation
cross-slip. It was found that the influence of the precipitate aspect ratio and orientation were reasonably
well captured by the simple Orowan model in the absence of the stress-free transformation strain.
Nevertheless, the introduction of the stress-free transformation strain led to dramatic changes in thedislocation/precipitate interaction and in the critical resolved shear stress to overcome the precipitate,particularly in the case of precipitates with small aspect ratio.

MMM 2018

133

(C4 oral)
Dislocation-precipitate interaction in Al-Mg-Si alloys*Inga Gudem Ringdalen1, Sigurd Wenner1, Jaime Marian2
1. Dept. of Materials and Nanotechnology, SINTEF, 2. Dept. of Material Science and Engineering, UCLA

Partial ageing of commercial Al-Mg-Si aluminum results in microstructures characterized byneedle-shaped Si/Mg-rich precipitates. These precipitates belong to the non-equilibrium β'' phase andare coherent with the fcc Al lattice, despite of which they can cause considerable hardening. We haveinvestigated the interaction between these β'' precipitates and dislocations using a unique combinationof modeling and experimental feedback. Dislocation-precipitate interactions including precipitate stressfields are simulated using discrete dislocation dynamics. The displacement fields due to the precipitates
are captured by expressions that satisfy elastic equilibrium and are fitted to high-resolution electron
microscopy measurements. The stress fields are derived from the displacements assuming isotropic
elasticity, and used to study the strength of individual precipitates to dislocation glide as a function of
precipitate size, orientation, and dislocation character and length. The dislocation dynamics results arethen used to parameterize a probabilistic analysis model to study the difference between the bulk
material and the precipitate free zones along the grain boundaries of these alloys.

134

MMM 2018

(C5 invited)
Plasticity and Fracture in Transition Metal Carbides*Giacomo Po1, Suneel Kodambaka2, Jeffrey M Wheeler3, Davide Sangiovanni4
1. University of California Los Angeles, Mechanical Engineering Department, 2. University of California Los Angeles,
Materials Science and Engineering Department, 3. Empa, Swiss Federal Laboratories for Materials Science andTechnology, Laboratory for Mechanics of Materials and Nanostructures, 4. Department of Physics, Chemistry andBiology, Linköping University

Current and future applications in hypersonic flight, re-entry vehicles, propulsion, and power production
create an insatiable demand for materials capable to perform in severe environments. Materials for theseapplications must possess a rare combination of properties, which include high specific strength, elevatedmelting temperature, high thermal conductivity, and low thermal expansion coefficient. Ultra-High
Temperature Ceramics are being considered for applications in extreme environments, especially whenoxidation is a major concern. Currently, the factor limiting the use of UHTCs as structural materials is their
low-temperature brittleness. This talk focuses on the plasticity and fracture mechanisms of the transition
metal carbide TaC, one of the highest melting temperature materials known to mankind. In-situ
micro-pillar compression experiments carried out at different temperatures and orientations reveal
unexpected intrinsic ductility of TaC, which contrasts its well-known bulk brittleness. These findings unveil
new properties of the material, such as a pronounced non-Schmid behavior and a remarkabletemperature/orientation dependence of the yield strength. A variety of multiscale modeling techniques
ranging from ab-initio to discrete dislocation dynamics simulations are employed to understand thesmall-scale behavior of TaC. Computer simulations shed light on the room-temperature brittleness of TaC
by explaining the link between plastic deformation and fracture.

MMM 2018

135

(C5 oral)
Atomic Scale Investigation of Plasticity in Laves phases*Julien Guenole1, Fatim-Zahra Mouhib1, Christoffer Zehnder1, James Gibson1, Blazej Grabowski2, Sandra Korte-Kerzel1
1. Institute of Physical Metallurgy and Metal Physics, RWTH Aachen University, 2. Max-Planck-Institut fürEisenforschung GmbH, Düsseldorf

The strengthening mechanisms in magnesium alloys are strongly related to the complex intermetallic
precipitates that spread all over the microstructure. In particular, the hard and brittle Laves phases [1]commonly observed in such alloys exhibit a complex atoms stacking with largely unknown plasticity
mechanisms [2].
We propose here a numerical atomistic approach at the cross-road of experiments and ab initio
simulations, to shed light on the responsible mechanisms for the plasticity in hexagonal Mg2Ca (C14)
Laves phases. Elastic and plastic properties of bulk Laves phase are investigated by both first principle
and classical atomistic simulations. Molecular dynamics/statics simulations with a robust MEAM
interatomic potential are performed to study the propagation of dislocations, by a combination ofnudge-elastic-band (NEB) methods and nano-mechanical tests. In particular, the cross-slip to pyramidal
slip planes, the propagation of dislocations in the basal plane by the so called synchroshear mechanism[3,4] and the influence of temperature on slip systems activation, are investigated at the atomic scale. All
these results are finally correlated and discussed by considering experimental findings, especially frommicro-pillar compression tests. Our goal is to improve the understanding of the largely unknown plasticityof Laves phases.
References:[1] –F. Laves and H. Witte, Metallwirtsch. Metallwiss. Metalltech. 15, 840 (1936).
[2] –M.F. Chisholm, et al., Science 307, 701 (2005)[3] –P.M. Hazzledine et al., MRS Proceedings 288, 591 (1992)[4] –P.M. Hazzledine and P. Pirouz, Scripta metallurgica et materialia 28, 1277 (1993).

136

MMM 2018

(C5 oral)
Nucleation of dislocation in ultra-hard ceramic nanoparticles
modelled by molecular dynamics and nudged elastic band simulations*Jonathan Amodeo1, Emile Maras2
1. MATEIS, Univ. Lyon 1, 2. SRMP, DEN, CEA, Gif-sur-Yvette

Micro- and nano-sized structures have attracted substantial interest due to their special mechanicalbehavior: they generally show an increased yield strength compared with the bulk material as well as an
improved ductility. Among them, nanoparticles (NPs) which are generally used for their shape-dependentfunctional properties also appear as perfect candidates for submicronic plasticity investigations due totheir broad range of sizes and their variety of shapes. While metallic and silicon NPs have been widely
investigated since almost two decades, less is known about the strength of ceramic NPs maybe due to thebrittleness of their bulk counterparts.
In the light of recent but preliminary in situ TEM observations, we investigate here the mechanicalbehavior of <100>-oriented MgO nanocubes. First, incipient plasticity mechanisms are investigated using
molecular dynamics simulations (MD) of virtual nanocompression tests at constant strain rate. Results
show that the plastic deformation of MgO nanocubes starts with the nucleation from the surface of perfect½<110>{110} dislocations under ultra-high stresses, one order of magnitude larger than what is generally
observed in fcc metals. However, as MD simulations can only be carried out at strain rates several ordersof magnitude larger than experimental ones,we used the nudged-elastic-band method to calculate theactivation energy for the nucleation of the dislocation as a function of stress, particle size and dislocation
nucleation site. With the help of the transition state theory, the nucleation stress can then be estimated asa function of temperature and experimental strain rate. We find that due to the large stiffness of theactivation energy vs.stress curve, the strain rate has a much smaller influence on the nucleation stressthan what is found for metallic nanocrystals and therefore, that the nucleation stress estimated using MD
is close to that corresponding to experimental strain rate.

MMM 2018

137

(C5 oral)
Multiscale discrete dislocation dynamics modeling of
nano-indentation near the grain boundary*Xu Zhang1, Songjiang Lu1
1. School of Mechanics and Engineering, Southwest Jiaotong University

Nano-indentaiton is a convenient method to investigate the mechanical properties of material by utilizing
the low-loads and small-scale displacements. However, grain boundary (GB) effect on nano-indentation
response needs detailed microstructure and mechanism analysis, which has been a long techniquechallenge. We developed a three-dimensional multiscale modeling framework, which couples thethree-dimensional discrete dislocation dynamics with finite element method, and use it to investigate theGB effects on nano-indentation of aluminum bicrystal. The interaction between dislocations and GB is
physically considered by introducing a penetrable GB, where dislocation pile-ups can penetrate throughGB and dislocation debris at GB can be re-emitted into grain interior. In the simulation, we confirmed two
experimentally observed phenomena, i.e., pop-in events and the dependence of indentation hardness on
distance to GB. The pop-in events are correlated with the activation and multiplication of dislocations,especially, the GB pop-in event results from the dislocations penetration through GB. By changing the
distances from indenter to GB, the simulation shows that the indentation hardness increases with thedecrease of GB-indenter distance. The size effect of nano-indentation results from the geometricallynecessary dislocation density distributed within the constraint volume between GB and the indenter.

138

MMM 2018

(C6 invited)
Effect of interstitial solutes on the structure and mobility of screw
dislocations in bcc metals
Berengere Lüthi1, Lisa Ventelon1, David Rodney2, Emmanuel Clouet1, Bernard Legrand1,
Fabienne Berthier3, *Francois Willaime4
1. DEN-Service de Recherches de Métallurgie Physique, CEA, Université Paris-Saclay, 2. Institut Lumière Matière,
CNRS-Université Claude Bernard Lyon 1, 3. Synthèse, Propriétés et Modélisation des Matériaux /Institut de Chimie
Moléculaire et des Matériaux d'Orsay, Université Paris-Saclay, 4. DEN-Département des Matériaux pour le Nucléaire,
CEA, Université Paris-Saclay

There is ample experimental evidence of the effects of solute interactions with dislocations, such as
Cottrell atmospheres and dynamical strain ageing (DSA). In this work we investigated, using Density
Functional Theory (DFT) electronic structure calculations, the interaction between interstitial soluteatoms, in particular carbon, and screw dislocation cores in body centered cubic (bcc) metals, in particulariron.
First considering carbon in bcc Fe, our calculations evidence a strong attractive interaction of carbon withscrew dislocation cores inducing an unexpected reconstruction towards a hard-core configuration, whichis unstable in pure metals. The carbon atoms are at the center of regular trigonal prisms formed by the Featoms. Around this decorated core, the 4th nearest neighbor octahedral sites are found to be also highly
attractive for carbon. The complex thermodynamic behavior of carbon segregation on these dislocationsites, as function of temperature and bulk carbon concentration, is modelled using mean field and Monte
Carlo methods. The parameters of the Hamiltonian, fitted to DFT calculations, account for the C-C
repulsions between neighboring sites. It is concluded that in the temperature range corresponding to theoccurrence of DSA, the carbon atoms occupy every other prismatic sites, while the other sites are
essentially empty. This configuration is then used to study the kink-pair mechanism suggested by in situ
straining experiments. The results obtained for the kink-pair formation energy and single kink migrationenergies are consistent with the activation energy of the end of DSA regime.
The solute segregation on dislocation-core prismatic sites was investigated for other solutes in Fe and for
C in other bcc metals. A behavior similar to Fe-C is exhibited for B, N and O in Fe and for C in group 6
metals, Mo and W, but with solute-solute interactions between prismatic sites which can be either
attractive or repulsive. By way of contrast, the configuration of lowest energy in group 5 consists of thedislocation in its easy core and the carbon atom in a fifth nearest neighbor octahedral site. We show that
this group dependence is consistent with the carbon local environment in the stable stoichiometriccarbide structures, namely cubic NaCl-type for group 5 and hexagonal WC-type for group 6.

MMM 2018

139

(C6 oral)
Effect of solutes on dislocation motion in dilute hcp and bcc alloys*Tomohito Tsuru1,2, Mitsuhiro Itakura1, Masatake Yamaguchi1,2, Tomoaki Suzudo1, Masato
Wakeda3, Shigenobu Ogata2,4, Daryl C Chrzan5
1. Japan Atomic Energy Agency, 2. ESISM, Kyoto University, 3. National Institute for Materials Science, 4. Osaka
University, 5. University of California, Berkeley

Unlike fcc metals, where dislocation motion associated with solutes is reproduced by the effect of soluteon the stacking fault energy and conventional hardening law, solutes in bcc and hcp metals induce moreunique effects such as softening and dramatic change in plastic deformation. Recently, modeling of
dislocations based on first-principles calculations was developed, and the modeling for various
crystallographic structure were systemized. Especially for BCC metals, solid-solution model wasestablished based on the thermally-activated process of double-kink nucleation and kink migration
related to Orowan’s relation. Furthermore, DFT calculations found to have a great role in providing
fundamental properties of these kink-related process and reproduced solution softening behavior. While
the Burgers vector of the primary slip system in HCP metals is generally dislocation, the slip plane differs
depending on the material, which mainly belongs to basal and prismatic plane according to the its
stacking fault energy. Solute atoms have a large variety of the influence on dislocation motion resulting indramatic change in plastic deformation.
In the present study, we implemented first-principles calculations to obtain corresponding data to thefundamental properties of dislocation motion. We will discuss the effect of transmutation product on solid
solution softening in BCC tungsten and the change in slip system caused by a specific solute in HCP
titanium.

140

MMM 2018

(C6 oral)
Investigation of the energy pathway for generation of dislocations insilicon at Σ3 grain boundaries with the kinetic Activation-Relaxation
Technique*Simen Nut Hansen Eliassen1, Normand Mousseau3, Mickaël Trochet3, Yanjun Li1, Jesper Friis2,
Inga Gudem Ringdalen2
1. Dept. of Materials Science and Engineering, Norwegian Univ. of Science and Technology, 2. SINTEF Materials andChemistry, 3. Dept. of Physics, Univ. de Montréal

Multicrystalline silicon (mc-Si) is widely used for solar cell applications due to the low production costs
and high efficiency. However, the crystallization processes of mc-Si induces different kinds of defects inthe structure which reduces the overall conversion efficiency. Regions containing a high density of defects
such as dislocations are especially detrimental. The origin of dislocations is an ongoing debate. Previous
studies indicates that generation of dislocations in mc-Si can occur at Σ3 grain boundaries; however, adetailed atomistic description of the mechanisms governing the generation of dislocations is lacking. To
cast light on the mechanisms behind generation of dislocations, we have deployed the kinetic
Activation-Relaxation Technique (k-ART), an off-lattice Kinetic Monte Carlo code with an on-the fly
cataloging of events. K-ART allows us to construct an extensive description of the energy landscapearound defects and obtain the energy barriers of diffusion pathways. Furthermore, this is achievable at atime scale and in a temperature regime relevant to experimental observations. In this work, we haveconstructed a model structure containing an asymmetric and a symmetric Σ3 grain boundary which arejoined together to form a kink. We present here an energy pathway for generation of dislocations from thekink with the associated energy barriers, and how the energy barriers is affected when the system issubjected to a shear force.

MMM 2018

141

(C6 oral)
Prediction of mechanical twinning in post-perovskite structure*Philippe Carrez1, Alexandra Goryaeva1, Patrick Cordier1
1. Lille University, Lab. UMET UMR-CNRS 8207

The plastic properties of MgSiO3 post-perovskite are believed to be one of the key issues for theunderstanding of the recorded seismic anisotropy at the bottom of the Earth in a thin layer at theboundary between Earth mantle and Earth core. Indeed, the seismic waves velocities are sensitive to thedevelopment of crystallographic preferred orientations directly related to the plastic properties of thephases. Experimental results from high pressure deformation experiments, have unfortunately led toseveral conflicting interpretations regarding slip systems and dislocation activities in the various
investigated post-perovskite material. Whereas, plastic slip in post-pervoskite has attracted much more
attention, twinning mechanism has not been addressed despite some experimental evidence on
low-pressure analogues such as CaIrO3 compounds.
In this work, we present a twin nucleation model in MgSiO3 and CaIrO3 post-perovskite based on ahierarchical mechanical model of the emission of 1/6<110> partial dislocations. Relying on
first-principles calculations, we show that {110} twin wall formation resulting from the interaction ofmultiple twin dislocations occurs at rather low stresses, suggesting that twinning is a strain producing
mechanism as competitive as dislocation activities in our understanding of the development of preferredorientations in post-perovskite materials.

142

MMM 2018

(C7 invited)
Predictive simulations of crystal plasticity: multiscale or cross-scale?
*Vasily V Bulatov1, Alexander Stukowski2, Luis A Zepeda-Ruiz1, Tomas Oppelstrup1
1. Lawrence Livermore National Laboratory, 2. Darmstadt University

Prediction of crystal plasticity from atomic motion has been regarded over the years as a poster childcontext for multiscale modeling, given that the relevant scales were inaccessible to direct MD simulations
and, at the same time, dislocation lines offer a natural, concise and accurate way to coarse-grain crystal
microstructure. The method of Discrete Dislocation Dynamics (DDD) is regarded as a key and mostchallenging element in the multiscale modeling hierarchy developed to enable such predictions. We will
discuss results of a recently completed direct Molecular Dynamic (MD) simulation of dislocation dynamics
in which a single crystal of tantalum was compressed at rate 105/s. Termed Livermore BigBig (LBB)
simulation, LBB is by far the largest MD simulation ever performed. LBB generated a fully dynamic
trajectory of over 2.1 billion atoms simulated over 5 microseconds of compressive straining. The
simulation generated nearly 80 exabytes of recordable trajectory data a fraction of which was saved on
disk in a highly compressed/post-processed form available for further analysis. As opposed to multiscale,
LBB simulation can be regarded cross-scale being sufficiently large to be statistically representative ofcollective action of dislocations resulting in the macroscopic flow stress and yet fully resolved to every
atomic “jiggle and wiggle”. Importantly, LBB is sufficiently large to be used as a critical test of the
DDD-based multi-scale approach. To enable an “apples to apples” comparison, we carefully calibrated
our DDD model (in ParaDiS) to match our LBB model at the level of single dislocation mobilities, shapeand dimensions of the simulation volume, straining rate, temperature, straining axis, positioning andshape of initial dislocation sources, etc. Our first comparisons of the so-calibrated DDD simulations with
LBB - which in this case serves as an exact benchmark for comparison –are far from satisfactorysuggesting, in particular, that in our DDD model the dislocations do not know how to multiply. The exact
origin of the uncovered discrepancies still unclear, we intend to make available to the community all the
relevant atomistic simulation data (including the LBB results) thus challenging the DDD practitioners tomatch our LBB predictions.

MMM 2018

143

(C7 oral)
Molecular dynamics simulations of dislocation avalanche emissions in
FCC and BCC crystals*Javier Varillas1,2, Jan Očenášek2, Jorge Alcalá1
1. New Technologies Research Centre, University of West Bohemia in Pilsen, 2. Department of Materials Science andMetallurgical Engineering, InSup, ETSEIB. Universitat Politècnica de Catalunya

Plastic deformation in crystalline materials is characterized by intermittent collective dislocation glide
processes, or dislocation avalanches [1, 2, 3]. Here we perform a comprehensive set of molecular
dynamics (MD) simulations to investigate the influence of strain-rate effects upon the emission of
dislocation avalanches in body-centered cubic (BCC) and face-centered cubic (FCC) microcrystals. The
MD simulations comprise periodic (representative) cells containing dense dislocation arrangements
injected prior to the application of uniaxial displacement. Our results enable assessment of the transition
from a continuous deformation mode, developed under a large strain-rate, to the characteristic serrated
discontinuous mode (indicating dislocation-mediated crystal plasticity) under a lower strain rate. In thelatter, the stress-strain curves can be essentially probed under strict displacement control, enabling directcomparison with the avalanche-size distributions from quasi-static micropillar compression experiments[3]. Moreover, our findings shed new light on the role of strain hardening and dislocation cross-slip in
FCCs as well as on the thermally-activated glide of screw dislocations in BCCs upon the avalanche
distributions, including avalanche truncation at large plastic slip events [3].

[1] M. C. Miguel, A. Vespignani, S. Zapperi, J. Weiss and J.-R. Grasso, "Intermittent dislocation flow inviscoplastic deformation", Nature, vol. 410, 2001.
[2] R. Maaβ, M. Wraith, J. T. Uhl, J. R. Greer and K. A. Dahmen, "Slip statistics of dislocation avalanches
under different loading modes", Phys. Rev. E, vol. 91, no. 4, 2015.
[3] J. Alcala, J. Ocenasek, K. Nowag, D. Esqué-de-los-Ojos, R. Ghisleni and J. Michler, "Strain hardening
and dislocation avalanches in micrometer-sized dimensions", Acta Materialia , vol. 91, 2015.

144

MMM 2018

(C7 oral)
Scrutinizing screw dislocation glide initiation at finite temperatures in
BCC metals*Tomoaki Suzudo1, Takashi Onitsuka2, Ken-ichi Fukumoto2
1. Japan Atomic Energy Agency, 2. Fukui Univ.

Plasticity of body-centered-cubic (BCC) metals at low temperatures is determined by screw dislocation
kinetics. Because the core of screw dislocation in BCC metals has non-planar structure, its motion iscomplex and unpredictable. For example, although density functional theory (DFT) predicts slip on a{110} plane, the actual slip plane departs from the prediction at elevated temperatures, its mechanism
having been a mystery for decades. In the current study, we conducted molecular dynamics simulations at
finite temperatures using a recently-developed empirical potential that has the single-hump Peierls barrierfor 1/2<111> the screw dislocation jump and examined the dislocation jump process.
In our molecular dynamics, the dislocation glides on the {110} plane with the highest Schmidt's factor, aspredicted by DFT at low temperatures, while the dislocation approximately glides on a {112} plane at
higher temperatures. Thus, our molecular dynamics simulations successfully reproduced the transition of
the slip plane observed in experiments. To examine how the glide-plane transition takes place, we
particularly developed a post-analysis tool to scrutinize the initiation of screw dislocation jumps from one
Peierls valley to the next with high spatiotemporal resolution. The results indicated that heaved motion ofscrew dislocation line inside a Peierls valley sometimes causes a double-kink nucleation on a {110} planewith lower Schmidt's factor. Because of the twinning/anti-twinning asymmetry of the core structure, theaverage slip plane over the long kinetics departed from the original {110} plane and approximately
becomes {112}. This suggests that thermal fluctuation of screw dislocation line are responsible for thetransition of the slip plane.
We also observed a jerky dislocation motion at low temperatures similar to experiments. Close
investigation of this motion using the new post-analysis tool indicates that inertial effect causes the jerky
motion, i.e., a previous jump beyond the Peierls barrier can directly cause the double-kink nucleation for
the next jump.

MMM 2018

145

(C7 oral)
Uranium plastic deformation: A multiscale approach*Pavel A. Pokatashkin1, Denis K. Ilnitsky1, Alexei V. Yanilkin1
1. Dukhov Research Institute of Automatics (VNIIA)

Uranium is a key element in nuclear technology. We focus on studying plastic deformation of thehigh-temperature BCC phase. On the one hand, classical molecular dynamics is used for obtaining
material parameters, necessary for dislocation dynamics. Dislocation motion involves various mechanisms.
The significance of their contribution to plastic response is studied on the other hand.
We show dislocation climb can be observed at nanosecond timescale. Imposing external shear results ingliding of the dislocation, making dislocation climb even more significant. Studying of those mechanisms
is performed and quantitative parameters e.g. phonon drag coefficient, activation energy, activation
volume are obtained. We emphasize that qualitative results remain the same for all three interatomic
potentials considered in our study. Both discrete and continuum dislocation dynamics simulations are
performed based on the results obtained from atomistic simulations. Synergy between two approaches ofmultiscale modeling results in better understanding of uranium plastic deformation.

146

MMM 2018

(C7 oral)
Understanding the grain refinement mechanism in surface mechanicalattrition treatment of Fe14Ni18Cr by molecular dynamics simulations*Ali Rida1, Matthieu Micoulaut2, Emmanuelle Rouhaud1, Ali Makke 1,3
1. University of Technology of Troyes , 2. Sorbonne Université, 3. EPF, Engineering school

In surface mechanical attrition treatment the surface of a treated material is submitted to a severe local
plastic deformation. This deformation results from the random impacts of spherical shots with highvelocity on the surface of the specimen. Therefore, a nanocrystalline layer is generated on the surface.
The thickness of this layer can reach a depth of 50 micrometers. The grain refinement mechanisms thatlead to the formation of the nanocrystalline layer are still not well understood. In this work, molecular
dynamics simulation has been employed to simulate the compression of a single crystal Fe14Ni18Cr at
9 -1
strain rate of 5.10 s in order to analyze its main plastic deformation mechanisms. Therefore, therelationship between the impacts of the shots and the refinement mechanisms in a grain has been
investigated. A combination of dislocation slips and mechanical twinning have been found to be the mainrefinement mechanisms. High density of dislocations and twins nucleation are observed on the surface.
Moreover, the large density of generated twin boundaries modify the monocrystal into lamellar
twin-matrix blocks with a nanometer sized thickness. Finally, twin-twin and twin-dislocation interactions
lead to the transformation of the twin-matrix blocks to small grains.

MMM 2018

147

(C8 invited)
Thermal fluctuations of dislocations reveal the interplay between theircore energy and long-range elasticity.
*Pierre-Antoine Geslin1,2,3, David Rodney4
1. Mateis lab, INSA Lyon/CNRS, 2. Institute for Materials Research, Tohoku University, 3. Frontier Research Institutefor Interdisciplinary Sciences, Tohoku University, 4. Institut Lumière Matière, Université Lyon 1

Thermal fluctuations of dislocations control their mobility and set the time-scale of thermally activated
events such as cross-slip or obstacles by-pass. While dislocation vibrations have been previously
investigated using simplified line tension descriptions and numerical dislocation dynamics models, weanalyze them by the means of an analytical approach combined with atomistic simulations. Within theframework of the non-singular dislocation theory, we derived an analytical expression for the elastic
energy of a weakly perturbed dislocation, which controls the amplitude of the equilibrium thermal
fluctuations through the equi-partition theorem. Comparing this analytical prediction with molecular
dynamics calculations performed in aluminum shows that a core energy (proportional to the dislocation
length) has to be incorporated in addition to long-range elasticity. Adding this contribution allows to
reproduce very accurately the fluctuation spectra obtained from molecular dynamics simulations andyields quantitative estimates for the core parameter of the non-singular theory and for the magnitude of
the core energy. We also discuss the transferability of these parameters to the bowing-out of a dislocation
between obstacles. Finally, a deeper analysis of the time-dependence of the fluctuations yields valuable
insights on the dynamical behavior of dislocations, namely their mass and phonon damping.

148

MMM 2018

(C8 oral)
Tension of fluctuating dislocation lines*Max Boleininger1, Thomas D Swinburne2, Laurent Dupuy3, Sergei L Dudarev1
1. Culham Centre for Fusion Energy, 2. Theoretical Division T-1, Los Alamos National Laboratory, 3. DEN-Service deRecherches Métallurgiques Appliquées, CEA

Fusion materials will be subjected to high lifetime doses of irradiation at high temperatures. To accurately
predict changes in mechanical properties of materials under fusion conditions, we need to simulate
complex stochastically-driven evolution of radiation damage defect networks over a broad range oftemperature and timescales.
Recent analysis shows that fundamental concepts underpinning defect dynamics require re-examination.
In stark disagreement with atomistic simulations, linear elasticity theory predicts that straight dislocations
have negative line tension with respect to small fluctuations, suggesting that dislocations are unstable tobow-out. Consequently, a dislocation dynamics model based solely on linear elasticity is unable to copewith stochastic thermal fluctuations.
A consistent treatment of dislocation cores is a prerequisite for correctly describing dislocation line
tension in a continuum model. We present a new solution of the discrete Multi-String Frenkel-Kontorova(MSFK) model, which in the continuum limit unifies linear elasticity and the Peierls-Nabarro model for anedge dislocation. The core (misfit) energy and core width are controlled by a single parameter. We findthe continuum displacement fields in agreement with atomistic displacements derived from molecular
dynamics simulations of edge dislocations in bcc iron and tungsten. We investigate line tension of thecontinuum MSFK model with respect to bow-out and find that the misfit energy plays a crucial role incontrolling the stochastic line instabilities.
This work has been carried out within the framework of the EUROfusion Consortium and has received
funding from the Euratom research and training programme 2014-2018 under grant agreement No
633053. Also, it has been part-funded by the RCUK Energy Programme (Grant Number EP/P012450/1).
The views and opinions expressed herein do not necessarily reflect those of the European Commission.

MMM 2018

149

(C8 oral)
Stress-Dependent Activation Parameters for Cross-Slip in FCC Metals*Alon Malka-Markovitz1, Dan Mordehai1
1. Technion—Israel Institute of Technology

Cross-slip is a thermally activated process by which screw dislocations can change their glide plane.
Quantifying the activation parameters for cross-slip in a general stress field is imperative to modelcross-slip in dislocation-based simulations. In most discrete dislocation dynamics (DDD) simulations, only
the contribution of the resolved shear stress is considered. Recently, the contributions of Escaig stresseson the primary and cross-slip planes were accounted for. In this work, we propose a model for cross-slipof screw dislocations in Face-Centered Cubic (FCC) metals by employing the line-tension model tocalculate the free-energy barrier under a general stress field.
When only Escaig stresses are applied, we show that cross-slip is favorable only when Escaig stress in the
primary plane is larger than in the cross-slip plane. The free-energy barrier decreases nonlinearly with
Escaig stresses, whereas it decreases stronger with Eacsig stress in the primary plane. Using our model, weshow that there is typical length for cross-slip, which is a means to quantify the region that bows-in
towards constriction. The typical length scale varies when applying different Escaig stresses on both theprimary and the cross-slip planes.
Schmid stresses in the cross-slip plane break the symmetry of the solution, and the partial dislocations
bow-out in the cross-slip plane after constricting. The application of Schmid stresses is shown to remove
the divergence in the free-energy barrier and cross-slip is possible for all combination of Escaig stress. We
propose an Escaig and Schmid stress-dependent closed-form expression for the free-energy barrier for across slip in a large range of stresses without any fitting parameters. The proposed expression captures
qualitatively the essentials found in atomistic simulations and is in good agreement with previous models.
This closed-form activation energy function can be easily implemented in DDD simulations, owing to its
simplicity and universality.

Malka-Markovitz A. and Mordehai D., Philos. Mag. 98 (2018) 347-370.

150

MMM 2018

(C8 oral)
The influence of precipitate size and shape on the strengthening andhardening rate as observed within metallic alloys.
*Benjamin A Szajewski1, Joshua Crone1, Jaroslaw Knap1
1. Army Research Laboratory

The interaction between glissile dislocations and precipitates within a continuum is responsible formarked increases in material strength. Due to their desirable engineering features, dislocation-obstacleinteractions have been the subject of theoretical study for many decades. Despite ongoing efforts, thesestudies have often been limited to crude approximations whose validity is difficult to assess. Towards
enhancing our understanding of these complex interactions, we employ a three dimensional coupleddislocation dynamics and finite element method computational scheme to directly compute the strength(i.e. bypass stress) of a variety of dislocation-precipitate interactions. The coupled framework accounts forelastic mismatch between the host matrix and precipitates via the inclusion of elastic image stresses andstress concentrations computed through the finite elements. Our simulations span a range of elastic
mismatch, linear obstacle densities, degrees of ellipticity and size. Through our simulations, wedemonstrate the influence of these four parameters on both the dislocation-precipitate strength, andhardening rate. We devise a simple mechanical model which yields insight into the results of both ournumerical simulations as well as others found within the literature.

MMM 2018

151

(C8 oral)
Molecular dynamics simulations of dislocation dynamics
Eyal Oren1, *Guy Makov1
1. Ben-Gurion University of the Negev

Dislocations are of fundamental importance in plasticity and mechanical deformation of solids and theirproperties are required for understanding of numerous phenomena related to mechanical response.
Molecular dynamics simulations allow the study of dislocation motion at the short time and length scales
which are difficult to access by experimental means, but require careful convergence for physicalmeaning. Highly converged simulations were performed on long dislocations in large FCC copper crystal
supercells with full periodic boundary conditions, incorporating a dislocation dipole with zero net Burgers
vector. Kinematics were studied by accelerating dislocations at constant stress and temperature. The
relation between stress and terminal velocity at cryogenic and room temperatures for both screw andedge dislocations was obtained. A transition from viscous behaviour to near sonic asymptotic behaviour
was identified and the velocity dependence of the drag force was characterised. At higher stresses a shift
into the transonic regimes occurs and the mechanism and stability of this transition is examined. The
thermally activated process of cross-slip was simulated multiple times creating statistical data from which
the kinetics and activation parameters were calculated. The effects of stresses in the cross slip and in the
glide planes on the activation volumes were determined. Finally, the rate controlling step of the cross-slip
process was identified producing insight into the cross-slip mechanism.

152

MMM 2018

(C9 invited)
Interstitial Shuffling Mechanism for Solute-Induced Embrittlement of
Titanium*Daryl C. Chrzan1
1. Department of Materials Science and Engineering, University of California, Berkeley

The exceptional sensitivity of the strength of Ti alloys to low concentrations of interstitial O arises from amechanism akin to steric hindrance [1]. The equilibrium interstitial sites of the O atoms are highly
distorted and compressed during the passage of a dislocation. The presence of an O atom at this site
resists this distortion and compression inhibiting the passage of the dislocation and strengthening themetal. However, this strength increase is also correlated with embrittlement of the metal. We identify adislocation-induced interstitial shuffling mechanism that can explain the observed O-induced
embrittlement of Ti [2]. Molecular dynamics simulations show that the passage of a dislocation can movea fraction of the O interstitials from their equilibrium octahedral sites into adjacent hexahedral sites. The
excess number of O interstitials in hexahedral sites is metastable and will eventually return to equilibrium.
However, simulations of the dislocation/O interstitial interaction predict that an O atom in the hexahedral
site presents a weaker obstacle to dislocation slip than an O atom in the octahedral site. Under typicalstrain rates, the time between dislocation passage on the slip plane is less than the estimated typicalresidence time for an O atom in the hexahedral site. The implication is that the passage of the first
dislocation on the slip plane leads to softening for the subsequent dislocations. Subsequent dislocations
may the further soften the slip plane through the same mechanism. This slip plane softening is expected
to lead to planar slip, and the stress concentrations associated with planar slip then lead to theembrittlement of the metal. This work is supported by the U. S. Office of Naval Research under grant
N00014-16-1-2304 and employed computational resources of the National Science Foundation under
grant ACI-1053575.

[1] Q. Yu et al., Science 347, 635-639 (2015).
[2] M. Poschmann et al., to be published.

MMM 2018

153

(C9 oral)
Screw dislocation mediated solution strengthening of substitutional
α-Ti alloys - First principles investigation*Piotr Kwasniak1
1. Warsaw University of Technology

The mechanical properties of hexagonal Ti alloys depend substantially on the glide of <a> type screw
dislocations. The configurations and stabilities of these line defects are, however, known only in pure Ti
and Ti + O solutions, where the locking-unlocking mechanism and a strong pinning effect control theiractivity. In this study, we investigated the unclear, screw dislocation mediated solution strengthening of
substitutional α-Ti alloys. To this end, a first principle computational scheme was used to determine thestructures and energies of the considered line defects during planar and cross-slip processes in thevicinity of the solute element. Two phenomena were determined that are crucial in terms of plastic
deformation: (i) enhanced polymorphism of the dislocation cores leading to multiple new coreconfigurations, and (ii) relatively large positive and negative interaction energies between the solutes andthe line defects. Both these effects are strongly affected by the valence configuration of the alloying
elements. Due to their pronounced structure and energy variations, dislocation planar and cross slip
processes can occur under different scenarios, through diverse non-planar core geometries. The
calculations performed also indicate In as a potential alloy element for improving both the strength andductility of Ti by stabilizing a special, compact core geometry able to spread on an arbitrary glide planewith a low energy barrier. All of the above effects are discussed in terms of the physical factors (solute size
misfit, stacking fault energy and electronic structure) that affect the energy and geometry of dislocation
cores.

154

MMM 2018

(C9 oral)
Microstructural effects on strain rate sensitivity in dual-phase titanium
alloys*Sana Waheed1, Zebang Zheng2, Daniel S. Balint1, Fionn P. E. Dunne2
1. Dept. of Mechanical Engineering, Imperial College London , 2. Dept. of Materials, Imperial College London

Cold dwell fatigue failure in titanium alloys is a major concern in aero-engine applications and isinherently linked to the strain rate sensitivity (SRS) of the material. Recent crystal plasticity finite element(CPFE) work has shown the material SRS to be influenced by a number of microstructural factors [1].
However, these higher-level modelling studies do not include discrete aspects of slip and considerably
overestimate the material rate sensitivity as compared to experimental studies. Thus, the significance andmechanism of microstructural effects on experimentally observed strain rate sensitivity in titanium alloys
remain unclear.
In this study, a planar discrete dislocation plasticity (DDP) model is set up explicitly incorporating
dislocation penetration across phase boundaries and material heterogeneity arising from alpha and betatitanium phases to simulate realistic microstructures. Stress relaxation tests are performed to investigate
microstructural effects, such as the presence of colony or basketweave microstructures, textured or
non-textured crystallography and grain size effects, on the material strain rate sensitivity. The SRS
coefficients obtained from the simulations match closely with experimental values. It is found thatchanging the microstructure from pure alpha, colony to basketweave, and reducing alpha grain sizes
leads to a significant reduction in material rate sensitivity, whereas dislocation penetration is found to not
be as significant as previously considered for small hold strains. The mechanistic basis for these effects isargued to be changes in dislocation mean free-path and the total amount of plasticity in the specimen.
Finally, DDP results are compared with corresponding CPFE simulations, to show that discrete aspects ofslip and hardening mechanisms have to be accounted for to capture experimentally observed ratesensitivity. These results provide increased understanding of the mechanism of cold dwell fatigue failurein titanium alloys and are relevant for the design of dwell-insensitive microstructures.

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155

(C9 oral)
Understanding thermal alleviation in cold dwell fatigue in titanium
alloys using crystal plasticity model*Zebang Zheng1, Adam Stapleton2, Kate Fox2, Fionn P.E. Dunne1
1. Dept. of Materials, Imperial College London, 2. Rolls-Royce plc

Dwell fatigue facet nucleation has been investigated in isothermal rig disc spin tests and underanisothermal in-service engine conditions in titanium alloy IMI834 using α-HCP homogenised andfaithful α-β lamellar microstructure crystal plasticity representations. The empirically observed facet
nucleation and disc failure at low stress in the isothermal spin tests has been explained and originatesfrom the material rate sensitivity giving rise to soft grain creep accumulation and hard grain basal stresses
which increase with fatigue cycling until facet nucleation. The α-HCP homogenised model is not able tocapture this observed behaviour at sensible applied stresses. In contrast to the isothermal spin tests,
anisothermal in-service disc loading conditions generate soft grain slip accumulation predominantly in thefirst loading cycle after which no further load shedding nor soft grain creep accumulation is observed,
such that the behaviour is stable, with no further increase in hard grain basal stress so that facet
nucleation does not occur, as observed empirically. The thermal alleviation, which derives from in-service
loading conditions and gives the insensitivity to dwell fatigue dependent on the temperature excursions,has been explained. A stress-temperature map for IMI834 alloy has been established to demarcate theranges for which the propensity for dwell fatigue facet nucleation is high, threatening or low.

156

MMM 2018

(C10 invited)First Principles Calculations of Dislocations in Model Engineering
Alloys (Ni, Ni3Al, hcp-Ti, and a Refractory Metal BCC-HEA)*Christopher Woodward1
1. Air Force Research Laboratory

In order to understand the “chemistry of deformation” an adequate description of the strain field near
the center (core) of dislocations is required. While continuum elasticity methods have been very
successful in describing long-range stress fields of dislocations these methods diverge in the core region.
Atomistic methods have shown that the forces produced at the dislocation core and their coupling to theapplied stress can have a dramatic effect on plasticity. However, atomistic methods are limited by the
fidelity of the assumed interaction model and for this reason are at best semi-empirical. Here the Lattice
Greens Function is used with Density Functional Theory to calculate the equilibrium core structure ofisolated screw dislocation in four model-engineering alloys. These include Ni, L12 Ni3Al, and a c+apyramidal dislocation in hcp Ti. Pipe diffusion along the dissociated partial dislocations in Ni has been
evaluated for self-diffusion and Co solutes. Recent progress extending a 3-d lattice Greens function tomodel screw dislocations in a refractory bcc high entropy (high concentration solid solutions) will be
reviewed.

MMM 2018

157

(C10 oral)
Hybrid QM/MM study of dislocation glide in tungsten in the presenceof plasma components*Petr Grigorev1, Tom Swinburne2, James Kermode1
1. Warwick Centre for Predictive Modelling, 2. Los Alamos National Laboratory

Tungsten has been chosen as the diverter armour material in ITER and is the main candidate material for
plasma-facing components for future fusion reactors.Interaction of plasma components with the material
is unavoidable and will lead to degradation of the performance and the lifetime of the in-vessel
components.From this point of view it is important to gain an understanding of dislocation mobility as wellas the influence of fusion plasma components (H and He) on the mechanism.
Dislocation glide is governed by the localised rearrangement of atoms inside the dislocation core but istightly coupled to the stress and strain fields on larger length scale.Thus, a correct description of theprocess requires chemically accurate treatment of the dislocation core atoms together with the use of thesystems large enough to accommodate the corresponding stress fields.On the one hand, Density
Functional Theory (DFT) provides an accurate description of the interatomic bonding, however thecomputational cost of the method does not allow one to study large enough systems and requires usageof periodic cells (i. e. with dislocation dipoles and quadrupoles with complex symmetry). On the other
hand, classical interatomic potentials are much more computationally affordable allowing to study
systems of required size, but the accuracy and transferability of the description of dislocation core is poor,
especially for multi-element systems.
In this study we use a hybrid multiscale approach, namely quantum mechanics/molecular mechanics(QM/MM) combining an accurate local QM description of the dislocation core atoms with a classicalmodel for the rest of the system. We apply a recently developed QM/MM implementation of the virtual
work principle to compute energy barriers. The effect of H and He atoms in the material on the dislocationcore structure together with the energetics of dislocations and impurities are investigated. The obtained
results are analysed by comparison with pure DFT studies from literature together with machine learning
based Gaussian Approximation Potential (GAP) model.

158

MMM 2018

(C10 oral)First-principles Calculations of Deformation Twins in Hexagonal
Titanium Alloys*Daisuke Matsunaka1, Ryosuke Amano1
1. Dept. of Mechanical Engineering, Shinshu Univ.

Because of the excellent strength-to-density ratio and the high corrosion resistance, titanium (Ti) and Ti
alloys have attracted much attention as aerospace and biomedical materials. One of the methods toimprove mechanical properties of Ti is addition of solid solution atoms. For example, the tensile andfatigue strengths of Ti increase with oxygen content, although compromising ductility and toughness. For
further development of titanium alloys, detailed understanding of effects of alloying elements on
individual deformation modes, various slip and deformation twinning, in hexagonal alpha-titanium alloys
is important.
In this study, we investigate twin boundaries and interaction of solute atoms such as Al in alpha-titanium
alloys by first-principles calculations. For {11-22} twin, the mirror glide structure is stable, compared withthe mirror reflection one and the {10-12} twin boundary. While the {10-12} twin boundary can migrate
easily, the energy barrier for migration of the {11-22} twin boundary is relatively high. The interaction ofsolute atoms with the {10-12} twin boundary depends on the distance and the corresponding boundary
sites, i.e., expansion and compression sites. On the other hand, the interaction of solute atoms with the{11-22} twin boundary is complex due to the mirror glide boundary structure.

MMM 2018

159

(C11 invited)
A Multi-Scale Dislocation Language - Data Mining, Statistical Analysis,and Steps Towards a Community-Driven Data Base
*Stefan Sandfeld1
1. TU Bergakademie Freiberg

Dislocations are simplistic objects: they are one-dimensional, their motion is constrained by the
crystallography and they are surrounded by a stress field that decays with 1/r. However, once dislocations
start to interact with themselves or with other microstructures, their collective behavior becomes
extremely complex. This - despite the apparent simplicity of the individual object - is still far fromcompletely being understood.
Simulation methods, such as the Molecular Dynamics (MD) or the Discrete Dislocation Dynamics (DDD)
have been very successfull in predicting the evolution of dislocation microstructures along with theresulting structure-property relations. However, up to now, there is no common "language" that allows to
directly compare different dislocation structures - not even if they are obtained from the same simulation
method. This makes statistical analysis and data mining on the level of the dislocations difficult.
An overview over state of the art methods for analyzing systems of dislocations will be given.
Subsequently, the "Multi-scale Dislocation Language" (MuDiLingo) will be introduced, which allows to
extract important geometrical properties of dislocations along with the corresponding energies. Based on
examples from MD and DDD it will be shown (i) how this approach can be used to quantitatively compare
dislocation microstructures, (ii) how valuable input for simulation methods on higher length scales can beprovided by data-mining, and (iii) how MuDiLingo will enable us to create a data base of dislocation data,
which might be of great benefit to the community.

160

MMM 2018

(C11 oral)
The effect of dislocation character on dislocation line tension in bcc
tungsten and its impact on kink-pair enthalpy*David Cereceda1, Vasily Bulatov3, Jaime Marian2
1. Dept. of Mechanical Engineering, Villanova University, 2. Department of Materials Science and Engineering,
University of California Los Angeles, 3. Physical and Life Sciences Directorate, Lawrence Livermore National
Laboratory

In addition to the well-characterized elastic contribution, the energy of a dislocation contains an inelastic,or ‘core’, term that reflects the loss of validity of elasticity theory at dislocation segments. While theelastic part is known to be symmetric about its maximum value for the edge orientation (minimum forscrew), in bcc metals, the core energy displays an asymmetry that can be characterized using atomistic
calculations. In kink-pair configurations on screw dislocations, this asymmetry leads to a difference inenergy between ‘right’ and ‘left’ kinks that is not captured in elastic models. In this work, we
calculate dislocation segment self-energies as a function of dislocation character in bcc tungsten andkink-pair enthalpies as a function of stress. To avoid finite-size artifacts in atomistic simulations, wedevelop continuum models of kink-pair configurations based on full elasticity and line tension
approaches, parameterized with a substrate Peierls potential and dislocation self-energies obtained fromatomistic calculations.
The elastic and line tension models represent specific situations of the environment of these kink-pairconfigurations, and we discuss our results in terms of the range of validity of each as well as the effect ofself-energy asymmetry on kink-pair enthalpy. To match the continuum results to direct atomistic
simulations, we vary the core radius of elasticity theory and discuss the implications of the values
obtained.

MMM 2018

161

(C11 oral)
Atomistic modeling of thermally activated plasticity in UO2Aurélien Soulié1, *Jean-Paul Crocombette1, Antoine Kraych1, Frédérico Garrido2, Gaël Sattonnay
2, Emmanuel Clouet1

1. CEA Saclay, SRMP, 2. CSNSM, CNRS-IN2P3-Université Paris-Sud

We investigate the thermally activated glide mobility of dislocations in uranium dioxide (UO2) from anatomistic point of view using a variable charge many-body empirical potential, the Second Moment
Tight-Binding potential with charge equilibration (SMTB-Q). In order to determine the main glide system,
we model the dislocation core structures for edge and screw orientations lying in different glide planes.
Uncommon core structures with a double periodicity and a charge alternation are obtained. Straightdislocations motion is first considered to obtain the Peierls stress of each dislocation. We then address
the thermally activated motion of the dislocations by the nucleation of kink pairs. Atomistic simulations
give us the structure as well as the formation and migration energies of kink pairs. This information is
finally combined with an elastic interaction model for kink pairs to obtain the dislocation velocities andthe evolution of the critical resolved shear stress as a function of temperature. These quantities arecompared to experimental critical resolved shear stress and dislocation velocity measured on urania
single crystals.
Reference : Acta Materialia 150, 248-261 (2018)162

MMM 2018

(C11 oral)
FTMP-based Simulations and Evaluation of Geometrically-Necessary
Boundaries (GNBs) of Dislocation*Shiro Ihara1, Tadashi Hasebe1
1. Dept. of Mechanical Engineering, Kobe Univ.

Rational as well as practically-feasible treatments of spatio-temporally distributed discrete dislocation
ensembles have been a long-standing issue and still a challenging problem inevitable in completion ofmultiscale modeling of metallic materials. Unlike 2D cases, which cannot be simply reduced down tocontinuously-distributed density function-like pictures, 3D counterparts need to deal, more or less, with
its configurational complexities explicitly. We have recently been tackling these based on FTMP (Field
Theory of Multiscale Plasticity), focusing on continuum descriptions of dislocation aggregates. Amongothers, quantitative stability/instability assessments of wall structures are critically important, in the sensethat they substantially dominates both the micro/macro mechanical properties of the material systems
concerned. The present study targets GNBs (Geometrically Necessary Boundaries) in terms of theirstability/instability criteria and some dynamic interactions with in-coming dislocations, whose details
about the consisting dislocations recently have been experimentally identified and theoretically evaluatedby Hong, Winther, et al. Dislocation dynamics simulations on five typical GNBs, i.e., GNBs 2, 3, 4, 7 and 8,
are conducted by utilizing Zbib code and ParaDis code, and FTMP-based evaluations are performed
against them. Duality diagram representations revealed a possible overall picture that governs the GNBs,i.e., all the GNBs exhibit a common tendency to converge ultimately to a single point, i.e., the ideal value
for a hexagonal network-based GNB that yields the lowest energy as well as the smallest incompatibility,
located in the most lower-left on the diagram.

MMM 2018

163

(C11 oral)
Improved phase field model of dislocation intersections*Songlin Zheng1, Yong Ni2
1. China Academy of Engineering Physics, 2. University of Science and Technology of China

Revealing the long-range elastic interaction and short-range core reaction between intersecting
dislocations is crucial for understanding dislocation-based strain hardening mechanisms in crystalline
solids. Phase field model (PFM) has shown great potential in modeling dislocation dynamics with
employing the continuum microelasticity theory to describe the elastic interactions and incorporating the-surface into the crystalline energy to enable the core reactions. Since the crystalline energy isapproximately formulated by linear superposition of interplanar potential of each slip plane in theprevious PFM, it does not fully account for the reactions between dislocations gliding in intersecting slip
planes. In this study, an improved PFM of dislocation intersections is proposed through updating the
crystalline energy by coupling the potential of two intersecting planes, and then applied to study thecollinear interaction compared with the previous PFM model. Collinear annihilation captured only in the
improved PFM is found to strongly affect the junction formation and plastic flow in multislip systems. The
results indicate that the improvement is essential for PFM of dislocation intersections.

164

MMM 2018

(C12 oral)
Modeling the interaction between martensitic phase transformations
and dislocation dynamics*Rachel Derby1, Michael Budnitzki1, Stefan Sandfeld1
1. TU Bergakademie Freiberg

Two of the most important deformation mechanisms in TRIP-Steel are the martensitic phasetransformations (MT) and elastic/plastic deformation due to the motion of dislocations. Martensitic phasetransformations are desirable because they enhance the strength of TRIP-steels, however, dislocations
can lower the strength of the material. These two phenomena are strongly coupled due to theircontributions of strain to the system, resulting in an effect on the final macroscopic properties of thematerial. Thus, both dislocations and phase transformations must be considered simultaneously whenmodeling TRIP-Steels.
While MD simulations are inherently able to treat both dislocations and MT, such simulations suffer from ahigh computational cost and, hence, are not able to reach relevant time scales. Phenomenologicalcontinuum models for plasticity can be coupled with phase field approaches for the MT and can reachlarger time scales; they do not capture the dynamics of the dislocations.
In our model, the dynamics of dislocations is described by a continuum dislocation dynamics (CDD)model, where instead of individual dislocations, a dislocation density is considered. This approach
reduces the computational cost and enables the modeling of larger length scales. We use a coupled
CDD/phase field approach to study the interaction of dislocation plasticity and the martensitic phasetransformations. Our main focus is on the influence of different initial values and on the history
dependence of the material. This will allow us to discuss, among others, what happens to dislocations inregions that undergo a MT –an information of big importance for continuum models operating on larger
length scales.

MMM 2018

165

(C12 oral)
The effect of microscopic slip deformation on the yield stress indispersion hardened alloys analyzed by using crystal plasticity FEM*Yelm Okuyama1, Masaki Tanaka1, Tatsuya Morikawa1, Tetsuya Ohashi2
1. Dept. of Materials Science and Engineering, Kyushu University, 2. Dept. of Engineering, Kitami Institute ofTechnology

The relationship between yield stress and the distribution of microscopic plastic deformation wasinvestigated in dispersion hardened alloys using crystal plasticity FEM. Dispersed particles were randomly
distributed in a 3D box in the calculation model. A plate region was extracted from the 3D box, and wasused as a finite element model. The length factor in the Orowan equation was determined form theparticle distribution in the 3D box. The critical resolved shear stress in local was set to a function of thelength factor in the model. The length factor was defined as the average spacing of the particles, whichsurround the element, on the slip plane. The crystal plasticity analysis elucidates that the initiation andpropagation of local slip deformation depends on the length factor. In this study, the relationship between
the length factor and macroscopic mechanical response, i.e. yield stress and work hardening rate, isdiscussed.

166

MMM 2018

(C12 oral)
Deformation Behaviour for Two-Phase Composites under Large
Deformations using Micromechanical Analysis*Srihari Dodla1
1. Madanapalle Institute of Technology (MITS) MadanapalleThis study presents a micromechanics approach for the elasto-viscoplastic behaviour of eutectic materials
and structures. In this approach, the high-fidelity generalised method of cells (HFGMC) is applied for theprediction of the effective behaviour of two cold-drawn Cu-Ag composites with different drawing strains
and to obtain the field (deformation gradient and stress) distributions in the composite. Both phases (Cuor Ag) are rate-dependent crystal plasticity material constituents. HFGMC is applied for analysing thedeformation behaviour of two-phase Cu-Ag composites under uniaxial compression. The
micromechanical analysis has been verified by comparison with experimental and finite element
simulation results. Results in terms of deformation behaviour and field distributions are given for two
different cold-drawn composites.

MMM 2018

167

(C12 oral)
Dislocation dynamics simulation of FCC single crystals in high strain
rate deformation*Ronan MADEC1
1. CEA, DAM, DIF

In the range of high strain rate deformations from 104 to 106 s-1 or more, the mobile dislocation density
significantly increases and some dislocations may reach a large fraction of sound velocity. The extent of
this mobile dislocation multiplication stage depends upon material, experimental or simulation
conditions. It also strongly depends on the initial dislocation density. When the total dislocation density
reaches a certain value, it may saturate [1].
Molecular dynamics simulations are well suited for very high strain rates (107 s-1 or more). Hoverver, owing
to the extremely small volumes and simulation times, it cannot deal with smaller strain rates. As aconsequence, another type of simulation is needed for strain rates below 106 s-1 and dislocation dynamics
simulations appears to be the most suitable method for this purpose.
The first dislocation dynamics studies at high strain rates were performed in small volumes (2-5 microns)of copper single crystals [2-5]. The dislocation fluxes and artefacts such as self annihilations were notcarefully handled [6]. In addition, plastic strains were quite small (less than 0.2%) for such volume sizes
and the saturation of the mobile dislocation density was difficult to reach even for the lowest strain rate.
Nevertheless, the effect of inertia above 103 s-1 [2] and, above all, the importance of cross-slip [3-5] were
established.
With growing computing power, larger volumes and deformations can be reached and the end of themultiplication stage of high strain rate deformation can be reached. So this topic deserves being revisited.
Results about dislocation velocities, mobile and total dislocation densities, as well as the anisotropic
response of copper single crystal as a function of stain rate and the effect of cross-slip will be discussed.
[1] L. Zepeda-Ruiz, A. Stukowski, T. Oppelsturp, V.V. Bulatov, Nature 550, 492-495 (2017)[2] Z.Q. Wang, I.J. Beyerlein, R. Lesar, Phil. Mag. 87, 2263-2279 (2007)[3] Z.Q. Wang, I.J. Beyerlein, R. Lesar, Model. Simul. Mater. Sci. Eng. 15, 675-690 (2007)[4] Z.Q. Wang, I.J. Beyerlein, R. Lesar, Phil. Mag. 88, 1321-1343 (2008)[5] Z.Q. Wang, I.J. Beyerlein, R. Lesar, Int. J. Plast. 25 26-48 (2009)[6] R. Madec, B. Devincre, L. Kubin, Solid mechanics and its applications 115, 35-44 (2004)

168

MMM 2018

(C12 oral)
Comparison of two methods to cross-slip modeling by means ofmathematical theory of moving curves*Miroslav Kolar1, Jan Kratochvíl1, Petr Pauš1, Michal Beneš1
1. Czech Technical University in Prague

The contribution focuses on the comparison of two different methods of dislocation cross-slip modeling,both based on the theory of evolving curves. The cross-slip is understood as deterministic, stress-driven
dislocation process. In certain geometrical configuration, the criterion based on evaluation of driving
stresses in the primary plane and in the cross-slip plane is employed. As an illustrative example, we
consider a scenario where the cross-slip is forced by the repulsive stress exerted by a spherical obstaclewith realistic repulsive stress. The result of this scenario is the double cross-slip.
The employed model for the dislocation motion can be schematically written as the curvature driven flowin the form
normal velocity = curvature + force.
In the first method, the dislocation motion law is based on the isometric projection. The cross-slip plane istilted to the primary plane so that dislocation motion remains planar. When the dislocation enters the
cross-slip plane, the appropriate physical quantities are recalculated accordingly.
The second method is based on the exploitation of the model of curves evolving on smooth surfaces anddriven by the geodesic curvature. The smoothness of the the glide surface is ensured by regularizing the
sharp interfaces between the primary plane and the cross-slip plane.
For both methods, the dislocations are described as parametrized curves. In numerical simulations, theflowing finite volume method is used. The results of the numerical simulations for both methods arecompared to demonstrate their agreement.

MMM 2018

169

(C13 invited)
Mesoscopic studies of slip and twinning processes in hcp
polycrystalline materials*Irene Beyerlein1, Marat Latypov1, Jason Mayeur, Wyatt Witzen1, Mitra Taheri2, Tresa Pollock1
1. University of California at Santa Barbara, 2. Drexel University

Under deformation, strain and lattice orientation gradients can develop near crystalline heterogeneities,
such as grain boundaries, deformation twins, and voids. These important effects have been studied
intensely most often in materials that deform by a single slip mode. In this presentation, we discuss
multiscale crystal plasticity modeling efforts to understand the development of lattice orientation
gradients induced near and at the boundaries of microstructural defects within crystals of materials witheither a BCC or HCP crystal structure. These two material classes commonly deform by multiple slip
modes and this distinction is taken into account in the modeling. The results presented in this talk will
highlight the effects of microscopic differences in the strengths of these slip modes on the evolution of
dislocation density.

170

MMM 2018

(C13 oral)
Crystal plasticity formulation involving volume fraction-based
deformation twinning model*Yuichi Tadano1
1. Department of Mechanical Engineering, Saga University

Deformation twinning is an important deformation mechanism of metallic materials as well as slip
deformation, especially in hexagonal closed-packed (HCP) metals such as magnesium and titanium.
Therefore, understanding twinning behavior is essential to improve mechanical properties of HCP metals.
HCP metals generally have a strong anisotropy in crystalline scale. For example, in case of magnesium,
plastic deformation due to slip along the c-axis is hard to occur because of much higher critical resolved
shear stress of pyramidal slip system, and deformation twinning occur to enhance plastic deformation
along the c-axis. The most dominant twinning in magnesium is {10-12}<11-20> tensile twinning system,
the number of which is six. As several experimental results showed that untwinned and twinned regions
can simultaneously exist in a grain, the author proposed a crystal plasticity model considering the volume
fraction of deformation twinning. In the practical problems, multiple twinning systems may besimultaneously activated in a grain because of inhomogeneity of deformation. Therefore, the deformation
twinning model has been extended to incorporate the volume fraction of multiple twinning systems. To
represent the polycrystalline behaviors of materials, the homogenization-based finite element method isadopted, so that the mechanical behaviors of polycrystalline magnesium can appropriately be
reproduced. Numerical investigation using the proposed model is performed, and the adequacy of themodel is validated.

MMM 2018

171

(C13 oral)
An Elastic-Viscoplastic Crystal Plasticity Modeling for Plane Strain
Deformation of Pure Magnesium*Weidong Song1, Liansong Dai1
1. Beijing Institute of Technology

A rate-dependent elastic–viscoplastic constitutive model is proposed. The model of plane deformation of
single crystal magnesium is established by considering seven orientations with different loading
directions. The basal, prismatic and pyramidal slip systems in the parent grain, compressive twinning (CT)and tensile twinning (TT) are incorporated in the model. The constitutive descriptions of CT and TT are
distinguished to better characterize their effects on the overall hardening of magnesium. The
contributions of different deformation modes to the macroscopic plastic deformation of magnesium single
crystals in the seven cases are presented. These computational predictions are compared with theircorresponding macroscopic experimental observations (stress-strain responses) and other numericalresults. These results prove that it is necessary to distinguish different twinning systems and theirassociated hardening laws for the plastic deformation of magnesium and its alloy.

172

MMM 2018

(C13 oral)
The Minimum Energy Pathways Identifications of Twinning Dislocation
Loop Nucleation of Extension Twinning in Magnesium*Xiao-Zhi Tang1, Qun Zu2, Lixin Sun4, Yue Fan3, Ya-Fang Guo1
1. Inst. of Mechanics, Beijing Jiaotong Univ., 2. Schl. of Mechanical Engineering, Hebei Univ. of Tech., 3. Dept. of
Mechanical Engineering, Univ. of Michigan, 4. Dept. of Nuclear Science and Engineering, MIT

The twinning dislocation loop nucleation on the coherent twin boundary in magnesium is investigated bypotential energy approaches, and also the dynamic simulations at various temperatures and strain rates.
Based on our numerical results of extension twinning, the single lattice transformation within the coherent
twin boundary migration is identified to be the elementary migration process, and its stress-free potential
energy barrier is lower than the thermal energy at room temperature. The migration can be triggered
thermally once the shear stress makes the transformed lattice at a low energy state. Both the nucleationenergy and the critical radius of a twinning dislocation loop are calculated. We believe our discoveries
pursue a better understanding of the plasticity of hexagonal close-packed metals.

MMM 2018

173

(P1-08)
Pressure effects on dislocation core structures in Mg2SiO4 olivine:
insights from atomic-scale modeling*Philippe Carrez1, Srinivasan Mahendran1, Patrick Cordier1
1. Lille University, Lab. UMET UMR-CNRS 8207

Olivine (Mg,Fe)2SiO4, a silicate with orthorhombic structure, is one of the most common minerals. As anabundant phase of the upper Earth mantle, its plastic properties strongly constrained the thermal
convection of Earth mantle in its upper part. Plastic deformation of olivine involves two types of
dislocation corresponding to the shortest Burgers vector [001] and [100]. At low temperature and highstress, [001] slip is supposed to be the most predominant slip system whereas at high temperature andlow stress, [100] dislocations dominate. Over the last decades, a number of different slip systems havebeen thus identified in various domain of temperature, strain rate or pressure.
In this study, we revisit plasticity of this silicate by computing at the atomic scale the intrinsic properties of
dislocation in Mg2SiO4 single crystal. All the calculations rely on a parametrized potential combining
coulombic interactions and a core-shell interaction model for oxygen atoms. We performed a systematicinvestigation of [100] dislocations metastable configurations and possible dissociations.
Our calculations show that at low pressure, the atomic arrangement within the dislocation core iscompatible with the [100](010) slip system observed experimentally. Also we show that the occurrence ofseveral metastable core configurations allows to various cross slip events for which the cross slip energy
barrier have been computed. Finally, we will show that the various core configurations are strongly
sensitive to pressure leading to some change in the relative metastable states of the dislocation core andultimately inhibiting some known slip plane at higher pressure.

174

MMM 2018

(P1-09)
Structural and magnetic properties of long-period stacking ordered(LPSO) structure of iron; A first-principles study*Takao Tsumuraya1, Ikumu Watanabe2, Takahiro Sawaguchi2
1. Magnesium Research Center/POIE, Kumamoto Univ., 2. Research Center for Structural Materials, National Institutefor Materials Science

Fe-Mn-Si based alloys is known to exhibit a shape-memory effect associated with deformation-induced
martensitic transformation from face-centered cubic (fcc) γ-austenite to hexagonal closed packed (hcp)ε-martensitic phase. The Fe-Mn-Si-based alloys with modified chemical compositions appeared to haveoutstanding properties of low-cycle fatigue lives, and a Fe-15Mn-10Cr-8Ni-4Si (mass%) alloy developed ispractically used in a seismic damping component of architectural constructions. Recently, under cyclic
push-pull loading of Fe-Mn-Si-based alloys, a new phase different from ε-phase was found bytransmission electron microscopy. The new phase shows electron diffraction spots at the 1/3 position ofthe {10-11} spots of the ε-phase, which suggests the existence of a long-period stacking ordered (LPSO)
structure. In 1960s, a similar phase was reported after several times of thermal cycles of γ-ε phase
phase transitions in Fe-Mn-C alloys. However, actual stacking sequence of the LPSO phase and relative
stability with γand ε-phases still remain unclear. To understand these issues, we proposed several
structural models of LPSO structure of pure Fe, such as 4H, 6H1 and 6H2, and discuss structural andmagnetic stabilities among the candidate of LPSO structures with first-principles calculation methods. [1]L. I. Lysak, and B. I. Nikolin, Fiz Met. Mettaloved 20, 547 (1965), ibid. 23, 93 (1967).

MMM 2018

175

(P1-11)
Dislocation transmission behaviors of bi-crystal BCC Tantalum withhigh and low angle symmetric tilt grain boundaries: Multiscale
simulation study*Moon Sunil1, Kang Keonwook1
1. Dept. of Mechanical Engineering, Yonsei Univ.

In this research, we conducted multiscale (molecular dynamics and dislocation dynamics) simulations tostudy dislocation transmission behaviors of bi-crystal BCC Tantalum (Ta) with high and low angle
symmetric tilt grain boundaries. To investigate dislocation transmission behaviors, we introduced adislocation loop to describe the Frank-Read source, one of the dislocation multiplications in molecular
dynamics simulation. We put dislocation loop on the slip plane where maximum resolved shear stressoccurs. In molecular dynamics simulation, we observed interactions between dislocation and grainboundary such as dislocation transmission and absorption. Furthermore, we performed dislocation
dynamics simulation to observed similar dislocation transmission behaviors. Finally, we analyzed
mechanical property changes (e.g. stress-strain response) as dislocation interacts with grain boundary.

Acknowledgement
This work was supported by the Basic Science Research Program through the National Research
Foundation of Korea (NRF) funded by the Ministry of Education (2016R1C1B2016484).

176

MMM 2018

(P1-12)
Ab Initio Dislocation Core Calculations using a Cluster Model*Jonas Frafjord1,3, Bjørn Holmedal4, Jesper Friisj2,3, Randi Holmestad1,3, Ingeborg-Helene Svenum
2, Inga Gudem Ringdalen2

1. Dept. of Physics, Norwegian University of Science and Technology, 2. SINTEF Industry, Trondheim, 3. Centre forAdvanced Structural Analysis, SIMLab, 4. Dept. of Materials Science and Engineering, Norwegian University of Scienceand Technology

Solute strengthening is an important mechanism contributing to the strength of metallic alloys. In order tocreate an accurate framework for strength calculations, this mechanism must be fully understood. In thatregard, studies have shown that it is crucial to include the core region of a dislocation in models [1-3], as
the core interacts strongly with solute atoms through short-range interactions, and affects the dislocation
mobility. The core region can be treated from first principles to include electronic effects. One challenge
has been to correctly describe the dissociation of a dislocation at atomic level [4,5]. Dissociation of aperfect dislocation results in a more favourable configuration by reduction of the elastic energy. The
equilibrium separation of the partial dislocations generated is due to the cost of the intrinsic stacking
fault. This separation width is important for the strengthening mechanism of solutes since it affects thesolute-dislocation interaction-energy map.
In this work, a cluster model for dislocation core structures is applied. The model takes the shape of acylinder, where the dislocation line is elongated through the centre [6]. It consists of an outer region
surrounded by vacuum, where the atoms are fixed by an elastic displacement field, and a core region thatis relaxed. This model is strongly dependent on the assumption that the elastic theory is respected within
the fixed region. The advantage of this model is the simplicity, and its transparency towards theassumptions that are taken. The aim is to improve the capability to set up an atomistic model for
dislocations in fcc metals using a cluster model with sufficient accuracy. The setup has been tested for
Aluminium with a pure edge dislocation. It will be extended further to screw dislocations.
Using this simple model, the static configuration of a dislocation core structure can be compared toexperimental observations and results using the flexible boundary condition applied by other groups [1,4].
[1] doi.org/10.1016/j.actamat.2016.09.046[2] doi.org/10.1016/j.scriptamat.2014.04.018[3] doi.org/10.1016/j.actamat.2010.06.045[4] doi.org/10.1016/j.msea.2005.03.039[5] 10.1103/PhysRevLett.100.045507[6] doi.org/10.1016/j.actamat.2016.09.049

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Anelastic recovery of a steel wire under pure shear strain with various
magnetic treatments*Shigeo Kotake1, Takuro Murata1
1. Dept. of Mechanical Engineering, Mie Univ.

Elastic property of a metal under pure shear strain is quite different from that under other strain, since theformer doesn’t have the volume change of the specimen. Without the volume change, elasticity under
the pure shear strain is mainly caused from bowing-out deformation of dislocations in slip planes. After
external force is removed, the solid will immediately return to its original size. It indicates deformed
dislocation lines are straitened instantly in this condition. On the other hand, people has observed slow
recovery of the size of the specimen after unloading, which is called anelasticity. When anelastic
phenomenon occurs under pure shear strain, it indicates some dislocations slowly recover its bowing-out
shape because of the existence of weak pinning sites. Since dislocations are known to interact withmagnetic domain walls in ferrous materials, the domain walls can partially act for weak pinning sites foranelasticity.
To prove this mechanism, we will observe anelastic recovering of steel compression coil spring, whose
strain is mainly pure shear state, under various magnetic treatments. The residual elongation cannot be
explained from simple relaxation phenomenon of macroscopic spring dynamics. Since the amount ofanelastic recover is influenced with magnetic treatment during compression condition, magnetic domain
walls can work as weak pinning sites for some dislocations. As temporal change of elongation islogarithmically changed, resultant number of bowing-out dislocations is inversely proportionallydecreased with time during recovering.
At last, we will discuss the mechanism of interaction between dislocation and magnetic domain wall. In
iron, easy-magnetization axes are <100> and 180-degree magnetic domain walls are located along {011}plane. Slip planes of bcc iron are {110} plane, and dislocation lines are perpendicular to {110}. Since[01-1] dislocation line, which is perpendicular to a magnetic domain wall, has no interactions to the wall,
most dislocation lines across the 180-degree wall in this manner. We will discuss the origin of weakpinning sites of dislocation from the distribution of Bloch wall and Bloch line in the domain wall. And wewill show the possibility to understand the interaction from magnetic force caused by swirling flow ofelectron along dislocation line.

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Possible explanation of sudden stress drop at upper yield point in ironby magnetostriction and unpinning dislocations with law of approachin residual magnetization*Shigeo Kotake1
1. Dept. of Mechanical Engineering, Mie Univ.

Unique mechanical properties of iron is one of important resources of engineering in modern technology.
Since elastic deformation is dominant below the yield stress in iron, people can design a machine understrength of materials. Although sudden drop of stress at the upper yield point is one of prominent
phenomena in iron, its mechanism has been a point of controversy in the study of mechanical properties.
On the other hand, Pitman (1990) reported sudden changes in residual magnetization of iron, which shifts
towards an anhysteresis curve after applying mechanical stress. Jiles (1995) and Jiles and Li (2004)
studied the magnetomechanical effect and discovered the existence of the “law of approach” inresidual magnetization, in which the hysteresis converts to an anhysteresis curve under applied stress.
In this study we will try to explain the sudden drop of stress at the upper yield point from magnetostriction
and unpinning dislocation with the sudden change of residual magnetization under the law of approach.
Because of the positive magnetostriction coefficient of iron, specimen will show sudden drop of stressfrom the sudden decrease of residual magnetization, in case the initial magnetization is perpendicular tothe direction of the applied force. Since the decreased magnetization has been kept, the sudden drop ofstress has not been observed just after the first tensile test. Recovery of the phenomenon can be
explained from remagnetization of the specimen during the heat treatment of annealing. It will beremaining study to explain the effect from temperature or strain rate to the upper yield stress.
Moreover, it has been noticed there are pinning effects between dislocation lines and magnetic domain
walls. The law of approach has been explained from unpinning effect of the dislocations from themagnetic domain walls. Therefore, at the upper yield point, unpinned dislocations can move freely untilcontact another pinning site to enhance the decrease of stress. And we will show the possibility tounderstand the pinning interaction from magnetic potential between magnetic domain wall anddislocation line, where swirling electron flows.

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179

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Investigation of dislocation core structure in Aluminum by using ageneralized Peierls-Nabarro model*Hideki MORI1
1. College of Industrial Technology

We investigate the dislocation core structure in Aluminum (Al) by using generalized Peierls-Nabarro (PN)
model.
Same as classical PN model, a generalized PN model expresses the total energy of dislocation core as asum of local misfit energy and non-local elastic energy.
To integrate micromechanics in classical PN model, the generalized PN model can flexibly and efficiently
evaluate the non-local elastic energy [1].
Especially, in isotropic case, the generalized PN model can evaluate the non-local elastic energy with highaccuracy [1,2].
In this work, we calculate the generalized stacking fault energy surface by density functional theory (DFT)
calculation and empirical atomic potential and evaluate dislocation core structure in Al.
Then we discuss the accuracy of generalized PN model by comparing to result of empirical atomic
potential.
We also demonstrate that the generalized PN model is a useful tool to investigate the mesoscopic
dislocation behavior.
[1] H.Wei,Y.Xiang and P.Ming: Commun. Comput. Phys. 4 (2008) 275–293[2] V.V.Bulatov and W.Cai: Computer simulations of dislocations (Oxford,2006)

180

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(P1-16)
Crystal orientation evolution analysis during deformation using
molecular dynamics*Keisuke Kinoshita1
1. Nippon Steel & Sumitomo Metal Corporation

Usually, metallic materials are anisotropic material, for example, in a tensile testing of single crystal, stressand strain relationship depends on tensile direction. Although, when grains in polycrystalline metals havefully random crystal orientation, respectively, anisotropy of the metals is reduced. However, in the actual
metallic materials, the grains don’t have fully random crystal orientation and the grains have been
limited to specific crystal orientations that depends on crystal structures and/or plastic deformation
processes, etc. In other words, the polycrystalline metals have the texture. Mechanical properties of thepolycrystalline metals that have texture is depended on the texture. Therefore, controlling the texture is
one of important problems for improving the mechanical properties of metals. The mechanism of the
texture formation during deformation is rotation of each grain, macroscopically. An elementary step of therotation of the grains is dislocation movement, microscopically. The dislocation movements depend on
the crystal structures and the stress state, etc. In result, the texture depends on the crystal structures, etc.
Although, the dislocation movement is the elementary step of the texture formation, the texture formation
should be reproducible by molecular dynamics. But the simulation result reports are not many.
In this study, we will show molecular dynamics simulation results for the crystal rotation of body-centeredcubic iron during deformation at 300 K. We will show two results. First, an algorithm that is determined
equivalent crystal orientation from atomic configures is described and inspected. Second, crystal rotation
evolution during tensile deformation on molecular dynamics simulation using the algorithm will be shown.
As a result, we will show two results, 1) the crystal rotation of the simulation is corresponded to thetheoretical result, and 2) when multi slip systems are activated, crystal orientation is stable, but whensingle slip system is activated, crystal orientation is changed.

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181

(P1-17)
Nanoindentation of Nanoparticles –A Molecular Dynamics andDiscrete Dislocation Dynamics Simulations Study
Roy Shyamal1, Riccardo Gatti2, Benoit Devincre2, *Dan Mordehai1
1. Mechanical Engineering, Technion - Israel Institute of Technology, Haifa, Israel, 2. LEM, UMR 104, CNRS-ONERA,
29 Av. de la Division Leclerc, 4 Chatillon, France

When decreasing the size of metallic specimens into the sub-micrometer scale, they can drasticallychange their mechanical properties. While plasticity at this scale is commonly studied using compression
tests, nanoindentation can reveal the importance of free surfaces on the depletion of dislocations during
the deformation. In this talk, we present a computational study on how the size and shape of defect-freenanoparticles affect the mechanical response to nanoindentation. Using Molecular Dynamics (MD)simulations and Discrete Dislocation Dynamic (DDD) simulations, we simulate nanoindentation of Au
faceted nanoparticles in a height range of 9-150 nm. The DDD simulations are coupled with a finite
element analysis, in order to reproduce the stress field generated during nanoindentation. Since thenanoparticles are initially pristine (dislocation free), a nucleation criterion for dislocations is introduced in
the DDD simulations. When indenting along a <111> direction, the simulations show that shear
dislocation loops are nucleated beneath the indent on all {111} slip planes. Those shear dislocation loops
interact and form v- and u-shaped dislocations or prismatic loops that glide towards the lower part of thenanoparticles, facilitating their escape from beneath the indent. Two other mechanisms are shown tocontrol the escape of dislocations laterally from the upper part of the nanoparticle. In one, segments ofshear dislocation loops, that are nucleated on the {111} slip plane parallel to the upper surface of thenanoparticle, cross-slip and escape from the upper surface, forming two half prismatic loops that glides
parallel to the upper facet. In another mechanism, dislocations on two different slip planes interact andform a glissile dislocation segment that glides lateraly and escapes the nanoparticles. The effect of size on
the various dislocation mechanisms is discussed.

182

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(P1-18)
Machine learning interatomic potentials for molecular dynamics
simulations of dislocations*Eyal Oren1, Guy Makov1
1. Dept. of Materials Engineering, Ben-Gurion University of the Negev

The mechanical response of crystalline materials is largely controlled by the mobility of dislocations. It is
possible to study the mobility of individual dislocations using molecular dynamics simulations, e.g. [1], but
the validity of these studies depends on the quality of the interatomic potentials employed. In the present
contribution we employ machine learning techniques for the construction of interatomic potentials using
ab-initio data as input, validate the interatomic potential accuracy against experimental and ab-initio
observables and apply to the study of dislocations.
[1] E. Oren, E. Yahel, G. Makov, Dislocation kinematics: a molecular dynamics study in Cu, Model. Simul.
Mater. Sci. Eng. 25 (2017) 25002.

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183

(P2-18)
Prediction of Biaxial Tensile Deformation Behavior of Aluminum Alloy
Sheets using Crystal Plasticity Finite Element Method and Machine
Learning*Kota Koenuma1, Akinori Yamanaka1, Ikumu Watanabe2, Toshihiko Kuwabara1
1. Tokyo University of Agriculture and Technology, 2. National Institute of Materials Science

Aluminum alloy sheets are used for automotive body parts to reduce the weight of automotive. However,
because the formability of aluminum alloy sheets is much lower than that of steel sheets, the plastic
deformation behavior of aluminum alloy sheets during plastic forming processes has been predicted bysheet metal forming simulations based on the finite element method. In order to predict the plastic
deformation behavior of an aluminum alloy sheet subjected to a biaxial stress state in a sheet metal
forming process, we proposed the numerical biaxial tensile test (NBT) methodology using the crystal
plasticity finite element method (CPFEM) and the mathematical homogenization method. The NBT
method has been already applied to predict the contour of equal plastic work of a 5000-series aluminumalloy sheet. The results demonstrated that the contour of equal plastic work (the yield locus) calculated by
the NBT method shows a good agreement with that measured by the experimental biaxial tensile test
using a cruciform specimen. Unfortunately, because we need to perform multiple NBTs to calculate thecontour of equal plastic work, the disadvantage of the NBT method is its high computational cost. In thisstudy, in order to predict the biaxial tensile deformation behavior of aluminum alloy sheets more
efficiently, we propose a new numerical material testing methodology by combining the NBT method withmachine learning methods. The deformation behavior of aluminum alloy sheets predicted by the
proposed method is verified by experimental multi-axitial material tests.

184

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(P2-19)
MobiDiC: A 3-D Dislocation Dynamics Simulation*Ronan MADEC1, Laurent COLOMBET1, Ladislas KUBIN2
1. CEA, DAM, DIF, 2. LEM, UMR 104 CNRS-ONERA, Université Paris Saclay

In 3D lattice-based dislocation dynamics (DD) codes, dislocation segments are discretized on anunderlying lattice in which they move by discrete jumps. The first version of such codes goes by the nameof microMegas and dates back to the beginning of the 1990s [1]. It makes use of a line model in which
dislocations are discretized in their slip planes into a succession of straight segments with edge and screw
characters. Two more sophisticated versions of the initial code are currently in use, Tridis, which retains
the 'screw-edge' line model [2] and an evolved version of microMegas with two additional mixed line
directions [3].
MobiDiC (for 'Mobile Dislocation Colony') derives from this last version of microMegas and is devoted tomass mesoscopic simulations involving a large number of perfect dislocations. It is designed to overcome
some limitations of the parent code in order to provide an improved and efficient framework for DD
simulations that can further evolve.
The specificities of this code will be presented, in particular the ability to handle complex crystallographic
structures and dislocation reactions. Indeed, MobiDiC is considerably more flexible than otherlattice-based DD codes because there is no limitation to the number of vectors per slip system. In
addition a semi-nodal approach is used. For instance, the movements of segments connected to a highconnectivity node, like triple nodes of a junction, are coordinated to move the node with the adequate
degree of freedom.
Examples of applications will be given as well as results obtained with OpenMP and hybrid parallelisms
using many-core processors.
[1] L. P. Kubin, G. Canova, M. Condat, B. Devincre, V. Pontikis and Y. Bréchet, Solid State Phenomena.,
23-24, 455 (1992).
[2] M. Verdier, M. Fivel and I. Groma, Model. Simul. Mater. Sci. Eng. 6, 755-770 (1998).
[3] B. Devincre, R. Madec, G. Monnet, S. Quereau, R. Gatti, L. Kubin, Modeling crystal plasticity withdislocation dynamics simulations: the 'microMegas' code, in Mechanics of Nano-Objects, O. Thomas, A.
Ponchet, S. Forest, (Eds.), Presses des Mines, Paris 2011, pp. 81-89.

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185

(P2-20)
Temperature dependence of fatigue crack growth in Ti-6Al-4V*Bhargavi Rani Anne1, Masaki Tanka1, Tatsuya Morikawa1
1. Dept. of Materials Science and Engineering, Kyushu University

Fatigue properties of dual phase (α+ß and primary α) Ti-6Al-4V were studied with respect to the range oftemperatures and stress intensity range. Fatigue tests were conducted with both high and low stressintensity ranges in the temperature range between room temperature and 550K. Micro crack propagation
was observed. Crack growth rate (da/dN) was measured where a is a crack length and N is a number ofcycles, changing temperature. It was found that da/dN was increased with temperature. It is assumed thatda/dN is the Arrhenius type of equation as it shows temperature dependence. Activation energy
calculations were attained from Arrhenius plot between the logarithm of da/dN and the reciprocal of
temperature. The dependence of activation energy on stress intensity range was also obtained, which
provides the information on the dislocation mobility controlling the fatigue crack growth. The comparisonof fatigue crack growth rate with the temperature dependence of dislocation motion was also studied indetail.

186

MMM 2018

(P2-21)
Parametric model of discrete dislocation dynamics based onmathematical theory of moving curves*Miroslav Kolar1, Jan Kratochvíl1, Petr Pauš1, Michal Beneš1
1. Czech Technical University in Prague

In this contribution we present the comprehensive model of discrete dislocation dynamics based on themathematical theory of moving curves. The purpose of our model is the precise and mathematicallyrigorous description of the dynamics of dislocations, which are represented as smooth curves evolving intheir respective slip planes.
Dislocations are described by parametric curves and their motion is governed by the curvature driven
flow.
The parametric model is coupled with the model of tagential velocity for increased stability, and withalgorithms for topological changes which allow modeling of complex effects as merging, splitting,self-replication or interaction with obstacles.
The numerical algorithm is based on the flowing finite volume method.
We present qualitative and quantitative results of our numerical simulations. We demonstrate thecapabilities of our model in the predicting of dipole formation and consequent estimation of theendurance limit, in the modeling of several interacting dislocations, and in the application of our methodon complex scenarios involving moving dislocations interactiong with several obstacles.

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187

(P2-22)On Visualization of Multiscale Information Transfer/Exchange
Processes via FTMP-based Duality Diagram Representation Scheme*Takuya Takagi1, Tadashi Hasebe1
1. Dept. of Mechanical Engineering, Kobe Univ.

It may safely be said that one of the critical issues about multiscale polycrystalline plasticity modeling are
ultimately consolidated into those about “information transfer and exchange” concurrently taking place
among plural scales of spontaneously evolving kinds. In tackling these, we make an attempt here to
introduce the duality diagram-based scheme of FTMP into multi-grained models under tension, where
FTMP stands for Field Theory of Multiscale Plasticity. The model used is composed of systematic
combinations of representative crystallographic orientations, i.e., three multi-slip [100], [110] and [111],
and two single-slip [123] and [125]. Strongly orientation-dependent intragranular substructure evolutions,
successfully reproduced solely via FTMP-based finite element analyses, result in distinct overall
deformation/fracture modes, including, e.g., local instability-induced brittle-like fracture modes.
Corresponding duality diagrams, i.e., the spatial trace of the incompatibility tensor versus fluctuation of
the elastic strain energy, are drawn and are compared with their rate versions. Found first is a similarity
between the two diagrams, i.e., the normal and their rate forms, when they are constructed via grain-wise
net sum basis, without taking the absolute values as we have done so far. Phase space diagrams are newly
introduced both for the incompatibility and strain energy fluctuation to further examine those similarities
in general. Demonstrated thereby is the models yielding “in-phase” responses roughly correspond tothe cases that exhibit relatively stable and ductile deformation/fracture modes. The phase shifts, on theother hand, are shown to be closely related to instability outset, based on which the construction of a
possible measure of the “degree” of instability is also attempted.

188

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(P2-23)
Rotational Field Evolutions based on Field Theory of Multiscale
Plasticity (FTMP)*Tadashi Hasebe1, Yasutaka Matsubara2
1. Kobe Univ., 2. Graduate School of Kobe Univ.

FTMP-based modeling and simulations have revealed its prominent descriptive capability of
deformation-induced evolving inhomogeneities, e.g., band-like dislocation substructures, which are
generally accompanied by misorientation across them. Three representative cases are examined in detail,i.e., (a)spontaneously evolved dislocation substructures, (b)eigenstrain-driven misorientation development
for modeling lath martensite block structures, and (c)alternative degrees of freedom-driven lattice
rotations enriched by incompatibility-based modified spin. For (a), we discuss deformation-induced
spontaneous evolution of misoriented dislocation substructures, comparing dislocation density contours
for BCC and FCC models, while, for (b), comparison is made of the eigenstrain-driven misoriented lathblock structures among three incompatibility conditions, where screw dislocation networks are commonly
evolved, yielding twisted lath boundaries. For (c), we examine emerging “kink-like” patterns associatedwith tensile twining assisted by incompatibility-based modification of rotation, comparing
component-wise contributions of incompatibility tensor on evolving rotation fields with “kink-like”
morphology for Mg single crystal under c-axis tension. In addition to these,“crystallization-like” patterns
evolved in single crystal sample are extensively examined, where initially-prescribed laminatedmisorientation distributions and/or hardening moduli are introduced to the model used in (c).

MMM 2018

189

(P2-24)
A 2D MESOSCALE STUDY OF DISLOCATIONS, CRACKS AND
MARTENSITIC PHASE TRANSFORMATIONS*Rachel Derby1, Michael Budnitzki1, Stefan Sandfeld1
1. TU Bergakademie Freiberg

TRIP-Steels are of commercial interest due to their exceptional strength. This is a result of the
microstructure; a matrix of both austenite and martensite that when plastically strained, exhibits strain
hardening behavior. Plastic deformation arises from the presence of dislocations and the martensitic
phase transformations (MT) that take place inside the metal. To understand the macroscopic properties of
TRIP-Steels, it is necessary to understand the underlying dynamics occurring at the mesoscale.
Most models do not take into account the interactions between martensitic phase transformations anddislocations, and therefore are missing the impact that these two phenomena exert on each other. While
MD simulations can resolve both phenomena naturally, they are prohibitively expensive for larger crystal
sizes or time scales. On the contrary, continuum models cannot resolve the motion of dislocations inside
the material.
We combine dislocation dynamics and martensitic phase transformations to study the interplay between
the two phenomena. For the dislocation problem we use a continuum dislocation dynamics (CDD) model,
allowing us to reduce computational cost and increase the system size. CDD is coupled with a Phase Field
approach, which we use to model martensitic phase transformations. This allows us to study how
dislocation motion can be influenced in the presence of an MT. Additionally, simple crack geometries canalso be modeled as special dislocation configurations, which helps to understand how cracks and voids
may trigger or inhibit MT.

190

MMM 2018

(P2-25)
Effects of Stress Distribution on the Plastic Deformation of Metallic
Glasses under Different Geometries*Chih-Jen Yeh1, Hsuan-Teh Hu1, Chang-Wei Huang2, Yu-Chieh Lo3
1. National Cheng Kung University, 2. Chung Yuan Christian University, 3. National Chiao Tung University

Metallic glasses (MGs) generally have diverse mechanical properties, such as high strength and poor
ductility. The deformation behaviors of MGs, to a large extent, depend on the ambient conditions. For
example, the deformation of MGs is homogenous at high temperature and low stress. On the other hand,the deformation of MGs is inhomogeneous at low temperature and high stress. However, theinhomogeneous deformation further causes the shear band which is an accumulation of local plastic
deformation. The formation of shear band always leads to a catastrophic failure with the increasing plastic
deformation. In order to study the generation of shear band, a multiscale model which considers the
microscopic shear transformation in macroscopic deformation is inevitable. In this paper, a mesoscalemodel combines the finite element method and the kinetic Monte Carlo method is used to investigate theproperties of the metallic glasses. More mechanical properties and deformation behaviors of the MGs canbe explored through the proposed model. In addition, the geometries of pores in composite MGs areinvestigated. Stress concentration and redistribution due to the different shapes and sizes of pores incomposite MGs are discussed. These results can improve the design of the MGs and prevent MGs fromcatastrophic failure.

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191

Symposium D(D1 invited)
Data-Driven Discovery of new materials*Isao Tanaka1
1. Dept. Materials Science and Engineering, Kyoto Univ.

Challenges for accelerated discovery of materials with the aid of data science have been welldemonstrated. One of the approaches uses materials database generated by density functional theory(DFT) calculations. A large number of DFT calculations with the accuracy comparable to experiments canbe used for high throughput screening (real screening). Another approach called virtual screening uses
machine-learning technique to select predictors for making a model to estimate the target property. The
whole library can then be screened. Verification process is generally required to examine the predictive
power of the model. Models and the quality of the screening can be improved iteratively through Bayesian
optimization. The virtual screening is useful when real screening based upon the DFT data is not practical,
i.e. when the computational cost for the descriptors is too high to cover the whole library within thepractical time frame. This is the same if one needs to explore too large space to cover exhaustively.
Discovery of new low lattice thermal conductivity crystals can be shown as an example of the applicationof the virtual screening technique [1]. Another approach relies only on inorganic crystal structure
database (ICSD) that collects literature data obtained mostly by experiments. We have demonstrated thatmatrix- and tensor-based recommender systems are very powerful for discovery of currently unknown
chemically relevant compositions (CRCs) of inorganic compounds from vast candidates [2]. A Tucker
decomposition recommender system shows the best discovery rate of CRCs. For ternary and quaternarycompositions, approximately 60 and 50 of the top 100 recommended compositions are found to be
CRCs, respectively. The high discovery rate with neither DFT database nor other prior physical/chemical
knowledge should be noteworthy.
[1] A. Seko et al, Phys. Rev. Lett. 115, 205901 (2015).
[2] A. Seko et al, Phys. Rev. Mater. 2, 013805 (2018).

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Machine Learning and Materials Discovery**Gus Hart Hart1
1. Brigham Young University

The relative accuracy and speed of density functional calculations have transformed computational
materials science and enabled the creation of large databases of computed materials properties. But true
"materials by design" or in-silico materials discovery has not yet been realized, though there are isolated
success stories. It seems likely that into make computational discovery of new materials possible, or to
discover materials engineering routes to improve already-deployed materials, a brute force approach will
not be practical---some other paradigm will be required. Machine learning, so successful in some otherapplication areas, is an intriguing and promising idea, but there are hurdles to overcome. There are two
important differences between the standard machine learning problems of image recognition, voicerecognition, etc., and materials prediction. In the first instance, we cannot afford the typical accuracy
tradeoff---materials predictions are not useful without meeting a high accuracy target; the energy
difference of competing phases is often very small, requiring high fidelity in the models. The second
difference is the amount of training data---we don't have "big data''. How do we move forward? In this talkI will review the state of the art in this emerging discipline and show some results from BYU's Materials
Simulation Group efforts in this area.
*This work is supported by ONR (MURI N00014-13-1-0635).

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(D1 oral)
Data-Driven Materials Design in an Industrial Environment*Thomas Eckl1, Lothar Kunz1, Benedikt Ziebarth1
1. Robert Bosch GmbHWe will present how a combination of an automated simulation workflow, including high-throughput DFT
calculations, textmining, utilization of available data bases and machine learning can be used to design
tailored materials with respect to cost efficiency and performance. We will discuss the necessary
requirements from an industrial perspective and show first results in the field of battery materials andpermanent magnets for electric engines. Possible extensions to go beyond the atomistic description andinclude the microstructure of materials are also briefly discussed.

194

MMM 2018

(D2 invited)
Using Machine-Learning to Create Predictive Material Property Models*Chris Wolverton1
1. Northwestern University

Rational, data-driven materials discovery has the potential to make research and development efforts far
faster and cheaper. In such a paradigm, computer models trained to find patterns in massive chemical
datasets would rapidly scan compositions and systematically identify attractive candidates. Here, we
present several examples of our work on developing machine learning (ML) methods capable of creating
predictive models using a diverse range of materials data. As input training data, we demonstrate ML on
both large computational datasets of DFT calculations, as implemented in the Open Quantum Materials
Database (oqmd.org), and also experimental databases of materials properties. We construct ML models
using a large and chemically diverse list of attributes, which we demonstrate can be used as an effective
tool to automatically learn intuitive design rules, predict diverse properties of crystalline and amorphousmaterials, such as formation energy, specific volume, band gap energy, and glass-forming ability, andaccelerate combinatorial searches.

MMM 2018

195

(D2 oral)
Designing mesoscale structures of Li-ion battery electrode using
FIB-SEM image via machine learning*Yoichi Takagishi1, Tatsuya Yamaue1
1. Kobelco Research Institute Inc.

Optimizations of the mesoscale structure of Li-ion battery electrode have been demonstrated by using theadvanced simulation method by single 2D slice image (Quasi-3D modeling) and machine learning. The
mathematical model is based on the electrochemistry and physics model1), and developed in order tocalculate Li/Li+ concentration on the 2D plane, in consideration of virtual 3D structure. In this study, we
firstly confirm the validity of the Quasi-3D model, and secondly optimize the electrode structure inmesoscale using Bayesian optimization, a method of machine learning.
In order to confirm the validity of our proposed model, full 3D discharge simulations with random packed
active material particles have been performed and compared. By use of an appropriate value of“connection factor”, quasi-3D model reproduce well a sliced Li/Li+ concentration calculated by the full
3D model in charge/discharge process, in addition that this model makes it possible to reduce
computation time dramatically. Next, we have carried out optimizations of the mesoscale structure of thepositive electrode Li(Ni1/3Mn1/3Co1/3)O2 based on the actual FIB-SEM image via Bayesian optimization. As
a result, statistical parameters of the optimized meso-scale structures, including the dispersion of active
material size and location, remarkably differ depending on the objective functions for high rate
charge/discharge performance or for long cycle performance.
1) M.Doyle et al. J. Electrochem.Soc. 1996, Vol.143, No.6, p.1890

196

MMM 2018

(D2 oral)
Stability Engineering of Halide Perovskite via Machine Learning*Wan-Jian YIN1, Wanjian YIN
1. Soochow University

Perovskite stability is of the core importance and difficulty in current research and application of
perovskite solar cells. Nevertheless, over the past century, the formability and stability of perovskite still
relied on simplified factor based on human knowledge, such as the commonly used tolerance factor t.
Combining machine learning with first-principles density functional calculations, we proposed a strategy
to firstly calculate the decomposition energies, considered to be closely related to thermodynamic
stability, of 354 kinds halide perovskites, establish the machine learning relationship between
decomposition energy and compositional ionic radius and investigate the stability of 14190 halide double
perovskites. The ML-predicted results lead us to rediscover a series of stable rare earth metal halide
perovskites (up to ~103 kinds), indicating the generalization of this model and further provide elemental
and concentration suggestion for improving the stability of mixed perovskite.

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197

(D2 oral)
Systematic evaluation of ionization potentials of divalent cation binaryoxides*Yoyo Hinuma1,2,3, Hiroyuki Hayashi2,3, Yu Kumagai4, Isao Tanaka2,3, Fumiyasu Oba2,5
1. Center for Frontier Science, Chiba Univ., 2. Center for Materials Research by Information Integration, National
Institute for Materials Science, 3. Department of Materials Science and Engineering, Kyoto Univ., 4. Materials
Research Center for Element Strategy, Tokyo Institute of Technology, 5. Laboratory for Materials and Structures,Institute of Innovative Research, Tokyo Institute of Technology

Finding materials with suitable surface properties is unarguably important for applications including(photo)catalysis and crystal growth. Modeling of surface properties requires identification of surfaces thata given material will preferentially adopt. In a data-driven materials discovery and design approach,reasonable models of surfaces with various orientations and terminations must be obtained with minimum
human intervention. Moreover, the surface energy as well as any relevant surface property, such as theionization potential (IP), needs to be calculated systematically and results have to be stored in a database.
Ultimately, reducing the number of costly first-principles calculations is desirable by using regression orsome other means to estimate a certain surface property and eliminate sampling of the search space thatis highly likely to have an unfavorable surface probability.
In this presentation we use first-principles calculations to systematically investigate the IPs of divalentcation binary oxides. The algorithm to make nonpolar and stoichiometric slab-and-vacuum models byHinuma et al. [1] is used to construct such models. Identification of the drivers that affect the IP, which is afundamental surface property, will be discussed. The insight obtained from this study would assist the
search of descriptors that determine surface properties.
[1] Y. Hinuma, Y. Kumagai, F. Oba, I. Tanaka. Comp. Mater. Sci. 113, 221 (2016).

198

MMM 2018

(D3 invited)
Finding the needle in the haystack: Materials discovery throughhigh-throughput ab initio computing and data mining*Geoffroy Hautier1
1. Universite catholique de Louvain

Essential materials properties can now be assessed through ab initio methods. When coupled with theexponential rise in computational power, this predictive power provides an opportunity for large-scalecomputational searches for new materials. We can now screen thousands of materials by their computed
properties even before the experiments. This computational paradigm allows experimentalists to focus on
the most promising candidates, and enable researchers to efficiently and rapidly explores new chemical
spaces.
In this talk, I will present the challenges as well as opportunities in materials discovery in high-throughput
ab initio computing using examples from transparent conducting materials. I will especially highlightcomputational predictions which have been followed by experimental synthesis and characterization. In
addition to allowing the ability to navigate through a large volume of materials data to identify promising
compounds, high-throughput computing also offers unprecedented data mining opportunities to detect
new relationships between chemistry, structures, and properties. I will illustrate examples of these
relationships through our recent work in crystal structure descriptors and automatic local environment
identification, which merge traditional solid-state chemistry and materials science concepts throughmodern informatics.
The impact of high-throughput computing is multiplied when the generated data is shared with free andeasy access. I will finish my talk by presenting the Materials Project (http://www.materialproject.org), a
collaborative project which precisely targets such a data dissemination.

MMM 2018

199

(D3 oral)
High-entropy alloys investigation using machine-learned potentials*Tatiana Kostiuchenko1, Alexander Shapeev1
1. Dept. of Material Science and Engeneering , Skolkovo Institute of Science and Technology, Moscow

High-entropy alloys (HEAs) are a class of materials promising for their potential durability and highheat-resistance. HEAs are defined as alloys consisting of at least four different components that form
single-phase solid solutions due to high entropy of mixing.
It is rather difficult to find which components would form a HEA since solid solutions alloys tend to split
into binary or mono-atomic alloys at low temperatures and can form metallic glasses at high temperatures.
This makes it difficult to experimentally investigate HEAs. Hence there is a need in computational
approaches to the design of HEAs.
In this work we propose a computational framework for predicting the temperature of the order-disorder(i.e., intermetallic-solid solution) transition for HEAs. We first construct a very computationally efficientmachine-learning on-lattice model of interatomic interaction (an alternative to the cluster expansion
model) [Shapeev A., 2017]. The model parameters are fitted to quantum-mechanical (DFT) data withaccuracy of about 1meV/atom. Then we perform canonical Monte-Carlo simulations for b.c.c.
(body-centered cubic), h.c.p. (hexagonal close-packed) and f.c.c. (face-centered cubic) lattices. We
validate our results against the published works [Körmann F. et al, 2017][Huhn W.P. et al, 2013][Ferná
ndez-Caballero A. et al, 2017]. Particular approach in atomistic simulations significantly reduces
calculation time, enables to increase explored atomic configurations range and preserves the accuracy
level in comparison with ab initio calculations. In the case of successful development of the method of
constructing phase diagrams using machine learning, human labor can be excluded from the routine
process of studying phase diagrams. This will also reduce the computational costs consumed byconventional calculations from the first principles.

200

MMM 2018

(D3 oral)
Stability evaluation of high-entropy alloys via accurate on-lattice
model*Evgenii Meshkov1, Ivan Novoselov1, Alexander Shapeev2, Alexey Yanilkin1
1. All-Russian Research Institute of Automatics (VNIIA), 2. Skolkovo Institute of Science and Technology

High-entropy alloys (HEA) are a new promising class of metallic materials. Some of them demonstrate
excellent mechanical properties at elevated temperatures, outstanding corrosion resistance, and highradiation tolerance. However, atomistic modeling of HEAs is hindered by their high chemical diversity. A
novel approach is required to perform accurate and computationally feasible atomic-scale description of
HEA.
In our approach, energy of the system is represented as a sum of contributions of local atomic
environments. These contributions are parameterized by low-rank multidimensional tensors in order toreproduce DFT results [1]. This model is then embedded into kinetic Monte-Carlo to perform large-scale
simulations. This approach gives an accurate description of alloys with a large number of elements.
We employ the proposed approach to evaluate thermodynamic stability of several multicomponent alloys,
and to search for new single-phase compositions.
[1] Shapeev, A. (2017) Computational Materials Science, 139, 26-30

MMM 2018

201

(D3 oral)
Atomistically informed atomic mobility databases for continuum
diffusion simulations*Katrin Abrahams1, Daniel Gaertner2, Maximilian Grabowski3, Irina Roslyakova1, Oleg Shchyglo1,
Sergiy V. Divinski2, Ingo Steinbach1
1. Scalebridging Thermodynamic and Kinetic Simulation (ICAMS), Ruhr-University Bochum, 2. Institute of Materials
Physics, University of Münster, 3. Atomistic Modelling and Simulation (ICAMS), Ruhr-University Bochum

Diffusion is a key aspect for microstructural evolution and has to be solved in full-field models like thephase-field model. Many phase transformations are diffusion controlled that their kinetics depend
crucially on mobilities of the diffusing elements as well as their thermodynamic factor. Both parameters,
the Gibbs energies and the atomic mobilities are temperature, composition and pressure dependent.
Parameters representing these dependencies are stored in CALPHAD (CALculated PHAse Diagrams) typedatabases.
In this talk a new approach for the assessment of the temperature and composition dependence of theatomic mobility data using atomistic and experimental data is presented. The new model takes into
account the physical meaning of three parameters on the basis of a mono-vacancy diffusion mechanism:
the frequency factor, the vacancy formation energy and the migration energy. The temperaturedependence is given by an Arrhenius equation, where the activation energy consists of the migrationenergy and the vacancy formation energy. The temperature dependence of both parameters isinvestigated using Kinetic Monte Carlo simulations. This information is used to deduce a generaldescription of the temperature dependence of the activation energy based on these parameters.
Additionally, the composition dependence of the pre-exponential factor and the activation energy isinvestigated separately using experimentally determined self-diffusion coefficients obtained from thetracer method and calculated self-diffusion coefficients based on kinetic Monte-Carlo simulations.

202

MMM 2018

(D3 oral)
The European Materials Modelling Council: standardization,interoperability and data management tools for materials modelling*Luca Bergamasco1, Emanuele Ghedini2, Gerhard Goldbeck3, Eliodoro Chiavazzo1, Pietro Asinari
1

1. Politecnico di Torino, 2. Università di Bologna, 3. Goldbeck Consulting Ltd

The European Materials Modelling Council (EMMC) was established in 2014 as a new European network
to coordinate and support the industrial uptake of materials modelling in Europe. The EMMC community
consists of different types of stakeholders, namely: manufacturers (industrial end-users); software owners(academic and commercial software providers); Translators (skilled professionals able to translateindustrial problems into modeling activities) and academic model developers (of electronic, atomistic,
mesoscopic and continuum models). The main EMMC objectives are: to enhance the interaction andcollaboration between all stakeholders; to identify materials model gaps in industry and to steer model
development; to build strong foundations to ease model interoperability, integration and data
management; to promote modelling in industry based on beneficial economic arguments. These activities
rely on wide stakeholder consultation, with the ultimate goal to integrate materials modelling into the
product life-cycle management, enhancing innovation and global competitiveness.
The aim of this talk is to present the EMMC activities, with particular regards to those related tostandardization, interoperability and data management strategies for materials modelling. First, theReview of Materials Modelling (RoMM), edited by the European Commission, will be presented as thereference document for standardization. Based on this preamble, the CEN Workshop Agreement (CWA)
for materials modelling terminology, the Modelling Data (MODA) sheets to document modelling instandard form, and the European Materials Modelling Ontology (EMMO) will be presented. Particularemphasis will be given to the discussion of physics-based vs data-based modelling approaches, showing
how they fit into the abovementioned tools. Available results on the data-management regulation will bealso shown and discussed. Finally, an outlook on how this framework will help materials discovery anddesign will be given.
This project has received funding from the EU H2020 project EMMC-CSA H2020-NMPB-2016-2017 GA
n. 723867

MMM 2018

203

(D4 invited)
Exploration of large ab initio data spaces to design structural materials
with superior mechanical properties*Joerg Neugebauer1, Jan Janssen1, Tilmann Hickel1, Blazej Gabowski1
1. Max-Planck-Institut fuer EisenforschungModern engineering materials have evolved from simple single phase materials to nano-composites that
employ dynamic mechanisms down to the atomistic scale. The structural and thermodynamic complexityof this new generation of structural materials presents a challenge to their design since experimental
trial-and-error approaches as successfully used in the past are often no longer feasible. Ab initio
approaches provide perfect tools to new design routes but face serious challenges: Finite temperature
free energies of the various phases are almost degenerate, requiring advanced theoretical formalisms that
accurately capture all relevant entropic contributions. In addition, their hierarchical nature with respect tolength and time makes it challenging to explore the large range of chemical compositions. We havetherefore developed a python based framework pyiron that allows in a highly automated way to combine
accurate finite temperature first principles calculations with big data analytics. The flexibility and thepredictive power of these automated approaches will be discussed for examples ranging from the designof ductile Mg alloys to the discovery of general rules for interstitials in metals.

204

MMM 2018

(D4 oral)
Toward a machine learning aided interatomic potential for
multi-element alloys: Application to binary compounds*Doyl Dickel1, David Francis1, Christopher Barrett1
1. Mississippi State University

As the chemical complexity of novel material systems continues to increase, the need for the rapid
development of predictive, scalable interatomic potentials has grown as well. Machine learning, neuralnetworks, and other data-driven techniques has shown promise in condensing large amounts of data fromfirst principles and density functional theory calculation into classical dynamical equations with linear
scaling, making them ideal for molecular dynamics simulation. A formalism for the development of amulti-species interatomic potential is presented and applied to binary metals. The resulting potential isthen analyzed in terms of predictive power, validity, performance, and development time.

MMM 2018

205

(D4 oral)
Machine Learning potentials for modeling iradiation defects in ironand tungsten*Alexandra Goryaeva1, Jean-Bernard Maillet2, Mihai-Cosmin Marinica1
1. DEN-Service de Recherches de Métallurgie Physique, CEA, Université Paris-Saclay, 91191 Gif-sur-Yvette, 2.
CEA-DAM DIF, 91297 Arpajon Cedex

Prediction of condensed matter properties requires an accurate description of a material at the atomic
scale. Ground state properties of a material are often described well within the Density Functional Theory(DFT) while studying irradiation-induced damage requires a length scale that is pushed beyond ab initio
level of theory. Unachievable CPU cost of such calculations have fueled the search for alternatives,
accounting for reasonable approximations, which has led to development of various empirical potentials,
ranging from pairwise potentials to embedded atom model and tight binding. Although these potentials
have been successful in making radiation damage feasible, inconsistency of the results from different
potentials is a major shortcoming that hinders conclusive theoretical predictions for such importantfunctional materials as Fe and W.
Here we present a new strategy to achieve machine learning interatomic potentials for metals thatapproach accuracy of DFT calculations and at the same time preserve a reasonable balance between
precision and CPU cost. Targeting to model irradiation-induced defects and plasticity, the potentials aretrained on the extensive DFT database that includes EOS, elastic deformation, planar defects (GSF),
self-interstitial atoms (SIA), vacancies, and liquid state.
The new potentials for Fe and W are applied to investigate the complex energy landscape of defects
under irradiation such as clusters of SIA. We aim to predict the relative stability of large SIA clusters up tonanometric-size, with a particular focus on to the relative stability of the conventional dislocation loops aswell as the C15 clusters [1]. The present approach enables us to account for the effect of temperature [2].
Moreover, as a perspective development, the potential will be tested to reproduce high-pressure bcc-hcp
transition in Fe.
[1] M.C. Marinica et al. (2012) Phys Rev Lett 108, 025501[2] T.D. Swinburne, M.C. Marinica (2018) Phys Rev Lett 120, 135503

206

MMM 2018

(D4 oral)
Effect of friction and ductility on relaxation dynamics and mechanicalmemory of crumpled materials*Mehdi Habibi1, Eric van Bruggen1
1. Wageningen University

Crumpled structures are ubiquitous in nature and technology. They combine low-density structures withsurprising mechanical strength and theability to absorb mechanical energy. This combination of properties opens doors
to use them as mechanical metamaterials for a variety of applications.
However, in order to rationally design metamaterials, a thorough physical understanding of their uniquefeatures is needed. One remarkable physical property observed in crumpled structures is their slow
mechanical relaxationand their ability to carry a long lasting memory of previous mechanical states.
We experimentally investigate the role of material properties (ductility and friction) on relaxation dynamics
of crumpled sheets bycomparing relaxation curves of different materials. We show that relaxation rates are not only dependent
on material’s elastoplastic properties, but also rely on friction and adhesion between surfaces.This isfurther explored by using a two-step compaction protocol, that allowed us to probe deeper into thematerial’s relaxation behavior. We study the material dependency of the non-monotonic aging in a
crumpled ball.
We show the normalized height of the non-monotonic aging peak depends
linearly on the time at which it arose with a slope that revealed a material property that
seemed to be correlated with the material dependent relaxation constant.

MMM 2018

207

(D4 oral)
Big-data insights into solute-GB segregation*Liam Huber1, Blazej Grabowski1, Joerg Neugebauer1
1. MPIE

Solute-grain boundary (GB) interaction plays a critical role in the evolution and stabilization of grain
structure and thus strongly impacts final material properties. Due to the large number of possible grainboundary configurations, there are a many inequivalent sites solutes can be incorporated at. Combinedwith the fact that there are many possible segregating chemical species in modern alloys, concepts toderive general trends in segregation are still missing. Using classical molecular statics, we perform
high-throughput calculations of 1.4 million segregation energies for six solutes to 38 different boundaries
in Al. The size of this data set is sufficient to apply machine learning techniques for building predictivemodels capable of predicting segregation energy to new GBs. It also provides useful insights into trends inthe atomistic mechanisms controlling segregation behaviour. We show that the resulting segregationenergy distributions can be interpreted analogously to electronic density of states and provides a usefulperspective to consider solute concentration enrichment at the GB and GB embrittlement.

208

MMM 2018

(D5 invited)
Computational exploration of strong permanent magnet compounds*Takashi Miyake1,2,3
1. CD-FMat, AIST, 2. ESICMM, NIMS, 3. CMI^2, NIMS

Modern strong magnets are rare-earth magnets in which high saturation magnetization and high Curie
temperature come from transition-metal 3d electrons, and strong magnetorcystalline anisotropy
originates from rare-earth 4f electrons [1]. There are various types of crystal structures and chemicalcomposition, and exploration of a new magnet compound is a hot topic. Among them, RFe12-type
compounds with the ThMn12 structure are attracting renewed interest because of their high iron content.
Recently synthesized NdFe12Nx film has higher saturation magnetization and anisotropy field than Nd2Fe14
B, although its bulk phase is thermodynamically unstable. I will present a first-principles study on theeffect of element substitution on magnetism and structural stability. I will also discuss how machinelearning accelerates magnetic-materials discovery. Application to the Curie temperature of RFe12-type
compounds shows that Bayesian optimization offers an efficient way to optimize chemical composition ofmagnet compounds. Kernel ridge regression using orbital-field matrix as a descriptor reproduces themagnetic moment and formation energy of thousands of transition-metal compounds in reasonableaccuracy [2], which can be utilized for virtual screening of new magnetic compounds. Bayesian
optimization approach to crystal structure prediction is also presented [3].
[1] Takashi Miyake and Hisazumi Akai, J. Phys. Soc. Jpn. 87, 041009 (2018).
[2] T.L. Pham et al., Sci. Tech. Adv. Mater. 18, 756 (2017).
[3] T. Yamashita et al., Phys. Rev. Mater. 2, 013803 (2018).

MMM 2018

209

(D5 oral)
A machine-learning approach for finding new hard-magnetic phases
Johannes J. Möller1, *Daniel F. Urban1, Christian Elsässer1
1. Fraunhofer IWM, Freiburg

Data-mining and machine-learning (ML) techniques play an increasingly important role in the discovery
and development of new materials. In this contribution, we use kernel-based learning methods to predict
optimal chemical compositions for new permanent magnets, which are key components in many
green-energy technologies. The magnetic-property data used for training and testing the ML models wereobtained by a combinatorial high-throughput screening (HTS) using density-functional theory
calculations. For encoding the structural and chemical information of the HTS data in a machine-readable
format, we use several existing and newly developed material descriptors and assess the predictive powerof the ML models built with them. The accuracy of the ML models with an optimal choice of descriptor
and model parameters enables the prediction of promising structurecomposition combinations for
substitutes of state-of-the-art magnetic materials like Nd2Fe14B - with similar intrinsic hard-magnetic
properties but no or less amounts of critical rare-earth elements.

210

MMM 2018

(D5 oral)
High-throughput optimization of finite temperature phase stabilities of
Ce-based hard magnetic materials*Tilmann Hickel1, Halil Soezen1, Fritz Koermann1, Joerg Neugebauer1
1. Max Planck Institut fur Eisenforschung

While the present search for novel materials concepts to improve hard magnetic applications isconcentrated on the magnetic properties of individual phases, their finite temperature phase stabilities
are largely unknown. In the Ce-Fe-Ti ternary system, for example, the promising phase CeFe11Ti iscompeting with various Laves phases as demonstrated by energy-dispersive X-ray spectroscopy (EDS)
measurements. We therefore use concepts of ab initio thermodynamics capturing vibrational, electronic
and magnetic entropy contributions in order to determine the Helmholtz free energy of all relevant
phases. Several additional elements have been considered within this formalism, to investigate their
partitioning and impact on the relative phase stabilities. The insights obtained demonstrate how thescreening of a complete set of transition metals in quarternary hard magnetic materials can be performed
most efficiently and which alloying elements improve the stability of promising phases such as CeFe11Ti
most substantially.

MMM 2018

211

(D5 oral)
Understanding pairwise magnetic interactions in Fe-based materials
with machine learning techniques*Osamu Waseda1, Omkar Hegde1, Tilmann Hickel1, Jörg Neugebauer1
1. MPIE

Magnetic interactions are crucial to the stability of structural phases as well as for various thermophysicaleffects such as magnetocalorics. Despite their importance, there is no experimental procedure whichallows for the understanding of magnetic interactions at the atomic level, and there is no exact theoreticalmodel capable of describing them precisely except for expensive ab initio methods. It has been recently
suggested that the Heisenberg Landau model has sufficient versatility to map the contribution of complex
magnetic interactions of Fe-based materials to the free energy. Its original form, however, contains a highnumber of parameters which make it prone to overfitting. In this study, we mapped the magnetic
interactions created from spin-polarised DFT calculations to extended Heisenberg-Landau models via
various machine learning regression techniques. The free energy contribution of the magnetic interactions
is then determined through Monte Carlo simulations for millions of atoms which would otherwise not be
achievable with ab initio methods. The results enable us to understand the overall effects of impurities
contained within microstructures on the magnetism in iron based materials.

212

MMM 2018

(D6 invited)
Novel two-dimensional materials: Materials discovery, data
provenance, and workflow reproducibility.
*Nicola Marzari1
1. EPFL

Two-dimensional materials have seen in the past decade very dedicated experimental and theoreticalefforts in the quest for novel physics and functionalities. Here, we systematically explore with
first-principles calculations known inorganic materials, to identify those that could be exfoliated into
two-dimensional layers. We start by curating experimental materials databases, collecting reliable data for
110,000 unique compounds. Then, we identify those that appear layered according to simple geometricand bonding criteria, and launch high-throughput calculations - based on van-der-Waals
density-functional theory and 2D density-functional perturbation theory - to characterize binding
energies, stability, and properties. Remarkably, we find close to 2000 inorganic compounds that could be
exfoliated into novel two-dimensional materials, and recover in the process the known ones - fromgraphene to transition-metal dichalcogenides to boron nitride and black phosphorus. I’ll provide aperspective on the promising properties we are uncovering - topological, electrical, magnetic, chemical while highlighting the need for scalable approaches to data and calculations, able to persist and query thefull provenance of the data and ensure reproducibility of the calculations’ workflows.

MMM 2018

213

(D6 oral)
In silico screening of Metal-Organic Frameworks for adsorption driven
heat pumps and chillers*Mate Erdos1, Martijn F. de Lange1, Freek Kapteijn2, Othonas A. Moultos1, Thijs J. H. Vlugt1
1. Engineering Thermodynamics, Process & Energy Department, Faculty of Mechanical, Maritime and Materials
Engineering, Delft University of Technology, 2. Catalysis Engineering, Chemical Engineering Department, Delft
University of Technology

20% of the total energy demand is attributed to hot water production and space heating/cooling ofbuildings. Although, it is intended to produce more and more energy from clean low carbon sources forheating/cooling, 75% of it is still acquired by using fossil fuels. To ameliorate the dependence on fossil
fuels and increase energy efficiency, several technologies have emerged recently, utilizing renewableenergy sources for heating/cooling applications. Among these, adsorption driven heat pumps and coolers(AHP/ACs) are promising candidates because these devices can utilize thermal energy instead ofelectricity. However, a drawback of AHP/ACs systems is the lower performance compared to heat pumps
based on absorption or chemical reactions. One possible way to improve the energy efficiency of
AHP/ACs is by finding adsorbents with enhanced adsorption and thermophysical properties.
Metal-Organic Frameworks in conjunction with alcohols (methanol, ethanol) are promising working pairs
for this application.
A computational screening of MOFs is carried out to find the best performing structures for AC
applications with methanol and ethanol as working fluids. Molecular simulations using Monte Carlo
method are conducted to investigate 2930 structures. An efficient screening methodology is devised with
4 subsequent screening steps. The commensurate adsorption behavior of the adsorbates is exploited toreduce computational effort. In the last screening step, the 6 best performing MOFs with high deliverable
working capacities (∼0.6 ml working fluid in 1 ml structure) and diverse adsorption step locations are
selected from the original 2930 structures for each adsorbate. The finally selected structures show higherdeliverable working capacities than the reported highest values (∼0.45 ml working fluid in 1 ml structure).

214

MMM 2018

(D6 oral)
Machine learning assisted by first-principles calculations for designingintermetallic-typed metallic glasses*Tokuteru Uesugi1, Manami Sakai 1, Yorinobu Takigawa1, Kenji Higashi1
1. Dep. of Materials Science, Osaka Prefecture Univ.

Metallic glasses have interesting properties such as low Young’s moduli, high corrosion resistance andhigh wear resistance and are considered new materials in many fields. The factors that contribute to theglass-forming ability (GFA) are an important consideration in the design of new metallic glasses. Ternary
metallic glasses are classified into three types on the basis of their atomic size distribution. The factorsthat affect the GFAs of the metallic glasses are presumed to vary among the three types. However, metallic
glasses with two alloying elements exhibit irregular composition ratios or atomic radii; thus, in thesecases, the contributing factor is unclear. The binary metallic glasses usually crystallize only intermetallic
crystalline phase after heating. For example, Ti-Cu and Ti-Ni binary metallic glasses crystallized as only
TiCu and Ti2Ni intermetallic compounds. Therefore, these metallic glasses such as Ti-Cu and Ti-Ni areable to be called as intermetallic-typed metallic glasses (IMG). In this work, we aimed to elucidate thedesign criteria of IMG and design new metallic glasses based on the machine learning assisted by the
first-principles calculations. First, the first-principles calculations were performed on intermetallic
compounds and then, explanatory variables were calculated. Second, a regression systems of GFA wasconstructed using artificial neural networks (ANN) and logistic regression analysis (LRA). The results of
ANN validation showed very high accuracy than the results of LRA validation. The partial regression
coefficients of LRA indicated the design criteria of intermetallic-typed metallic glasses. The obtained
regression systems were applied to intermetallic compounds whose GFA are unknown, and the alloy
systems of a novel metallic glass were predicted.

MMM 2018

215

(P2-26)
Generalized nano-thermodynamic model for predicting
size-dependent surface segregation in multi-metal alloy nanoparticles
from smaller particles*Abhijit Chatterjee1
1. Dept. of Chemical Engg., Indian Institute of Technology Bombay

Multi-metal alloy nanoparticles (NPs) offer new avenues for exploration and design ofnanoscale-properties, e.g., catalytic, electronic and optical, by virtue of their tunable composition.
Unfortunately, a method that can aid such exploration by accurately predicting the size-, shape- andcomposition-dependent elemental distribution associated with nanomaterials is crucially missing. A
nano-thermodynamic model based on distribution coefficients Δis introduced to fill this gap. Δisemployed to predict surface segregation in NPs as a function of the NP size and composition.
Interestingly, we find Δto be independent of size for NPs beyond 2 nm. This key finding motivates theconstruction of thermodynamic tables for distribution coefficients using segregation observed with one ormore NP sizes. The tables can enable accurate prediction of phase diagrams for nanomaterials across awide-range of sizes. Key concepts of this new theory are demonstrated with Au-Pt-Pd, Ag-Au-Pd and
Ni-Pt-Pd, which are found to exhibit complex size-dependent segregation behavior for 2-6 nm NPs andrelatively weaker size-dependence beyond 6 nm. Numerically well-converged values of Δare calculated
for small NPs using Monte Carlo simulations in the canonical ensemble. Simulations are based on anembedded atom method (EAM) potential for metal alloys.
[1] S Divi, A Chatterjee, RSC Advances 8, 10409, 2018.

216

MMM 2018

(P2-27)
Development of thermodynamic database of Nd-Fe-B-based
permanent magnet alloys*Jiang Wang1, Lei Xu1, Kaicheng Yang1, Qingrong Yao1, Guanghui Rao1, Huaiying Zhou1
1. School of Material Science and Engineering, Guilin University of Electronic Technology

Nd-Fe-B permanent magnets with excellent magnetic properties have been used in the industrial
applications including medical apparatus and instruments, electrical machinery, aerospace, permanent
magnet motor and wind power. In order to fully balance the application of rare-earth resource in magnetic
materials and reduce the costs, it is a promise way to introduce high abundant rare earth (RE) metals (e.g.
La, Ce, Y) into Nd-Fe-B permanent magnets [1-3]. Phase diagrams and thermodynamic information of the
RE-Fe-B alloys are necessary to understand the effect of the abundant rare earth metals on phaseformation, microstructure and magnetic properties of Nd-Fe-B permanent magnets.
In this work, the RE-Fe, RE-B and Fe-B sub-binary systems were reviewed firstly in the published literature.
After that, the RE-Fe-B (RE=La, Ce, Pr, Nd) ternary systems were assessed using CALPHAD method on thebasis of thermodynamic data and phase equilibria data, which is fundamental to obtain thethermodynamic database of multi-component RE-Fe-B alloy systems. The calculated results including
liquid projects, isothermal sections and vertical sections as well as the solidification path of some alloys
were compared with the experimental results using the thermodynamic database obtained. The
thermodynamic database of the Nd-Fe-B-based permanent magnet alloy systems with high abundant rareearth elements is developed finally, which is necessary to design alloy composition and heat treatments ofnovel Nd-Fe-B permanent magnets with good magnetic properties and low costs.
References[1] X.J. Cao, L. Chen, S. Guo, J.H. Di, G.F. Ding, R.J. Chen, A.R. Yan, K.Z. Chen, Aip Adv. 8 (2018) 056222.
[2] L. Liu, H. Sepehri-Amin, T. Ohkubo, M. Yano, A. Kato, T. Shoji, K. Hono, J. Alloy. Comp. 666 (2016)432-439.
[3] J.Y. Jin, T.Y. Ma, M. Yan, Y.J. Zhang, B.X. Peng, Y.S. Liu, F.H. Cao, J. Alloy. Comp. 735 (2018)2225-2235.
Acknowledgements
This work was supported financially by National Key Research and Development Program of China(2016YFB0700901), National Basic Foundation of China (2014CB643703), National Natural Science
Foundation of China (51761008, 51461013), Guangxi Natural Science Foundation(2016GXNSFDA380015, 2016GXNSFGA380001) and Guangxi Project of Science and Technology(2017AD23031).

MMM 2018

217

(P2-28)
Design of proteins and biopolymers: role of directional interactions
and of water.
*Valentino Bianco1, Ivan Coluzza2
1. University of Vienna, 2. CIC biomaGUNE, center for cooperative research in biomaterials

The design of a (bio-)polymer is a rational scheme allowing to transfer the one-dimensional information
contained in the polymer sequence into the three-dimensional information contained in the polymer
folded conformation.
Proteins are an example of designable heteropolymers able to fold in unique target structures. The
stability of the native conformation depends on the protein sequence and on the thermodynamic
conditions of temperature and pressure.
In our work we use a multiscale approach to investigate how the geometry of the polymer backbone andthe properties of the surrounding water affect the selection and the stability of proteins and, more in
general, of artificial heteropolymers.
We find that the key actors are: i) the directional interactions along the backbone and ii) thehydrophilic/hydrophobic composition of the surface and core of the folded structure.
References:
V. Bianco, G. Franzese, C. Dellago, I. Coluzza, Phys. Rev. X 7, 021047, 2017.
V. Bianco, N. Pagès-Gelabert, I. Coluzza, G. Franzese, J. Mol. Liq. 245, 129, 2017.
C. Cardelli, V. Bianco, L. Rovigatti, F. Nerattini, L. Tubiana, C. Dellago, I. Coluzza, Sci. Rep. 7, 4986, 2017.
V. Bianco, G. Franzese, Phys. Rev. Lett. 115, 108101, 2015.
V. Bianco, S. Iskrov, G. Franzese, J. Biol. Phys. 38, 27, 2012.

218

MMM 2018

(P2-29)
Aquatic pollutant removal by adsorption in zeolite structures: An
experimental and molecular simulation study*Mate Erdos1, Nan Jiang2, Sebastian G. J. Heijman2, Othonas A. Moultos1
1. Process & Energy Department, Faculty of Mechanical, Maritime and Materials Engineering, Delft University ofTechnology, 2. Department of Water Management, Faculty of Civil Engineering and Geosciences, Delft University ofTechnology

Water contamination with micropollutants poses a serious threat to public health and the ecosystem.
Technologies based on adsorption are widely used to remove micropollutants (inorganic and organic)from wastewater. Zeolites show a promising potential as adsorbents in these applications. Zeolites are
crystalline, microporous aluminosilicates with well-defined 3-dimensional structure, composed oftetrahedral SiO4 and AlO4 clusters connected to each other by shared oxygen atoms. To compensate thecharge imbalance caused by the aluminum content of the framework, exchangeable cations (usually alkali
and alkaline earth cations) are located in the cavities of the structure. By removing the aluminum content
of the framework the hydrophobicity of the zeolite can be increased, providing favorable adsorption
characteristic to organic molecules. In this study, the effect of aluminium content of zeolite structures foraquatic pollutant removal are investigated. To that end, molecular simulations using Monte Carlo method
are performed. In comparison with experimental methods, these simulation techniques can provide
fundamental understanding of the nano scale behavior of the system which is crucial for designing newmaterials.
In this study, two types of zeolites (FAU, BEA) with different aquatic pollutants (2,4,6-trichlorophenol,
triclosan) are investigated experimentally and with simulations. The simulated and experimentallymeasured results show qualitative agreement. To obtain insights into the adsorption mechanisms, radial
distribution functions, and the distribution of adsorbates are calculated for each structure. The preferredadsorption sites and configuration of adsorbates (e.g., pi-pi stacking, H-bonding) are identified for eachadsorbates and structures. Based on the simulation and experimental results the relationship ofaluminium content and adsorption affinity can be determined and the performance of the different
zeolites can be evaluated.

MMM 2018

219

(P2-30)
Optimizing elastic moduli of the silicate glasses throughhigh-throughput atomistic modeling and machine learning techniques
Yong-Jie Hu1, Ge Zhao2, Tyler Del Rose1, *Liang Qi1
1. Dept. of Materials Science and Engineering, University of Michigan, 2. Department of Statistics, Pennsylvania State
University

Chemical design of the silicate glass with high elastic moduli is of great interest. However, it is difficult tofind a universal expression to predict the elastic moduli according to the glass composition before
synthesis since the elastic moduli are a complex function of several material properties at different lengthscales. This work presents a computational framework to efficiently predict the elastic moduli of thesilicate glass across a multicomponent compositional design space, including 11 types of additive oxides,
by integrations of high-throughput molecular dynamic (MD) calculations and machine learning (ML)techniques. Our newly developed ML model can predict the elastic moduli for k-nary silicate glass
systems, using the learning datasets generated from MD calculations for only binary and ternary systems.
The usefulness of our model is illustrated by identifying the most relevant materials descriptors that
determine the elastic moduli and screening for the silicate glasses with high stiffness.

220

MMM 2018

(P2-31)High pressure phase transition and structural stability of transition
metal compounds*Fanyan Meng1, Wandong Xing1,2, Rong Yu2
1. Dept. of Physics, University of Science and Technology Beijing, 2. School of Materials Science and Engineering,
Tsinghua University

The lattice, charge, spin and other degrees of freedom in transition metal compounds couple to one
another, giving these materials rich physics and properties. The extreme conditions of high pressure andhigh temperature provide a new way to create new structures that do not exist at ambient pressure, which
has become an important way for the discovery of novel transition metal compounds. In recent years,
important progress has been made in the theoretical prediction and high pressure synthesis of new
structures. First, this work suggests a lot of new materials to be investigated by changing stoichiometry inphase diagrams. Taking the V-C binary system as an example, here we report the first-principles
prediction of a new type of vanadium carbide, V5C3, which has an unprecedented stoichiometry in the V-C
system. It is demonstrated that the new phase is mechanically stable, and is energetically favorable than
known phases under high pressures. We believe that this work opens a door to materials design bychanging stoichiometry. And the relationship between the crystal structure, electronic structure andphysical properties are discussed. Secondly, the phase transition under high pressure will be analyzed tounderstand the structural stability of materials at high temperatures and high pressures, providing
important theoretical basis for optimizing high-pressure synthesis conditions.

MMM 2018

221

(P2-32)
Development of artificial neural network model for prediction ofelectronic density of states in atomistic systems*Atsushi Kubo1, Yoshitaka Umeno1
1. Institute of Industrial Science, the University of Tokyo

Recently, the artificial neural network (ANN) model has been intensively applied to interatomic potentials
for atomistic simulations. ANN-based potential functions possess basic characteristics; i.e., (1) they do nothave any physical background besides the least geometrical conditions, e.g., physical quantity
conservation against any coordinate transformation; (2) theoretically they can mimic any continuousfunctions. These features make ANN potentials applicable to complex atomistic systems, where various
crystal structures and phases are relevant.
Since an ANN potential can be simply regarded as a mapping from atomistic structure to a real number, it
is found that the application of ANNs does not have to be limited only for prediction of potential energy;i.e., ANNs are applicable to prediction of other physical quantities or material properties of the atomistic
systems, e.g., electronic density of states (DoS), magnetic moment, etc. However, to the best of ourknowledge, there has been no attempt to apply ANNs to prediction of physical quantities beside potential
energy in atomistic systems. It will be of great impact if we can evaluate physical quantities such as
electric or magnetic properties in huge atomistic structures using ANNs with the accuracy of thefirst-principles calculation.
In this study, we developed an ANN model to predict the DoS, as an exemplified case of prediction ofgeneral physical quantities in atomistic systems with ANNs. The ANN was optimized to reproduce the DoS
in various crystal structures of silicon-carbon system that was obtained by first-principles calculations
based on the density functional theory.

222

MMM 2018

(P2-33)
Development of First-principles Platform Technology for Energy
Research*Kanghoon Yim1, Chan-Woo Lee1, Jehyun Lee1, Incheol Jeong2, Yong Youn3, Seungwu Han3
1. R&D Platform Center, Korea Institute of Energy Research, 2. Dept. of Energy Science and Engineering, Daegu
Gyeongbuk Institute of Science & Technology, 3. Seoul National University

As future energy technologies such as high-capacity energy storage and renewable energy applications
require exceptional functionalities of host materials, the importance of employing a novel material isgetting bigger and bigger. However, discovering a new superior material is very hard to success though it
requires large costs and manpower. Recently, many researchers attempt to use an informatics technology
such as machine learning in materials screening to overcome the realistic limitations of conventional trial
and error method. The key to successive research using informatics technology largely depends on thequantity and quality of the considered database rather than technical details of informatics model. Since
property data from experiments are usually sparse or biased to favorable materials in industry, it is hard to
obtain a practical database for the informatics research. In that point of view, first-principles calculation isan excellent tool for generating systematic and reliable data of materials properties. However,first-principles calculation itself also requires considerable computational resources and many practicalproperties are hard to obtain by simple calculations. Therefore, a decent automation of first-principles
calculation can do a significant role to establish a successive database. In this talk, I’ll introduce thefirst-principles platform of Korea Institute of Energy Research aiming at providing a practical computing
platform for various researchers with different backgrounds. After introducing the importance ofwell-defined automation procedure in former materials design studies such finding novel high-k
dielectrics and p-type transparent oxides, I'll introduce the automated platform technology forsurface-adsorption reactions which have great importance in most energy applications.

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223

Symposium E(E1 invited)
Mechanism of Enhanced Ductility in Mg Alloys*W. Curtin1, Z. Wu2, R. Ahmad1, B. Yin1
1. EPFL, 2. IHPC

Pure Mg has low ductility due to strong plastic anisotropy and due to a transition of pyramidal
dislocations to a sessile basal-oriented structure [1]. Alloying generally improves ductility; for instance,
Mg-3wt.%RE (RE=Y, Tb, Dy, Ho, Er) alloys show relatively high ductility [2], and typically larger than mostcommercial Mg-Al-Zn alloys at similar grain sizes. Possible concepts for ductility in alloys include thereduction of plastic anisotropy due to solute strengthening of basal slip, the nucleation of from basal I1
stacking faults, the prevention of the detrimental transformation to sessile structures, and the weakeningof strong basal texture by some solute/particle mechanisms. Experiments and modeling do not strongly
support these concepts, however. Here, we introduce a new mechanism of pyramidal cross-slip from thelower-energy Pyr. II plane to the higher energy Pyr. I plane as the key to ductility in Mg and alloys [3].
Certain alloying elements reduce the energy difference between Pyr. I and II screw dislocations,accelerating cross-slip that then leads to rapid dislocation multiplication and alleviates the effects of theundesirable pyramidal-to-basal dissociation. A theory for the cross-slip energy barrier is presented, andfirst-principles density functional theory (DFT) calculations, following methods in [4], are used to computethe necessary pyramidal stacking fault energies as a function of solute type for many solutes in the diluteconcentration limit. Predictions of the theory then demonstrate why Rare Earth solutes are highly effectiveat very low concentrations, and generally capture the trends in ductility and texture evolution across thefull range of Mg alloys studied to date. The new mechanism then points in directions for achieving
enhanced ductility across a range of non-RE alloys.
[1] Z. Wu, W.A. Curtin, Nature 526 (2015) 62-67[2] S. Sandlobes, et al., Acta Materialia 59 (2011) 429-439; Acta Materialia 70 (2014) 92–104[3] Z. Wu, R. Ahmad, B. Yin, S. Sandlobes, and W. A. Curtin, Science 359, 447-452 (2018).
[4] B. Yin, Z. Wu, and W. A. Curtin, Acta Materialia 136 (2017) 249-261.

224

MMM 2018

(E1 invited)
Interaction of screw dislocations with interfaces during multiaxial
loading: large scale 3D simulations*maxime Dupraz1, S.I. Rao2, Helena Van Swygenhoven1,3
1. PEM-LSC-PSD, Paul Scherrer Institute, Villigen, 2. Materials Directorate, Air Force Research Laboratory, WPAFB,, 3.
NXMM-IMX-STI, École Polytechnique Fédérale de Lausanne

In spite of extensive numerical and experimental studies, there are still many open questions related tothe interaction of dislocations with interfaces. Large scale 3D Molecular Dynamics simulations of a screw
dislocation interacting with Coherent Twin Boundaries (CTB) are presented for Al, Cu and Ni bi-crystals
modeled with 6 different embedded atom (EAM) potentials. The simulation cell containing approximately
7.2 million atoms mimics a bi-pillar geometry subjected to compression. Two scenarii were investigated
for the interaction between the screw dislocation and the CTB. In the first one, we consider the case of asingle arm source: the dislocation is pinned at one end of the sample while interacting with the CTB. In
the second one, the dislocation is free to propagate into the incoming grain and to interact with the twinboundary over its entire length. It is shown that both the reaction mechanism and reaction stress dependon the material, the sign of the dislocation, the stacking fault energy, the potential chosen and can differ
significantly from the results reported for quasi-2D simulations. In Cu and Ni, screw dislocations canovercome the CTB at a much lower resolved shear stress than in the quasi-2D case by cross-slip using theFriedel-Escaig (FE) mechanism. In Al, the transmission of the screw dislocation into the twinned grain
occurs at a much larger stress and is achieved by a sequential mechanism using both Fleischer (FL) andFE mechanism. For all materials, the critical stress for transmission is affected by the dislocation line
length and curvature. Our results highlight the importance of directly modeling the slip transfer reactions
using full 3D-models.
Following these first results, we extended this study on two fronts. First, we evaluated the impact of theboundary structure on the interaction mechanism and on the critical stress for transmission, byconsidering the case of Incoherent Twin Boundaries containing ledges. Second, we investigated theinfluence of complex loading conditions on the GB-dislocation interactions. In particular we simulated
intergranular interaction by applying shear stress on the bi-crystal, and performing multiaxial loading tests
with different load ratios.

MMM 2018

225

(E1 oral)
The Multiscale calculations on the behaviors of some nuclear fuels
and cladding materials*shiyu du1
1. Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences

For the nuclear plants, the safety and efficiency of the reactors are the most conerned issues mainly
determined by the performance of the applied materials. Hence, the structural design of new nuclear fuels
and the development of new generation cladding materials have been the research focus for years. In ourworks, we have studied the crystalline structures of different members of uranium silicides by firstprinciple calculations. The impact of defects on the stability and lattice structure of uranium silicide frommicroscopic scale is investigated and predictions on the performance in the reactor are discussed. As thepromising cladding materials, SiC/SiC composite is studied in our group as well. Non-equilibrium
Molecular Dynamics simulations are performed to study the mechanism for the mechanical failure of thecoating-matrix and coating-fiber interfaces. It is found that the mechanical strength of interface is strongly
dependent on the temperature of the system. At 700-1000K, the shear strength is significantly reduced
due to the phase transition of the pyrolysis carbon coatings. Furthermore, the implanted He atoms arealso determined as a major factor that influence the mechanical behavior. The exsitence of He atoms in
the coating materials may cause a significant increase in shear strength and have a delaying effect on thehigh temperature failure. The phase field formulation has been employed to investigate the abnormal
grain growth behaviors for UO2 with pores in the final stage of sintering from mesoscale. The
microstructure evolution is found dependent on the total volume fraction and individual sizes of pores.
The grain growth rate is evidently suppressed when the porosity is high and it is found independent ofpore size at low porosity. Moreover, the smaller pores may cause worse abnormal grain growth at low
porosities and more significant stagnation effect at high porosities in the sintering of UO2.

226

MMM 2018

(E1 oral)
The connection between ideal strengths and deformation mechanisms
in BCC Refractory Metals*Liang Qi1, Chaoming Yang1
1. University of Michigan

An important theoretical criterion to evaluate the ductility of body-centered cubic (bcc) refractory metals
is the mechanical failure mode of their perfect crystals under tension along [100] directions. When thetensile stress reaches the ideal tensile strength, a perfect crystal of a group-6 element (Mo or W) fails by acleavage fracture along (100) plane so that it is intrinsically brittle, but a perfect crystal of a group-5
element (V, Nb or Ta) fails by a shear deformation along certain slip plane so that it is intrinsically ductile.
We have applied first-principles calculations and linear elastic fracture mechanics to find the alloying
strategy to change their intrinsic ductility/brittleness. However, how these ideal strength properties affectthe realistic deformation and fracture mechanisms of these refractory alloys are still unclear. Thus, weconstruct and find different modified embedded atom method (MEAM) interatomic potentials, which canduplicate the ideal strength behavior of those bcc refractory metals under multiple deformation modes.
Then we apply atomistic simulations based on these interatomic potentials to investigate the dislocationand fracture behaviors near the crack tips for refractory metals with different ideal tensile strength
properties, such as Mo and Nb. The results indeed show that the competitions between dislocation
activities and fracture propagations in different refractory metals indeed are controlled by their ideal
strength behavior in the corresponding perfect crystals. These results bring us new physical insights on
the ductility-brittle mechanisms of bcc refractory metals under extreme stress conditions.

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227

(E1 oral)
Nanoscaled Matrix-Inclusions-Composites*Konrad Schneider1, Swantje Bargmann2
1. Institute of Continuum and Material Mechanics, Hamburg University of Technology, 2. Chair of Solid Mechanics,
University of Wuppertal

Tailor-Made Multi-Scale Materials Systems are predicted to achieve ultimate properties such as
scratch-proof surfaces, ultra lightweight stable structures or resistance against extreme heat. However, theorigin of theses exceptional characteristics most likely emerges from the hierarchal structure of thesesubstances. Starting at the nano-scale hard, stiff, strong and at the same time very lightweight structures
are obtainable, cf. [1]. Often it is a vague procedure to explore the many unknown adjustment options likemorphology, material combination etc. which would result in desired properties.
This contribution focuses on so called matrix-inclusion nano-composites. Herein state of the art
generation procedures of randomized representative volume elements, featuring a fast, robust and highly
automatable algorithm with fully periodic finite element models, are employed [2,3]. Super stiff
nano-particles and a soft cross-linked matrix material are considered. We investigate non-linear effects ofmacroscopic responses which are directly linked to the employed material laws at the micro-scale andmight therefore lead to a more conclusive relation between the different scales.

[1] Georgopanos, Prokopios, Gerold A. Schneider, Axel Dreyer, Ulrich A. Handge, Volkan Filiz, Artur Feld,
Ezgi D. Yilmaz et al. "Exceptionally strong, stiff and hard hybrid material based on an elastomer andisotropically shaped ceramic nanoparticles." Scientific reports 7, no. 1 (2017): 7314.
[2] Schneider, Konrad, Benjamin Klusemann, and Swantje Bargmann. "Automatic three-dimensional
geometry and mesh generation of periodic representative volume elements for matrix-inclusioncomposites." Advances in Engineering Software 99 (2016): 177-188.
[3] Bargmann, Swantje, Benjamin Klusemann, Jürgen Markmann, Jan Eike Schnabel, Konrad Schneider,
Celal Soyarslan, and Jana Wilmers. "Generation of 3D representative volume elements for heterogeneousmaterials: a review." Progress in Materials Science (2018).

228

MMM 2018

(E2 oral)
Using IM3D to simulate nano-beam and nano-target effects in ion
radiation*Ju Li1
1. Massachusetts Institute of Technology

Full 3D simulations of ion irradiation are necessary in a wide range of applications to capture theincreasing effect of ion leakage out of surfaces or interfaces [Nuclear Fusion 57 (2017) 016038]. Using a
recently developed 3D Monte Carlo simulation code IM3D [Scientific Reports 5 (2015) 18130], we firstquantify the relative error of the 1D approach in three applications of nano-scale ion implantation: (1)nano-beam for nitrogen-vacancy (NV) center creation, (2) implantation of nanowires to fabricate pnjunctions, and (3) irradiation of nano-pillars for small-scale mechanical testing of irradiated materials.
Because the 1D approach fails to consider the exchange and leakage of ions from boundaries, its relative
error increases dramatically as the beam/target size shrinks. Lastly, the "Bragg peak" phenomenon, where
the maximum radiation dose occurs at a finite depth away from the surface, relies on the assumption ofbroad beams. We discovered a topological transition of the point-defect or defect-cluster distribution
isosurface when one varies the beam width, in agreement with a previous focused helium ion beamirradiation experiment. We conclude that full 3D simulations are necessary if either the beam or the target
size is comparable or below the SRIM longitudinal ion range. [Nanoscale 10 (2018) 1598]

MMM 2018

229

(E2 oral)First principles studies of H interaction with the face-centred cubic Al
Σ5 [100] twist grain boundary during a uniaxial tensile test
Flemming JH Ehlers2,1, Mahamadou Seydou2, David Tingaud1, Francois Maurel2, Yann Charles1,*Sylvain Queyreau1
1. Universite Paris XIII, Sorbonne Paris Cite, Laboratoire des Sciences des Procedes et des Materiaux, LSPM UPR
3407, 2. University Paris Diderot, Sorbonne Paris Cite, ITODYS, UMR 7086

For the theoretical examination of intergranular fracture, ab initio investigations of grain boundaries (GBs)subjected to loading play a central role. While the modelling of material failure is an inherently multi-scaletask, a first principles framework often provides unmatched accuracy for the description of atomic
rearrangements and bond breakage at the heart of the region of key interest. At the same time, it isintegral to a meaningful analysis that the atomistic traction-separation curve emerging from density
functional theory (DFT) based studies be coupled self-consistently to the stress field of the surrounding
bulk grain. The absence of a robust solution to this challenge has manifested itself as a “cell size
convergence problem” for the computed atomistic GB properties. In this talk, we first show how thisobstacle may be entirely circumvented for the modelling of metal GBs within a standard DFT framework.
(F. J. H. Ehlers et al., Comput. Mater. Sci. 139, 39 (2017)) In the procedure, we delimit a GB “local
region” outside which the system response to deformation bears no significant evidence of the presenceof a nearby GB. Then, we show through the example of H decoration of the fcc Al Σ5 36.87° [100] twist
GB how this platform may be used efficiently for quantifying the influence of impurities on a metal GB in a
multi-scale modelling scenario. The H formation energies at the various unequivalent sites in the vicinityof the GB evolve differently with the increase of tensile strain. Finally, the H impact on the full TSL curve
assuming fast diffusion of H atoms at the most stable sites during elongation is assessed.

230

MMM 2018

(E2 oral)
Investigation of the {10-11} twin boundary migration in Magnesium
Qun Zu1, Xiao-Zhi Tang2, *Ya-Fang Guo2
1. School of Mechanical Engineering, Hebei University of Technology, 2. Institute of Engineering Mechanics, Beijing
Jiaotong University

Due to the low symmetry and limited slip systems, deformation twinning plays an important role in theplastic deformation of hexagonal close-packed (hcp) metals. In this work, the {10-11} twin boundary
migration under different directions of shear is studied by molecular dynamics simulations. The twinboundary migration is found to be fulfilled by the two-layer twinning dislocation movement on the twinboundary, with both screw and edge components. The implementation of the TD-mediated migration
relies on the <1-210> screw component, while the direction of TB migration is determined by the<10-12> edge component. The two-layer twinning dislocation structure is further compared with theexperimental observation.

MMM 2018

231

(E2 oral)
Effect of Twins on Mechanical Properties of Silicon Nanowires*Zheng Qin1
1. Tianjin Univ.

The yield strength and tensile ductility of metals can be effectively increased by the existence of nano
twins, leading to a significant ductile-to-brittle transition in Au, for example. Similarly, in experiments,
silicon nanorods or nanowires containing twins have also been reported. Using molecular dynamics (MD)simulations, a modified embedded-atom-method (MEAM) potential was used to systematically study
twinned silicon nanowires (SiNWs) with different shapes, in order to characterize the effect of twins on
their mechanical properties. Our results indicate that twins improve the yield strength and tensile ductilityof SiNWs, which contribute to the further understanding of mechanical responses of low-dimensional
silicon materials, and certify that they have promising potential applications in the design andmanufacture of silicon-based nano devices.

232

MMM 2018

(E2 oral)
Intrinsic Ductility of Alloys from Nonlinear Elasticity*Ian Winter1,2, Maarten de Jong1,3, Daryl Chrzan1,2
1. University of California, Berkeley, 2. Lawrence Berkeley National Laboratory, 3. SpaceX

The direct computation of the ideal strength of an alloy is confounded by the lack of formal crystalline
symmetry. A simple analytical method to estimate the ideal strength and to study intrinsic ductility of acrystalline solid using higher-order elastic constants is presented. Since the method is rooted inparameters that are easily calculated, even for disordered systems, it can be applied to study theproperties of alloys. This method estimates the stress and strain associated with elastic instability andyields the detailed mode of the instability. It is noted that ductility and brittleness are relative. A parameter
gauging the relative intrinsic ductility of a material is introduced, and is shown to be consistent withexperimental measurements of elongation for a number of materials. Finally, the model is applied to thestudy of a chemically complex alloy, W-Nb-Mo-Ta-V, and is used to suggest shifts in composition that will
increase the ductility of the alloy. The work is supported by the U.S. Department of Energy, Office of
Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division under Contract No.
DE-AC02-05-CH11231 (Materials Project program KC23MP).

MMM 2018233

(E2 oral)
Tailoring the stability of {10-12} twin in magnesium with solutesegregation at the twin boundary and strain path control
Heyu Zhu1, *Ziran Liu1
1. Dept. of Phyiscs, Hunan Normal University, Changsha, Hunan{10-12} twinning in magnesium is commonly activated at the room temperature under mechanical loading
to accommodate arbitrary deformation. Effect of solute segregation at twinning boundary on the stabilityof {10-12} twinning which is dependent on the strain path was investigated by employing first-principles
calculations. A model of simulating twinning under external stress is proposed to predict the stability oftwinning with solid solutes under strains. The calculations reveal that the stability of {10-12} twinning
could be tailored by applying external stress on twinning with or without solid solutes at boundary. The
modeling well match the previous experimental results. Effective solute could be selected based on theelectron work function to substitute a certain position along the {10-12} twinning boundary in order tostabilize the twinning.
Reference:
1. Z.R. Liu, D.Y. Li, Acta Materialia 2015;89:225.
2. Z.R. Liu, D.Y. Li, Computational Materials Science 2015;103:90.
3. H. Zhu, Z. R. Liu, Submitted to Computational Materials Science, 2018.

234

MMM 2018

(E2 oral)
Contribution of defects on the anisotropic diffusion behaviour ofhydrogen in nickel single crystals*Arnaud Metsue1, Guillaume Hachet1, Abdelali Oudriss1, Xavier Feaugas1
1. LaSIE UMR 7356 CNRS

It has been proven that hydrogen contributes significantly to the embrittlement of metals. In particular,
several experimental studies have shown that hydrogen promotes the formation of superabundantvacancies, which can act as an embrittlement mechanism. Recently, anisotropic hydrogen diffusion
behaviour has been put in forward experimentally in external stress-free Ni single crystals [1]. Therefore, it
has been suggested that defects, including superabundant vacancies and vacancy clusters, and theirelastic displacement fields may be responsible to the observed anisotropic diffusion behaviour.
In this study, we conduct DFT-based ab initio calculations to study hydrogen diffusion in an elastic strain
field induced by the solute, vacancies and vacancy clusters in Ni single crystal. Temperature effects havebeen taken into account from the extension of the calculations to the free energy including vibration andelectronic excitations contributions. The diffusion tensor in the solid under stress is determined from thehydrogen elastic dipole and the total strain field induced by the defect. The latter is calculated in an
anisotropic elasticity media according to the previous work of Larché and Cahn [2]. We found that theelastic displacement fields induced by the solute and the vacancies are not strong enough to reproduce
the anisotropic diffusion behaviour observed experimentally. Therefore, we turn our study to the effect ofthe vacancy clusters, which can act as gas bubbles. These vacancy clusters have been observed using
TEM on H-charged Ni single crystals [3]. The presence of such clusters and their elastic displacement
fields can reproduce the diffusion coefficient tensor observed experimentally and suggests the large
contribution of these defects on the anisotropic diffusion of hydrogen in Ni single crystal. Finally, we
discuss some implications of the presence of vacancy clusters on the initiation of embrittlement based on
the instability of such defects according to the work of Bockris and Reddy [4].
[1] J. Li, et al., Sci Rep, vol 7, 45041, 2017[2] F. C Larché et J. W Cahn, Acta Metall, vol 30, p 1835-1845, 1982
[3] G. Hachet, et al., Acta Mater, vol 148, p 280-288, 2018[4] J. O’M Bockris and S. U. M. Khan, Surface Electrochemistry: A Molecular Level Approach, Plenum
Press, 1993

MMM 2018

235

(E2 oral)
Development of simplified model for one-sided mechanical joining of
dissimilar materials*Jaeho Kim1, Heungjae Choi1, Dongchoul Kim1
1. Dept. of Mechanical Engineering, Sogang Univ

There are needs for one-sided mechanical joining dissimilar materials in automotive industry because it isefficient to fasten dissimilar materials for manufacturing light-weight vehicles. The full car crash model
applying the joining technology with solid elements leads to an impractical computational cost due to thecomplexity of the model. Thus, we develop a simplified model with shell elements of the joining
technology, which is efficient enough to simulate a full car model. We analyze and optimize theparameters of the simplified model that determine the behavior of the joining technology. The developedmodel is validated with lap shear and cross tension tests.

236

MMM 2018

(E3 invited)
3D mesoscopic study of the stability of three-dimensional short cracks
in FCC metals using the Discrete-Continuous Model
Laurent Korzeczek1, Riccardo Gatti1, Arjen Roos2, *Benoit Devincre1
1. LEM, CNRS-ONERA, 2. Safran Tech

The erratic behaviour of short cracks propagation under low cyclic loading in ductile metals is commonly
attributed to a complex interplay between stabilisation mechanisms that occur at the mescopic scale.
Among these mechanisms, the interaction with the existing dislocation microstructure play a major role.
The dislocation microstructure is source of plastic deformation and heat transfer that reduce thespecimen stored elastic energy, screen the crack field due to its self generated stress field or change thecrack geometry through blunting mechanisms. For the first time, these mechanisms are investigated with
3D-DD simulations using the Discrete- Continuous Model, modelling three different crack orientations
under monotonic traction loading promoting mode I crack opening. Surprisingly, screening and blunting
effects do not seem to have a key role on mode I crack stabilisation. Rather, the capability of the specimen
to deform plastically without strong forest hardening is found to be the leading mechanism. Additional
investigations of two different size effects confirm those results and show the minor contribution of apolarised dislocations density and the associated kinematic hardening on crack stabilisation.

MMM 2018

237

(E3 invited)Role of cracks, voids and interfaces in hot spot formation and initiationof energetic materials
Michael Sakano1, Chunyu Li1, Nicolo Grilli2, Brenden Hamilton1, Camilo Duarte2, Marisol
Koslowski2, *Alejandro Strachan1
1. School of Materials Engineering, Purdue University, 2. School of Mechanical Engineering, Purdue University

The chemical initiation of high-energy (HE) materials following mechanical insults requires the excitationof chemical bonds with lengths of a few angstroms and sub-picosecond vibrational periods. This would bea nearly impossible task were it not for: i) the materials’ microstructure that localizes the input energy
into hotspots, and ii) the equilibration of inter- and intra-molecular degrees of freedom that transfers
input energy to the high-frequency bond vibrations responsible for chemistry. Thus, a predictive
understanding of the response of HE to strong mechanical insults requires identifying, characterizing andmodeling coupled processes at the microstructural level (interfacial friction, cracks, void collapse), crystal
level (dislocations and shear bands) and the molecular/electronic level (inter- and intra-molecular energy
transfer and chemical reactions).
While our large-scale molecular dynamics (MD) simulations recently provided an atomic picture of theformation of a steady deflagration wave following shock loading of a defective HE crystal, such simulations
cannot capture the complex microstructure of the materials of interest nor the size of the hot spots ofinterest in real applications. Thus, we developed a multiscale model that combines large-scale reactive
and non-reactive MD simulations with a continuum model capable of describing dynamical loading,plastic deformation, fracture and friction, together with thermal transport and chemistry. The MD
simulations provide insight and parameters to characterize energy localization as shock waves interact
with several pre-existing defects, including cracks and voids as well as interfaces. In addition, reactive MD
simulations are used to characterize thermal transport and develop chemical kinetics models. These
results inform the continuum model that is used to predict energy localization in microstructurally
complex systems of plastic bonded energetic formulations. These simulations enable us to characterize
the relative potency of various microstructural features to general hot spots that can result in sustained
chemistry. Both the atomistic and continuum simulations are validated against experiments capable ofcapturing the physics of interest at scales matching those of the simulations.

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(E3 oral)
Precipitation hardening effects on extension twinning in magnesium*Haidong Fan1, Jaafar El-Awady2, Dierk Raabe3
1. Sichuan University, 2. The Johns Hopkins University, 3. Max-Planck-Institut für Eisenforschung GmbH

Precipitation is an efficient method to strengthen metallic materials. While precipitation hardening effects
on dislocation slip have been studied extensively in the past, the influence of precipitates on twinning
mediated plasticity and the development of corresponding hardening models that account for
twin-precipitate interactions have received less attention. The interaction of {10-12} extension twinboundaries (TBs) in pure magnesium with precipitates of plate-, sphere- and rod-like shapes is studied
using molecular dynamics (MD) simulations. We find that TBs that engulf precipitates are absorbed by theprecipitate-matrix interfaces, and the precipitates are neither twinned nor sheared but deform elasticallyleading to their rotation. TBs can pass small precipitates (length ≈20 nm) and remain intact. In contrast
when TBs are interacting with large precipitates (length ≈50 nm), basal dislocations or stacking faults
nucleate from the interfaces, causing local plastic relaxation. The stress field around a plate-like
precipitate as calculated in the MD simulations suggests that a strong back-stress is imposed on the TBs.
We then coarse grain these mechanisms into an analytical mean field model of precipitation hardening on
twinning in magnesium alloys, which is based on the energy conservation during the TB-precipitate
interaction. The model is in good agreement with the current MD simulations and published experimental
observations. The hardening model shows that spherical precipitates have the strongest hardening effecton twinning, basal and prismatic plate-like precipitates have a medium effect while rod-like precipitates
exert the weakest influence. We also find that most types of precipitates show a stronger hardening effecton twinning mediated plasticity than on basal dislocation slip. Finally, prismatic plate-like precipitates are
predicted to have reasonable hardening effects on both twinning and basal slip. These results can help
guiding the development of magnesium alloys with enhanced strength and ductility.

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239

(E4 oral)
High-performance first-principles calculation software developmentwith applications to the zirconium*Haifeng Song1,2, Xingyu Gao1,2, Jun Fang1,2, Yafan Zhao2, Han Wang2, Deye Lin2
1. Institute of Applied Physics and Computational Mathematics, 2. CAEP Software Center for High Performance
Numerical Simulation

With the development of high-performance computer, first-principles calculation becomes to be afeasible way in many nuclear material research fields. In this talk, we will give two examples todemonstrate it. Firstly, we develop two high-performance oriented methods for wave functions in solving
the Kohn-Sham equations: a more parallel scalable FFT and optimal extrapolation order for the initial
guess at next time step. Armed with adaptive time stepping, we estimate the threshold displacement
energy of zirconium by large-scale first-principles calculations. We utilize CESSP to realize thecalculations of about 100 tasks on the Tianhe-2 supercomputer in 3 months, where each task is thefirst-principles molecular dynamic simulation of 9600 valence electrons and 1 picosecond. Compared toclassical molecular dynamic simulations, we arrive at the results closest to most recent experiment.
Secondly, we have combined a novel crystal structure search method based on the basin hopping
algorithm into our first-principles code. Several improvements were implemented, including a symmetry
structure generation algorithm based on the crystal space groups, a structure adjustment method basedon a virtual spring-force to adjust unreasonably structures. The method is also highly paralleled to makefull use of the computing resources. We have applied the method into the structure searching of Zr
hydrides (ZrHx, x= 0.5, 1.0, 1.5, 2.0). All experimentally observed Zr hydrides are reproduced, and several
new structures were found. The dynamic and thermodynamic stability of these Zr hydrides will also be
discussed.

240

MMM 2018

(E4 oral)
Catalyst design for ammonia synthesis of ammonia*Chenghua Sun1
1. Swinburne University of Technology

Ammonia (NH3) is a critical chemical widely used in modern agriculture and chemical engineering.
Currently, it is based on the Haber-Bosch process at high temperature and high pressure. This
computational work is to design novel catalysts to achieve ammonia synthesis at room temperature.
Single-atom catalyst and defects of low-dimensional materials will be particularly considered. As revealed
by first pricinple calculations, Mo-based various catalysts are promising for this applications.

MMM 2018

241

(E4 oral)
A solid-solution structure model for multi-component alloys*Xingyu Gao1,2, Fuyang Tian3, Yafan Zhao2, Deye Lin2, Haifeng Song1,2
1. Institute of Applied Physics and Computational Mathematics, 2. CAEP Software Center for High Performance
Numerical Simulation, 3. University of Science and Technology

First-principles calculations have been widely used to predict and design the properties of metallic alloys.
However, it is still a challenge to generate ab initio suitable structure for multi-component random alloys
in a finite supercell. In this talk, we propose a novel solid-solution structure model to mimic the similar
local atomic environment (SLAE) of the random alloys. In the SLAE model, local solid-solution environment
such as the random disorder, partial disorder, and short-range order can be calibrated via the standard
deviation of the pair distribution function and three-body correlation function. Taking the typicalhigh-entropy alloy CoCrFeMnNi, medium-entropy alloy CoCrNi, and continued solid-solution binary alloy
TaW as test cases, we evalueate the SLAE models by comparing between the ab initio predicted phasestability and some available experiments.

242

MMM 2018

(E4 oral)First-princples prediction of plastic deformation modes in alloys*Song Lu1, Levente Vitos1
1. KTH Royal Institute of Technology

First-principles alloy theory is used to establish the gamma-surface of alloys. Then we put forward atransparent model solely based on first principles simulations for mapping the deformation modes inalloys. The model bridges intrinsic energy barriers and different deformation mechanisms and resolves thecomplexity of the observed orientation-dependent deformation mechanisms in alloys. Examples of studies
are given in austenitic stainless steels and polysynthetic twinned gamma-TiAl alloys.

MMM 2018

243

(E4 oral)
The local-orbital construction of strongly correlated electrons basedon the PAW method*Yu Liu1,2, Xingyu Gao1,2, Jianzhou Zhao3, Haifeng Song1,2
1. Institute of Applied Physics and Computational Mathematics, 2. CAEP Software Center for High Performance
Numerical Simulation, 3. Southwest University of Science and Technology

The effects of electron strong correlation, spin-orbit coupling and multi-configuration bring out challenges
in the first principle studies of some nuclear materials and rare-earth materials. Density functional theory(DFT), however, cannot properly describe their electronic structures. For this reason, people havedeveloped Hubbard model based post-DFT method (DFT+X) to correct the strong correlation effect. The
coupler that connects the DFT and X still requires more intensive research. In this report, we will present
the most recent progress on the DFT+X coupler and its realization in the projector augmented wave
(PAW) method under the infrastructure of CESSP code. Firstly, there are three major schemes, namely P0,
P1 and P2, to construct the local orbitals of the strong correlated electrons. Prof. Haule from Rutgers
University demonstrate the advantages of P2 scheme under the linear augmented plane wave (LAPW)
method. However, we are going to show that under the PAW method, P2 is more appropriate for systems
with d electrons while P1 is more appropriate for systems with f electrons. Secondly, we realize thecorrection of spin-orbit coupling and crystal field splitting to the local orbitals, which improves theconvergence and accuracy of the DFT+X calculations. We further test our code in the typical strong
correlated materials SrVO3 and Ce.

244

MMM 2018

(E5 invited)
Use of FFT-based micromechanical modeling for analysis ofsynchrotron-based diffraction experiments
Vahid Tari1, Ricardo A Lebensohn2, Reeju Pokharel2, Rachel E Lim1, Darren C Pagan3, Yufeng
Shen1, Joel V Bernier4, Robert M Suter1, *Anthony D Rollett1
1. Carnegie Mellon University, 2. Los Alamos National Laboratory, 3. Cornell High Energy Synchrotron Source, 4.
Lawrence Livermore National Laboratory

This presentation describes use of image-based modeling to analyze synchrotron-based High Energy
Diffraction Microscopy (HEDM) stress-strain experiments. HEDM is both in-situ and non-destructive andwas used to measure micromechanical fields such as strain and orientation at the grain scale developed
under macroscopic tensile loading of Ti-7Al. Taking the 3D image of the experimentally measured initial
microstructure as input, elasto-viscoplastic modeling based on the Micromechanical Analysis ofStress-Strain Inhomogeneities with Fourier transforms (MASSIF) was used to compute themicromechanical fields that develop during loading. To validate the MASSIF calculations, we compared
the calculated fields with the ones measured by HEDM. The initial comparisons showed that MASSIF canreproduce the macroscopic stress/strain curve but poor agreement was found between calculated andmeasured fields at the grain scale. The differences at the grain scale were hypothesized to be caused by
the initial residual stress state that was induced during prior material processing, and which was notincorporated in the MASSIF calculation. We used eigenstrain concept to incorporate residual stress in the
MASSIF calculation by converting it to an initial eigenstrain field. The results reveal that incorporation ofresidual stress results in good agreement between calculated and measured fields at the grain scale,thereby validating the computational approach. MASSIF was further used to model an experiment inwhich a polycrystalline sample of Ti-7Al was cyclically deformed and mapped using boxbeam nearfield(nf)-HEDM and far field(ff)-HEDM. The ff-HEDM results show a decrease in residual elastic strain overthe first cycle followed by a steady increase in (elastic) strain. The initial residual strain in each grain wasanti-correlated with the change in that same strain component over the first cycle. The distribution in the
von Mises stress, which is a scalar measure of deviatoric stress, broadens as cycles accumulates anddevelops a long upper tail. Initially, a graph of hydrostatic stress against stress coaxiality angle shows apositive correlation but after about 65 cycles, the trend-line has rotated to a negative correlation.
Simulations with MASSIF are used to analyze and understand these results.

MMM 2018

245

(E5 oral)
Computational Design of Hysteresis-Free and Linear Super-Elastic, andUltralow Modulus Ferroelastic Materials*Jiaming Zhu1, Yipeng Gao2, Dong Wang3, Tong-Yi Zhang4, Yunzhi Wang2
1. City University of Hong Kong, 2. Ohio State University, 3. Xi'an Jiaotong University, 4. Shanghai University

A unique characteristics of ferroelastic materials is their highly non-linear stress-strain curves withplateaus and large hysteresis. We show by computer simulations how to render ferroelastic materials
nearly hysteresis-free and linear super-elastic (with an elastic strain limit of 2.7%), and ultralow modulus(<20 GPa) by creating appropriate concentration modulations (CMs) in the parent phase via spinodal
decomposition and by pre-straining. The CM causes phase stability modulations, suppresses autocatalysis
in nucleation, imposes nano-confinement on growth and hinders long-range order of transformation strain
during martensitic transformation (MT) and, thus, turns the otherwise sharp first-order transition into a
broadly smeared, macroscopically continuous transition over a large stress range. The pre-straining yields
retained martensitic particles that are stable at the test temperature without applied load and act as
operational nuclei in subsequent load cycles, eliminating the stress-strain hysteresis and offering an
ultralow apparent Young’s modulus. This study demonstrates a novel and universal approach to design
new ferroelastic materials with unprecedented properties.

246

MMM 2018

(E5 oral)
A biphasic continuum model for large deformation visco-elastic
mechanics of uncured carbon fibre prepregs*Amir Hosein Sakhaei1, Timothy James Dodwell1
1. College of Engineering, Mathematics and Physical Sciences, University of Exeter

Composite materials are widely used in high value manufacturing industries like aerospace and automotive. However, they are often compromised by high costs, long development time, and poor quality due
to manufacturing defects. Therefore, numerical simulation for composite manufacturing processes has
attracted much attention in academic and industrial communities, with the twin objectives of improving
product quality and decreasing production time.
Typically, carbon fibre composite components are made by layering a series of thin carbon fibre layers,
pre-impregnated with resin, onto a tool surface. During this lay-up process the stack of plies is
consolidated at moderate temperatures and pressures to remove air trapped between layers. The
consolidation of composite laminates prior to curing strongly influences the formation of defects which
ultimately hinder the components structural integrity.
The deformation of uncured composite laminates is complex. It depends on the individual constitutive
behaviour of the plies and interface, as well as the geometric constraints that arise from the layers fitting
and moving together. Theoretically, uncured composites can be modelled using detailed finite element
calculations in which each layer and interface is defined explicitly. Experimental characterization have led
to much better understanding of the shearing, bending, flow and consolidation mechanics of uncured
laminate composites. This data has been parameterized largely via simplified heuristic models and have
yet to be translated into rigorous 3D continuum models.
At In this study, we have developed a new continuum model for uncured carbon fibres plies to capture thenon-linear visco-elastic behaviour of uncured laminates considering shear, bending, flow, andconsolidation mechanisms based on the recent experimental characterization results. The model isdeveloped from a biphasic continuum theory allowing for coupling between the solid and fluid responses.
It includes the deformation mechanisms of the fibres as an anisotropic hyper-elastic material as well as
resin flow through the fibre network. This work is the first part of a larger research effort to develop an
efficient multi-scale numerical framework for composite manufacturing process of full scale components.

MMM 2018

247

(E5 oral)
Strong coupling of deformation and microstructure/microchemistryevolution in hot compression tests*Evgeniya Kabliman1, Johannes Kronsteiner1, Ernst Kozeschnik2
1. Light Metals Technologies Ranshofen, Center for Low-Emission Transport, Austrian Institute of Technology, 2.
Institute of Materials Science and Technology, TU Wien

When material deformation occurs at elevated temperatures, one has to take into account thesimultaneous evolution of grain, sub-grain structure and microchemistry (dissolution and precipitation ofphase particles). In the present work, we present current results on the implementation of acomputational strong coupling scheme between material deformation and microchemistry/microstructure
evolution. For this purpose, we introduce a Dyna2Micro Material Model, which is implemented into a
Finite Element Code (LS-DYNA of LSTC) and allows a simultaneous calculation of the (sub)-grain structure
and phase kinetics at every integration point and time step. Dynamic processes, such as, work hardening
and recovery are calculated by using a dislocation based model (MD2 Model) implemented into a user
defined subroutine, while the phase dissolution and precipitation is calculated by the thermo-kinetic
software package MatCalc. Computational efficiency is realized by a new remote control feature
implemented in MatCalc specially developed for this project as command interface between the user
subroutine and a running MatCalc session. The simulations are checked by simulation of multi-step hotcompression tests using a 3D axisymmetric cylindrical model of samples according to the experimental
set up. For the material, we choose an Al-Cu-Mg alloy, where precipitation and dissolution of Al2Cu and Al
2CuMg is considered. The whole simulation process from solidification, pre-heat treatment, deformationand post heat treatment is then controlled by the SProC (Smart Process Control) Toolkit.

248

MMM 2018

(E6 oral)
Phase Field Crystal Simulation of Crack Extension and Brittle-Ductile
Transition Behavior on Nano-Scale*Ying-Jun Gao1
1. Guangxi University

Phase field crystal (PFC) method is used to simulate cracks propagation as a function of grain orientation.
The results show that when the direction of tensile stress is perpendicular to the grain orientation, thecrack exhibits cleaving extension characteristics with straight and smooth edges. As the angle between
the grain orientation and the direction of the applied stress changes, the extension of the cracks begins topresent a structure with saw teeth on the edge. Then, with the change of the angle increasing, adislocation is emitted in front of the crack tip. Along with the dislocation slipping, it generates a series of
micro holes. These holes further develop and grow to connect each other, and form a crack, which
presents a ductile crack growth process. It can be seen that the change of the angle between the
direction of the applied stress and the orientation of the grains can lead to a transformation of theductile-brittle behavior of the fracture.

MMM 2018

249

(E6 oral)
Continuum elasticity and correlations of plastic strain fluctuations insheared glasses: The effect of hard boundaries*Muhammad Hassani1, Fathollah Varnik1
1. Ruhr-University Bochum

Spatial correlations of plastic strain fluctuations in a shear-driven amorphous solid are investigated in thevicinity of a hard wall with a molecular-scale corrugation [1]. Within thin slabs parallel to the wall plane,
normalized correlations are enhanced when the slab center is closer to the wall. The amplitude of these
correlations, however, is found to be suppressed by the wall. It is shown in this work that theenhancement of the normalized correlations quantitatively matches with the wall-induced enhancementof the elastic propagator within continuum elasticity [1,2]. The decrease of strain amplitude, on the other
hand, is shown to originate from molecular scale wall effects on the size of the nearest neighbor cage,
explored by particles on intermediate times scales [1]. These results highlight the fundamental role ofelasticity for the correlation of plastic activity in glassy systems [3].
[1] M. Hassani, P. Engels and F. Varnik, Wall effects on spatial correlations of non-affine strain in a 3Dmodel glsss (Europhysics Leters 2018, in press).
[2] A. Nicolas and J.-L. Barrat, A mesoscopic model for the rheology of soft amorphous solids, withapplication to microchannel flows, Faraday Discuss. 167, 567 (2013).
[3] F. Varnik, S. Mandal, V. Chikkadi, D. Denisov, P. Olsson, D. Vagberg, D. Raabe, and P. Schall,
Correlations of plasticity in sheared glasses, Phys. Rev. E 89, 040301 (2014).

250

MMM 2018

(E6 oral)
Modeling approaches to tetragonal-to-monoclinic transformations in
MgO partially stabilized zirconia*Michael Budnitzki1
1. TU Bergakademie Freiberg

Partially stabilized zirconia (PSZ) based ceramics are known for their excellent mechanical properties andbio-compatibility. The material's ability to undergo stress-induced phase transitions results in a very highfracture toughness, making it ideal for applications such as dental implants. In MgO-stabilized zirconia(MgO-PSZ) the martensitic phase transformation is restricted to lenticular inclusions of retained
tetragonal phase embedded in a cubic matrix.
We use semi-analytical models as well as phase-field methods based on Ginzburg-Landau theory to
account for various aspects of microstructure formation. Using phase-field it is possible to directly
simulate the phase morphology and investigate the influence of geometric and crystallographic aspects.
The semi-analytical model allows to study the effects of material parameters, such as surface energies, on
the transformation stress as well as the inelastic strain in the inclusions and allows for the application ofhomogenization techniques in order to assess the effective constitutive response. Both approaches
complement each other in order to achieve a more quantitative understanding of the micromechanics ofzirconia.

MMM 2018251

(E6 oral)
Delamination cracks in wire-drawn fully pearlitic steels.
*Masaki Tanaka1, Yelm Okuyama1, Tatsuya Morikawa1, Toshiyuki Manabe2
1. Kyushu Univ., 2. Nippon Steel & Sumitomo Corporation

Wire-drawn fully pearlitic steel was twisted at room temperature. As a result, a delamination crack
propagated along the longitudinal direction of the wire. The fracture surface of the delamination crack
was compared with the fracture surface obtained by an impact test at 120K, demonstrating that thedelamination crack is neither a simple cleavage nor ductile shear one. Backscattered electron andtransmission electron microscopy images indicated that the cementite lamellae immediately beneath thedelamination crack had vanished. In addition to that, a fine-grained structure was observed. This
indicated that the delamination fracture was not a brittle one but a shear one associated with local severeplastic deformation.

252

MMM 2018

(E6 oral)
FTMP-based Modeling and Simulations of Inhomogeneous
Recovery-Triggered Accelerated Creep Rupture in Lath Martensite
Structures*yasutaka matsubara1, tadashi hasebe1, yuta amano1
1. Kobe Univ.

Against highly complex hierarchical material systems as in high Cr heat-resistant ferritic steels exhibiting
inhomogeneous recovery-triggered accelerated creep rupture, the present study tackles a series ofproblems about their practically-feasible multiscale modeling and simulations based on FTMP. The
targeted system here is composed of martensite laths with high-dense dislocations (Scale A), lathblocks/packets (Scale B) embedded within a prior austenitic grain (Scale C). The objectives are(a)reproduction of the experimentally-observed accelerated degradation of the creep strength due toinhomogeneous recovery of the microstructures under relatively low stress conditions, and(b)identification of the minimal conditions for (a) to occur, focusing on the interactions between Scales A
and B. Creep analyses, considering the interior high-dense dislocations evaluated by the
spontaneously-evolved incompatibility tensor field in Scale B, are conducted for single lath block models
first. They exhibit pronounced local instability due to local recovery brought about by the interaction
incompatibility field. Thus-developed models are further combined to construct single packets andembedded packet models, respectively, and the same series of analyses are performed on them. The
fluctuating incompatibility field in Scale A, concurrently enhanced by the interaction term, isdemonstrated to promote the local recovery. We further clarify the contributions of the projection
directions of the incompatibility tensor in evaluating the Scale A dislocation density. The projection to theslip plane normal direction is turned out to play exclusively-dominant roles, implying the criticalcontribution of the climb-related elementary processes to the enhanced localized recovery. An attempt isalso made to replace the above recovery model to improve the consistency of the current approach.

MMM 2018

253

(E7 invited)
The Activation Parameters for Dislocation Nucleation in Molybdenum
Nanoparticles under Compression
Doron Chachamovitz1, *Dan Mordehai1
1. Mechanical Engineering, Technion, 32000 Haifa, Israel

Metallic faceted nanoparticles exhibit ultra-high strength, since their deformation involves the nucleationof dislocations on their surfaces. In this work, we employ molecular dynamics (MD) simulations to study
the strength of molybdenum faceted nanoparticles and its relation to the activation parameters for
dislocation nucleation [1]. We show that under compression, the nanoparticles yield by nucleating
dislocations at the vertices, which are points of stress concentration. For each temperature, the simulationis repeated 30 times with different atomic velocities. Since dislocation nucleation is a thermally-activatedprocess, the calculated strength varies between the different simulations. The strength distribution can be
exploited to calculate the activation parameters. We show that the distribution can be approximated by anormal distribution, with a standard-deviation that corresponds directly to the activation volume at thegiven temperature and stress. Accordingly, the activation volumes are calculated from the MD simulation
results at different temperatures and stresses. In addition, the dependence of the most-probablenucleation stress on the temperature is calculated. With the help of classical nucleation theory, theactivation free-energy and the activation entropy are calculated. We find that the dependence of theactivation free-energy on the stress obeys a power-law near the conditions for spontaneous nucleation,with a critical exponent that is equal ~1.5. This critical exponent is typical for simple bifurcation problems.
In addition, the activation entropies are found to be in the range of 0-15kB, with some deviations form the
Meyer-Neldel compensation rule. Based on the calculated activation parameters, the probabilistic natureof the strength at this scale is discussed.
1. Chachamovitz D, Mordehai D. Sci Rep 2018;8:3915

254

MMM 2018

(E7 oral)
Dislocation density-based crystal plasticity analysis for the evolution of
atomic vacancies during plastic slip deformation*Tetsuya Ohashi1
1. Kitami institute of technology

Density evolution of atomic vacancies during plastic slip deformation is evaluated by crystal plasticity
analyses where evolution of dislocation density is calculated by the model of Kocks and Mecking1 and thedislocation mean free path is given by an effective average distance of forest dislocations and a
microstructure length scale2, 3. We slightly modified the model by Essmann and Mughrabi4 for theevolution of atomic vacancy density and implemented to the crystal plasticity software code.
Analysis results5 show that the vacancy density sometimes reaches at the order of 1024 /m3, which is at
the order of 10-4 in terms of concentration. Increase rate of the atomic vacancy is shown to depend largelyon the microstructure length scale and slip multiplication on different systems.
Reference
1. Mecking H, Kocks U f.; Acta Metall. 1981;29:1865-1875.
2. Ohashi T. Dislocation Density-Based Modeling of Crystal Plasticity Finite Element Analysis. In:

Mechanics of Materials. Micromechanics. Springer Nature Singapore; 2018.
3. Ohashi T, Kawamukai M, Zbib H.; Int J Plast. 2007;23(5):897-914.
4. Essmann U, Mughrabi H.; Philos Mag A. 1979;40(6):731-756.
5. Ohashi T.; submitted.

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255

(E7 oral)
Structural Stability of Long-period Stacking Ordered Magnesium Alloys*Daisuke Matsunaka1, Yoji Shibutani2
1. Dept. of Mechanical Systems Engineering, Shinshu Univ., 2. Dept. of Mechanical Engineering, Osaka Univ.

Magnesium (Mg) alloys with long-period stacking ordered (LPSO) structures show excellent mechanicalperformances such as high yield strength and ductility. The LPSO structures consist of periodic
arrangement of basal stacking faults (SFs) and enrichment of solute atoms in the vicinity of the SFs.
HAADF-STEM measurements observed that the L12-type clusters of solute atoms are formed and aligned
at each SF. The L12 clusters at each SF have a solute-enriched region of four close-packed planes. As SFs
are periodically introduced, each type of the LPSO structures has the specifc close-packed layers between
the quadrople solute-enriched layers. While a previous first-principles study showed that the binding
energy of solute atoms to SF and the solute-solute pair interaction can describe the formation of the L12clusters, the physical origin of the periodic arrangement of the quadrople solute-enriched layers with theL12 clusters remains controversial.
In order to elucidate the periodic arrangement of the quadrople solute-enriched layers, we investigate
interactions mediated by electrons or phonons between the neiboring quadrople solute-enriched layers.
The formation energy of the L12 cluster evaluted by first-principles calculations shows that theelectron-mediated interaction is short-range repulsive with respect to the distance of the quadrople
solute-enriched layers. On the other hand, we investigate effects of phonon on the inter-planer orderingof the solute-enriched layers using the 1-dimensional chain model with mass change. For heavy masschange, the ordering of the mass changes is stabilized by phonons and the energy gain increases with theconcentration of the mass changes, i.e., the short LPSO period is favorable. Thus, a promising mechanismof the inter-planer ordering of the LPSO structures is the phonon-mediated interaction of the quadrople
layers where heavy solute atoms are enriched as the L12 clusters at SFs.

256

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(E7 oral)First-principles-based prediction of yield strength in the RhIrNiPdPtCuhigh entropy alloy*Binglun Yin1, William A. Curtin1
1. LAMMM, EPFL

High entropy alloys (HEAs) are random solid solution alloys with 5 or more components, usually of nearequi-composition. HEAs exhibit excellent mechanical properties, including high strength, high ductility,and high fracture toughness [1]. Guiding the design of new HEAs across the vast composition spacerequires an ability to compute necessary underlying material parameters via first-principle calculations.
Here, a methodology is proposed to compute, via density functional theory (DFT), the elemental misfit
volumes, as well as alloy lattice constant, elastic constants and stable stacking fault energy, in the fcc
noble metal HEA RhIrNiPdPtCu [2]. These properties are then used in a recently developed solutestrengthening model [3, 4] for temperature and strain-rate dependent yield strength, with the predictionof 563 MPa is in excellent agreement with the experimentally measured value of 527 MPa [5]. Thismethodology links the alloy composition with the yield strength prediction, indicating a general
methodological path for exploring new potential high-strength HEAs in this and other alloy classes.

[1] D. B. Miracle and O. N. Senkov, Acta Mater. 122, 448 (2017).
[2] B. Yin and W. A. Curtin, in preparation.
[3] C. Varvenne, A. Luque, and W. A. Curtin, Acta Mater. 118, 164 (2016).
[4] C. Varvenne, G. P. M. Leyson, M. Ghazisaeidi, and W. A. Curtin, Acta Mater. 124, 660 (2017).
[5] S. Sohn, Y. Liu, J. Liu, P. Gong, S. Prades-Rodel, A. Blatter, B. E. Scanley, C. C. Broadbridge, and J.
Schroers, Scr. Mater. 126, 29 (2017).

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257

(E7 oral)
Metal-coated carbon nanotube reinforced aluminum composites*Samaneh Nasiri1,2, Michael Zaiser1,2
1. Institute for Materials Simulation WW8, Deptartment of Materials Science, Friedrich-Alexander University
Erlangen-Nuernberg, 2. Cluster of Excellence EAM/FUMIN

Carbon nanotubes (CNT) possess excellent mechanical properties in terms of axial elastic stiffness andrupture strength. It is therefore an obvious question whether these properties can be harnessed forstructural applications by using such nanoparticles as fillers in low-melting lightweight metals such as Alor Mg nanocomposite. To this end, homogeneous dispersion of CNT in the metal matrix and strong
interfacial bonding are essential factors since agglomerated nanoparticles with weak bonding into thesurrounding metal matrix might act as flaws which actually deteriorate, rather than improve, themechanical properties of composites. However, good dispersion and strong interfacial bonding arehindered by the low affinity of CNT to Al or Mg. To overcome this problem, interface engineering
approaches such as coating CNT with an appropriate metal such as Nitweight , or by decorating themwith metal nanoparticiles, may offer a promising approach towards the efficient fabrication of lightweight
metal-CNT nanocomposites.
In this work we study how metal coated or metal decorated carbon nanotubes affect the mechanicalproperties of aluminium/CNT composites. In particular we look at the pullout behaviour of pristine as wellas Ni coated or Ni decorated CNT from an aluminium matrix. Our result shows that Ni coating produces
an extended interface (“intephase”) where a significant amount of energy is dissipated during CNT
pull-out, leading to a high pull-out force. We then investigate the interaction of embedded CNT with andwithout coating/decoration with cracks and with dislocations during simulated tensile tests. We discuss
the results in view of promising approaches for engineering CNT-metal interfaces such as to achieve highstrength and high toughness of the metal-CNT composites.

258

MMM 2018

(E8 oral)
Effect of hydrogen on the vacancy diffusion in metals*Jun-Ping Du1,2, W.T. Geng3, Kazuto Arakawa4, Shigenobu Ogata2,1
1. Elements Strategy Initiative for Structural Materials, Kyoto University, 2. Department of Mechanical Science andBioengineering, Osaka University, 3. University of Science and Technology Beijing, 4. Department of Materials
Science, Faculty of Science and Engineering, Shimane University

Hydrogen can be either an intentional constituent or unwelcome impurity in metals. The fact that excesshydrogen can enhance greatly the self-diffusion of atoms in metals has been explained by the appearanceof superabundant vacancies, because the vacancy formation energy decreases substantially withincreasing H concentration, while individual vacancy diffusion is supposed to be slowed down due to theincreased jumping energy barrier of H-vacancy complexes based on the previous first-principles studies.
Here, performing first-principles calculations of appearance probability of possible H-vacancy
configurations and activation energy of possible vacancy jumping pathways in combination withmolecular dynamics (MD) simulations, we found at certain H concentrations and temperatures, thediffusivity of vacancy in face-centered cubic Cu can be accelerated by H, which is caused by H-enhanced
diffusion attempt frequency and environmental H-assisted vacancy diffusion pathway. The MD simulations
demonstrated that the promoting effect of H can also be found in dynamical processes. The uncovered
H-vacancy diffusion processes in metals can advance our understanding of the H behavior in metals.

MMM 2018

259

(E8 oral)
Promotional effects of anisotropic strain on vacancy mobility intungsten: the independence on the sign of strain*Zhong Zhu Li1, Yu Hao Li1, Hong Bo Zhou1, Guang Hong Lu1
1. Department of Physics, Beihang University

Tungsten (W) is one of the most promising candidates for the plasma facing materials (PFMs) in future
fusion devices. Vacancy is the typical defect in W, and plays key role in the microstructure and mechanicalproperties of W. Vacancies can aggregate to form voids by migrating, and further lead to swelling,hardening and embrittlement of W. Therefore, the behaviors of vacancy in W have attracted many
attentions. Generally, the presence of vacancy is accompanied with the disappearance of normal W atom,
and induces local lattice distortion and forms stress field. Thus, one can expect that the externalstrain/stress field should have effect on vacancy behaviors in W, while little work focuses on this aspect so
far.
Here, we have investigated the migration of vacancy in W under strain using a first-principles method in
combination with the activation volume tensor and thermodynamic models. In general, vacancy inevitably
induces local tensile stress field, tending to contract the lattice. Thus, it is considered that the mobility ofvacancy responds to strain “monotonically”, i.e., the migration energy of vacancy will decrease(increase) with the increasing tensile (compressive) strain. The mobility of vacancy in W under triaxial
strain follows this rule. Surprisingly, we have discovered that the vacancy mobility can always bepromoted by anisotropic (biaxial) strain in W, independent of the sign of strain. In a wide range of strain
values, the vacancy mobility is enhanced in the strained W. This anomalous behavior is found to becaused by an unusual variation of the vacancy activation volume tensor induced by anisotropic strain,
which is originated from the Poisson effect. Further, it is found that the diffusivity of the vacancy in W with
5% tensile (compressive) biaxial strain will be increased by 3 (2) orders at 600K based on the Arrhenius
equation. Meanwhile, the onset temperature for vacancy diffusion will also be markedly reduced by biaxial
strain. Consequently, our finding suggests that anisotropic strain will significantly enhance vacancy
mobility in W and promote the formation and growth of hydrogen/helium bubbles.

260

MMM 2018

(E8 oral)
Ab initio investigation on the stacking fault energy and the c/a ratio in
hexagonal metals and alloys*Gang Zhou1, Hao Wang1, Dong sheng Xu1, Rui Yang1
1. Institute of Metal Research, Chinese Academy of Sciences

For structural materials with hexagonal close-packed structures, the stacking fault energy and the ratio of

c/a are key factors which have strong correlation with many properties, such as the critical shear stress of
dislocations and deformation twinning. We thus employed high-throughput ab initio calculations toinvestigate the stacking fault energy and the c/a ratio in hexagonal metals, Mg, Ti, Zr, Be and Zn, as wellas in alloyed Mg and Ti. The result indicates that the ratio of the unstable stacking fault energy on theprism plane over that on the basal plane, γp/γb, is primarily relevant to the c/a ratio. In Be, Mg and Zn,γp/γb increases with c/a leading to increasing preference of basal slip, whereas in Ti and Zr, γp/γbchanges unperceivably with c/a leading to the preference of prism slip. In alloyed Mg and Ti, alloying
varies both the stacking fault energy and the c/a ratio and certain alloying elements significantly affect-slip preference in Mg, while alloying exhibits negligible influence on -slip preference in hexagonal Ti.

MMM 2018

261

(E8 oral)
Role of vacancies and grain boundaries of 2D materials for thecatalytic ammonia synthesis*Qinye Li1, Lizhong He1, Chenghua Sun2,3, Xiwang Zhang1
1. Monash University, 2. Swinburne University of Technology, 3. Dongguan University of Technology

Ammonia has been extensively synthesized by human beings, dominatedly by Haber-Bosch process athigh temperature and high pressure. It results in large amount of carbon emisson and energy
consumption. This work aims to investigate the role of vacancies and grain boundaries for catalysts designused in ammonia synthesis. They are employed as active sites for the design of single-atom catalyst
design. As revealed by DFT calculations, single Fe/Co/Mo atoms can actively adsorb on these defects andoffer high performance for N2 activation. Fe-doped MoN2 has been identified as an excellent catalyst for
nitrogen reduction.

262

MMM 2018

(E8 oral)First-principles Investigation on the Stability and Oxygen Adsorption
Behavior of a Ti2AlNb/TiAl Interface*Yue Li1, Jianhong Dai1, Yan Song1
1. School of Materials Science and Engineering, Harbin Institute of Technology at Weihai

The stability of a interface between O-phase Ti2AlNb (1 -1 0) surface and B2-phase TiAl (2 1 -1) surface
was studied by first principles calculations to investigate the atomic matching behavior. A coherent twinboundary with extremely small misfit degree was found, The calculated formation energy and theelectronic structure illustrate that the interaction is strong. The adsorption of a single oxygen atom at theinterface was estimated thermally stable by adsorption energy. The present research indicates theexistence of O/B2 interface may lead to influences on both the mechanical strength and the oxidation
resistance of Ti2AlNb alloys.

MMM 2018

263

(E8 oral)
DFT predictions of hydrogen storage properties of Mg7TiX2 (X= F, O, S,
P and Cl)
*Yuying Chen1, Jianhong Dai1, Yan Song1
1. School of Materials Science and Engineering, Harbin Institute of Technology at Weihai

Stability and reversibly de/hydrogenation properties of presumed Mg7TiX2 (X= F, O, S, P and Cl) werepredicted using Density Functional Theory. Hypothetical Mg7TiX2 and its hydride are energetically stablewith respect to the individual elements involved and they may be synthesized experimentally. The stabilityof alloy highly relate to the formation energy of Mg-X compound because that distance of Ti-X is large
than that of Mg-X. The considered systems possess preferable hydrogen adsorbing capacity, and theirhydrogen adsorption energies lie in the range from -0.5 eV to -0.05 eV. The dehydrogenation of the Mg7
TiX2H14 were calculated and found that H-Mg bond and Ti-Mg bond have significant effect on
dehydrogenation process. The study of desorption energy of X atoms found that P atoms are less stable
than H atoms and will escape from system before H atoms that lead system collapse, thus Mg7TiP2 is notavailable to hydrogen storage. Our studies indicated that Mg7TiX2 (X= F, O, S and Cl) can realize
reversible hydrogen storage properties.

Keywords: Density Functional Theory; Hydrogen Storage; Stability; Reversibility; Mg7TiX2.

264

MMM 2018

(E9 invited)
The motion of a single dislocation from molecular dynamics
simulations and its physical interpretation*Yujie Wei1,2, Shenyou Peng1,2
1. Chinese Academy of Sciences, 2. School of Engineering Sciences, University of Chinese Academy of Sciences

The dependence of dislocation mobility on stress is the fundamental ingredient for the deformation in
crystalline materials. Strength and ductility, the two most important properties characterizing mechanicalbehavior of crystalline metals, are in general governed by dislocation motion. Experimentally, recording
the position of a moving dislocation in a short time window is still challenging, and direct observation to
deduce the speed-stress relationship of dislocations is still missing. Here we report the motion of an
obstacle-free twinning partial dislocation in face centred cubic crystals with spatial resolution at the
angstrom scale and temporal picosecond temporal information. The dislocation exhibits two limiting
speeds: The first is subsonic and occurs when the resolved shear stress is on the order of hundreds of
megapascal. While the stress is raised to gigapascal level, an abrupt jump of dislocation velocity occurs,from subsonic to supersonic regime. The two speed limits are governed respectively by the local
transverse and longitudinal phonons associated with the stressed dislocation, as the two types of phonons
influence dislocation gliding at different stress levels. In contrast, the kinetics of a screw dislocation is
distinct from that of edges. We demonstrate that a screw dislocation can move steadily at the speed ofshear wave velocity, or even move supersonically.

MMM 2018265

(E9 oral)
Atomistic study on the super-elasticity of single crystal bulk NiTi shapememory alloy under adiabatic condition*Bing Wang1, Guozheng Kang1, Qianhua Kan1, Wenping Wu2, Kun Zhou3, Chao Yu1
1. School of Mechanics and Engineering, Southwest Jiaotong University, 2. School of Civil Engineering, Wuhan
University, 3. School of Mechanical and Aerospace Engineering, Nanyang Technological University

The temperature-induced phase transition and the super-elasticity (from the stress-induced phasetransition) of equiatomic single crystal bulk NiTi shape memory alloys are investigated by the molecular
dynamics method. By the simulation to the thermo-mechanical response of the single crystal NiTi alloy
along the <001>B2 under the compression/unloading and an adiabatic condition, the temperaturechange and the nucleation and growth of martensite transformation during the compression/unloading
are discussed. The simulated results of molecular dynamics show that the single crystal bulk NiTi shapememory alloy exhibits a significant temperature change during the martensite transformation and its
reverse under an adiabatic condition; moreover, a localized instability occurs apparently in the process ofmartensite transformation, which is closely related to the nucleation and growth rates of martensite
phase; finally the effect of model size and strain rate on the thermo-mechanical response of the single
crystal bulk NiTi alloy is also discussed, and no instability is observed in the simulated stress-strain curves
if the model size is relatively larger, e.g., 8V0 and 13.824V0.

266

MMM 2018

(E9 oral)
Atomistic Modelling of Fracture with Non-Linear Elastic Boundary
Conditions*Punit Patel1, James Kermode1
1. University of Warwick

Atomistic simulations of crack propagation are key to understanding the fracture behaviour of materials.
Cracks involve strong coupling across the scales, with bond breaking on the quantum scale driven by longrange stress fields. Current QM methods are limited to simulation sizes too small to accurately describe
fracture dynamics and improved QM/MM methods are able to adequately capture elastic effects [1].
However these methods are still heavily limited temporally and to coarse grain into the continuum scale
while maintaining the overall fracture dynamics an expanded multiscale approach is required.
Applications in covalently bonded single crystals to complex alloys are underway. Screened classicalpotentials [2] predict a brittle response in both silicon carbide (SiC) and diamond, giving confidence intheir applicability. DFT SiC surface energy calculations produced predictions in good agreement withexperiment [3]. Extension of a multiscale approach for fracture includes a novel approach in correcting for
finite domain boundary conditions, in which a non-linear continuum boundary correction is applied tohelp reduce the required system size, in order to compute energy barriers for crack extension bridging
DFT and long time scale MD [4].

[1] N. Bernstein et al., Rep. Prog. Phys. 72, 026501 (2009).
[2] L. Pastewka et al., Phys. Rev. B. 87, 205410 (2013).
[3] G. Sernicola et al., Nat. Commun. 8, 108 (2017)[4] P. Patel, L. Pastewka, C. Ortner and J. R. Kermode, In Prep, (2018)

MMM 2018

267

(E9 oral)
Atomistic simulation of the deformation and crack nucleation
mechanisms in titanium alloys*Dongsheng Xu1
1. Institute of Metal Research, Chinese Academy of Sciences

Titanium alloys are widely used in aero-space and chemical industries, but due to the complicatedhierarchical microstructure formed in these systems, the deformation and fatigue life control is not easy.
Multi-scale simulations were carried out on the deformation and microstructure evolution of titanium
alloys in order to understand deformation and fatigue behavior under different conditions. The dislocation
nucleation, interaction, debris accumulation and their effects on the deformation and fatigue crack
nucleation were investigated. It was shown that, upon cyclic deformation, point defects and small
dislocation loops can be formed in slip band by dipolar reaction. This will consume mobile dislocations,and make the operating slip system hardened; at the same time, they can provide deformation mechanism
upon further increase of the stress. Comparing with perfect lattice, the lattice with point defects under
tension perpendicular to the slip plane can be weakened by these point defects, therefore they may serve
as fatigue crack nucleation sites and propagating paths. These understanding gained by simulations may
provide important clue to the fatigue resistance design for titanium alloys.

268

MMM 2018

(E10 oral)
Study of atomic trajectories during twinning transformation inmagnesium*Andrej Ostapovec1, Tomáš Káňa1, Václav Paidar2
1. Institute of Physics of Materials ASCR, 2. Institute of Physics ASCR

Deformation twinning is important mode of plastic deformation in magnesium and its alloys. Study oftwinning mechanisms has long tradition. However, there is still opened questions in this field. There issome speculations in the literature, which connects possibility of non-Schmid behavior of mechanicaltwinning to type of atomic trajectories during twinning transformation. However, such theoreticalconsiderations often use simplified descriptions of atomic motions. For instance, some authors madeconclusions about macroscopic behaviour of twinning on the basis of linear or circular atomic trajectories.
In a broader picture, real atomic paths should be quite complicated. They are consequence of
disconnection glide along twin boundary. In this study we would like to discuss atomic trajectories
obtained from modeling using EAM potentials as well as from ab-initio calculations.

MMM 2018

269

(E10 oral)
Understanding nanocontact plasticity through massive MD simulations*Jorge Alcala1, Javier Varillas2,1, Jordi Torner2, Jan Očenášek1
1. Universidad Politecnica de Catalunya, 2. New Technologies Research Centre

This presentation concerns comprehensive molecular dynamics (MD) simulations of nanocontact
plasticity in body-centered cubic (BCC) and face-centered cubic (FCC) crystals. The main focus is on theunderstanding of the evolution of the nanoscale material hardness with increasing tip penetration throughdetailed analyses of the distinct defect nucleation processes which result in the formation of a permanent
imprint. It is shown that the gradual development of an entangled defect structure essentially governs theevolution of material pileup at the contact boundary, a feature that explains the different FCC and BCC
imprint morphologies and topographies that develop depending on loading orientation. The present
analyses provide a fundamental background to understand why BCC surfaces are harder than FCC
surfaces at the nanoscale, including the role of the elastic response of the indented crystals. Novel MD
simulations for crystals containing a preexisting (dense) dislocation network further confirm the pivotal
role of the incepted defects upon indenter tip penetration. Our analyses contribute to the understandingof indentation size effects in submicrometer-sized material volumes, where a physical rationale to thevalidity of strain gradient plasticity and geometrically necessary dislocations is essentially lacking. Finally,
our investigation leads to the finding of a general correlation between the nanohardness and the yieldstrength measured at the nanoscale under uniaxial loading conditions.

270

MMM 2018

(E10 oral)
Atomistically informed mesoscale modeling of fracture*Hamad ul Hassan1, Johannes Möller2,3, Erik Bitzek2, Alexander Hartmaier11, Rebecca Janisch1
1. ICAMS, Ruhr-Universität Bochum, 2. Friedrich-Alexander Universität Erlangen-Nürnberg , 3. Fraunhofer Institut fürWerkstoffmechanik, Freiburg

At the engineering scale, continuum mechanics provides an efficient way to model fracture based onstresses, stress intensity factors, and energy release rates. At the atomic scale, in contrast, the breaking ofatomic bonds is caused by critical forces acting on individual atoms. Therefore, a scaling methodology isrequired to apply information from atomistic scale to the continuum level problems, which are governedby engineering length and time scales.
Fracture at the continuum level can be described using state of the art methods such as cohesivezone-based modeling, which requires a material-specific traction-separation law (T-S law). Here, we
present a mesh independent approach for atomistic-to-continuum level scaling of the stress anddisplacement measures, which are used in the T-S law. Our approach is based on a detailed analysis of
the forces acting between the atoms in front of a crack tip, as well as between two semi-infinite
half-crystals. Such a constellation is used to calculate the properties of cohesive zones based on atomistic
simulations. The analysis shows, that the interatomic forces at a crack tip can be directly related to therestoring tractions between the two planar surfaces. This allows for an unambiguous scaling of the criticalstresses and displacements, from GPa / Åon the atomic level, to the order of hundreds of MPa and nm on
the mesoscale. A series of finite element simulations are performed for KI loading based on the scaled
input data for the T-S law and the critical stress intensity factors are calculated and compared with resultsfrom the atomistic simulations. We demonstrate and examine the ability of the atomistically informed
finite element simulation to directly reproduce results from atomistic simulations.

MMM 2018

271

(E10 oral)
Lattice Distortion Effect on Cross-Slip in High Entropy Alloys andLennard-Jones Systems for Face-Centered Cubic*Chao-Chun Yen1, Junping Du2,3, Jia-An Lin4, Yu-Chieh Lo4
1. National Tsing Hua University, 2. Kyoto University, 3. Osaka University, 4. National Chiao Tung University

The superior mechanical properties of high-entropy alloys (HEAs) make an outstanding success inmaterial science and engineering. In recent years, more efforts have been devoted to the effect of severe
lattice distortion, which is one of the core effects of HEAs. Previous studies have shown the impacts ofsevere lattice distortion on plastic deformation, including the influences on nucleation and propagation of
dislocation. One of the most common mechanism is cross-slip in metals, which is a fundamental processof screw dislocation motion and plays an important role in dislocation annihilation and work hardening.
However, there is no sufficient reference so far that can provide a clear correlation between the cross-slip
and lattice distortion in HEAs. This may result from the difficulties of finding the cause of local latticestrain through experimental approach. Nevertheless, atomistic simulations can overcome the dilemma.
Here, we create a binary system containing two different sizes of atoms. In order to focus on the lattice
distortion caused only by size difference, a large-scale molecular dynamics simulation is performed using
Lennard-Jones potentials to provide a size-controllable system where different sizes of atoms are assumed
to be the same chemical potential. Therefore, we can systematically discuss the influence of the lattice
distortion on cross-slip of HEAs. Furthermore, we apply nudged elastic band method on modified
embedded-atom method potentials of CoCrFeMnNi and Lennard-Jones potentials to calculate theactivation energy of cross slip for screw dislocation respectively, and compare both results for further
investigation.

272

MMM 2018

(E10 oral)
Atomic structure of gamma/alpha2 interface and its influence onplastic deformation of lamellar TiAl alloys*Hao Wang1, Aidong Tu1, Yanxia Liu2, Dongsheng Xu1, Rui Yang1
1. Institute of Metal Research, CAS, 2. Dept. of Physics, Liaoning University

TiAl alloys with the γ-TiAl/α2-Ti3Al lamellar structure exhibit excellent high temperature performance.
However, the strong anisotropy of the lamellae leads to the strong dependence of mechanical properties,
in particular the fatigue behavior, on lamella orientation, thickness, volume fraction, etc. In the present
work, molecular dynamics with the embedded-atom potential is employed to investigate the energy ofboth the coherent and incoherent γ/α2 interfaces. The interface coherency is found to depend on thevolume ratio of the γlamellae over the α2lamellae, resulting in a critical volume ratio, below/above which
the interface is coherent/incoherent. Loading perpendicular and parallel to the lamella interface indicatesthat the yield strength of coherent interface is higher than that of the incoherent interface. Plastic
deformation mostly occurs first in the γregion and transmits to the α2 region via dislocations or twinning
across the γ/α2 interface. Subsequent fracture behavior also depends on the γ/α2 volume ratio. With
the coherency and incoherency interfaces, crack nucleates within the γregion and at the γ/α2 interface,respectively. The present result thus contribute to the evaluation of the structural stability and theimprovement of mechanical performance of TiAl alloys.

MMM 2018

273

(E11 invited)
Thermodynamically consistent directional distortional hardening of
wrought Mg alloys: experimental investigation and constitutive
modeling*Baodong Shi1, Yan Peng1, Chong Yang1, Jianliang Sun1, Fusheng Pan2
1. NECSR, School of Mechanical Engineering, Yanshan Univ., 2. NERC-MA, Chongqing Univ.

As a lightweight structural material, magnesium alloy exhibits strong anisotropy due to texture, which
limits its use in energy-saving lightweight structures. This pronounced anisotropy cannot be captured byonly classical isotropic or kinematic hardening due to the constant shape evolution of yield surfaces
during plastic deformation. Therefore, the shape evolution of yield surface, named as distortional
hardening is the main approach to capture the anisotropic behavior of Mg alloy. Moreover, stress state ischanging during forming process such as deep drawing, i.e., the loading history effect during plastic
deformation cannot be ignored. Thus, focusing on loading path dependent distortional hardening of AZ31
Mg alloy, experimental investigation on the evolution of anisotropy is performed, moreover, a constitutive
model with distortional evolution of yield surfaces is developed based on elasto-plasticity theory at finite
strain. Thermodynamical consistency is proved. The anisotropic mechanical behavior of AZ31 Mg alloy isdemonstrated after model parameters calibrated with experiments.

274

MMM 2018

(E11 oral)
A computational infrastructure for multiscale materials simulation*Xiaoyu Yang1
1. Computer network information center of the Chinese Academy of Sciences

Bridging materials models and passing materials-related data and information across length scales iscritical for the quantitative and predictive modelling of materials development. However, lacking of
acceptable linkage software and tools is an obstacle. It is recommended in the report by the Minerals,
Metals & Materials Society that “establishing an infrastructure for multiscale materials data is the firstprogrammatic recommendations”. Currently we are developing such an infrastructure to address thisneed based on a high-throughput computational infrastructure MatCloud[1]. The core of this multiscale
simulation infrastructure is a linking software tool and technologies that contains a workflow designer anda workflow engine. The workflow designer aims to support the design of simulations involving running
quantum mechanics scale ABINIT simulation and molecular scale LAMMPS simulation by drag & drop
approach. The workflow engine is responsible for parsing and processing the workflow of multiscalematerials simulation created by the workflow designer. As a test case, the multiscale materials simulation
linking tool will be used to develop a particular interatomic potential through data-driven approach. In
this talk, we will report our progress and challenges encountered.
Reference:[1]. Yang X. et al , MatCloud: A High-throughput Computational Infrastructure for Integrated Managementof Materials Simulation, Data and Resources, Computational Materials Science, Volume 146, 15 April
2018, Pages 319-333, https://doi.org/10.1016/j.commatsci.2018.01.039

MMM 2018

275

(E11 oral)
Using machine learning methods to homogenize damage from microto macroscale
Denise Reimann1, Hamad ul Hassan1, Tobias Glasmachers2, *Alexander Hartmaier1
1. ICAMS, Ruhr-Universität Bochum, 2. Institut für Neuroinformatik, Ruhr-Universität Bochum

Micromechanical models are in general able to describe microstructural influences, such as texture andgrain size distribution, on damage evolution. A homogenization from the micro to the macro scale is,however, conceptually demanding. Hence, a new approach involving machine learning algorithms issuggested. In this work, numerical data based on micromechanical simulations is used to train themachine learning algorithm, which in turn describes macroscopic damage evolution as function of loading
conditions and microstructure. The micromechanical simulations are based on representative volume
elements (RVE) of realistic microstructure models, using crystal plasticity and damage mechanics todescribe plastic deformation and damage evolution on the grain level.
Local quantities from these RVE simulations, such as stress, strain and damage, are homogenized into
global averages. The trained machine learning algorithm is then able to predict global damage evolution
as a function of the macroscopic loading state (e.g. equivalent strains, equivalent and hydrostatic
stresses), elastic-plastic material properties and microstructure information (e.g. grains size distribution,
crystallographic texture, etc.). The results are compared with well-accepted closed-form damage models
such as Chaboche or Lemaître for validation. Furthermore, different machine learning algorithms such as
artificial neural networks, support vector machines and random forest are used and their results arecompared with each other as well.

276

MMM 2018

(E11 oral)
Graph theory analysis of rich fiber-scale data yields very fast
simulations of damage evolution in composites
jerry quek2, *brian n cox1
1. arachne consulting, 2. IHPC

Simple mechanistic arguments suggest that the loci of cracks that initiate and propagate amongfine-scale, stochastic, heterogeneous material domains might be predictable by fast graph theoretic
methods, without executing explicit fracture mechanics simulations. Here we explore the usefulness ofgraph theoretic methods for analyzing the microcracking that develops in continuous fiber composites
loaded transversely to the fiber direction. We use graph theoretic methods to analyze rich data publishedelsewhere for two types of fiber composite. The data describe the irregularity of the spatial distribution of
the fiber population, and meandering of fibers within the population, which is seen when the fibers aretracked along the nominal fiber direction. Graph analysis yields very fast predictions of the likely sites ofcrack initiation in the fiber composite, as well as plausible indications of the likely direction in which an
initiated crack will grow, the developing shape of the crack, and the frequency of instances of fibers thatwill bridge the crack obliquely, thereby raising the composite fracture resistance. From the results of thegraph analysis, we infer a stochastic population of effective defects in the composite, whose location andeffective strength are related to the Euclidean and topological characteristics of the fiber population in
the vicinity of the defect. We propose that, if the predicted distribution of defects is entered as an initialmaterial condition in an homogenized finite element simulation of the composite, then the important
effects of the stochastic fiber distribution, in regard to crack initiation, preferred directions of growth, andtoughening due to fiber-bridging, can be captured in a simulation of tractable size. This strategy carries
the pertinent spatial information content of any measured random distribution of fibers into a simulationin which the fibers are not represented explicitly.

MMM 2018

277

(E11 oral)
A deep learning-based constitutive model for finite element analysis
Fariborz Ghavamian1, *Angelo Simone2,1
1. Delft University of Technology, 2. University of Padova

A deep learning model is simply a function that maps a given input to a desired output. In spite of thesimple nature of a deep learning model, over the past decade it has revolutionized natural language
processing (NLP) and image understanding (IU). One can name three fundamental reasons to explain thisastonishing accomplishment: 1) availability of large amount of data for NLP and IU over the past decade;2) the fact that the performance of a deep learning model keeps increasing by increasing the training data
(unlike other machine learning models); and 3) the recently available hardware and software
infrastructures which are tailored for deep learning.
In this contribution we leverage the power of deep learning to construct a deep learning-based
constitutive model. More specifically, we develop a novel neural network architecture which is tailored for
simulations of history-dependent material behavior, and we discuss its implementation in a finite element
model.
A possible application of this deep learning-based constitutive model is to serve as an efficient surrogate
to a representative volume element (RVE) in a multi-scale finite element analysis. In this application large
amount of training data can readily be generated from the RVE. As a proof of concept, we train our deeplearning model on a set of data collected from a viscoplastic constitutive model and employ it a finite
element model. Through numerical experiments we show that the structural level response of the finite
element model is adequately approximated when using the deep learning-based constitutive model inplace of the viscoplastic one.

278

MMM 2018

(E12 oral)
Diffusivities and atomic mobilities in bcc Ti-Mo-Nb-Ta-Zr alloys*Weimin Bai1, Guanglong Xu2, Libin Liu1, Ligang Zhang1
1. School of Materials Science and Engineering, Central South Univ., 2. Tech Inst. for Advanced Materials & School ofMaterials Science and Engineering, Nanjing Tech Univ.

Titanium and its alloys are widely used in biomedical field due to the low elastic modulus comparable tobone as well as high specific strength and high corrosion resistance. β-type (with bcc structure) and nearβtitanium alloys comprising non-toxic and non-allergic elements have been extensively investigated in
the past decades to achieve low Young’s modulus and good mechanical properties. Researchers
suggested that Zr, Nb, Ta and Mo are the most desirable alloying elements in β-type bio-Ti alloys whichhave ability to effectively increase strength and reduce elastic modulus according to electronic structural
calculations. Recently, a number of low-modulus biomedical β-titanium alloys have been developed, forexample, Ti–29Nb–13Ta–4.6Zr (TNTZ), Ti–24Nb–4Zr–7.9Sn and Ti–8Mo–4Nb–2Zr.
Diffusivities play important roles when we manipulate above phase transformations and control
microstructure development. The diffusion paths and phase fractions during homogenization andprecipitation are precisely predicted with the help of thermodynamic and kinetic data by means of the
Thermo-Calc and DICTRA software [2,3]. The microstructure evolution during heat treatment can be not
only statistically explained via the classical nucleation and growth model equipped with accurate
diffusivities, but also represented by the phase field modeling allying with diffusion kinetic database ofmulti- component and multi-phase systems.
In this work, the diffusivities of sub-ternary systems in Ti-Mo-Nb-Ta-Zr were extracted from composition
profiles of diffusion couples using Whittle-Green [4] and Hall [5] methods. Based on the experimental
results and thermodynamic descriptions, a self-consistent atomic mobility database of bcc
Ti-Mo-Nb-Ta-Zr alloys were assessed using DICTRA software. All the kinetic descriptions were further
verified by comprehensive comparisons between various model-predicted diffusion properties and theexperimental data. The general agreement validates the potential application of the present atomic
mobility database to simulate the diffusion in higher orders.
[1] J.-O. Andersson and J. Ågren, J. Appl. Phys. 72 (1992) 1350–1355.
[2] J.-O. Andersson et al., Calphad. 26 (2002) 273–312.
[3] D.P. Whittle and A. Green, Scr. Metall. 8 (1974) 883–884[4] L.D. Hall, J. Chem. Phys. 21 (1953) 87–89.

MMM 2018

279

(E12 oral)
Design of Ti-alloy by Integrating High Throughput Experiments andCalculations*Ligang Zhang1, Libin Liu1, Di Wu1
1. Central South University

The Speed for development of next materials is too slow and it has been emerging as the bottleneck forthe innovation of the manufacturing technology. However, application of the computer and information
technology to the materials science and engineering has made it possible for us to estimate the properties
for single phases, model the microstructure evolutions, and predict the material properties. On the other
hand, in order to verify the calculation results, we should develop and use the high throughput methods.
In this talk, we introduce some new progress in materials calculation and high throughput experiments,
especially the high throughput determination of the phase diagram, diffusion coefficients, andthermal-physical properties, and the high throughput verification of the response of the materials
microstructure and properties to the compositions and heat treatment temperatures. Some preliminary
results on the attempt of development of high strength and high toughness Ti alloys has been introduced.

280

MMM 2018

(E12 oral)
Simulation of Plasticity in Amorphous Solids*Shingo Urata1, Takahiro Murashima2, Shaofan Li3
1. Innovative Technology Research Center, Asahi Glass Co., Ltd (AGC)., 2. Dept. of Physics, Tohoku University, 3.
Dept. of Civil and Environmetal Engineering, Univ. of California Berkeley

A Multiscale model to simulate mechanical response of amorphous solids has been developed bycoupling molecular mechanics and finite element method. In this method, an atomistic-based
representative sampling cell (RS-cell) is embedded into each element to represent inelastic deformationsin amorphous materials. Because the method employs a Parrinello-Rahman molecular dynamics based
Cauchy-Born rule to construct an atomistically-informed constitutive model at continuum level, it ispossible to quantitatively measure amorphous plastic deformations. In other words, the method
intrinsically embeds a potential shear-transformation-zone (STZ), and thus the evolution of RS-cells cannaturally allow molecular clusters having irreversible microstructure rearrangements at microscale inresponse to applied loads without using any phenomenological modeling. By using the proposed method,
we obtained inelastic hysteresis loops for the amorphous materials under cyclic loading and also shear
band formation at macroscale by using the Lennard-Jones binary glass (LJBG) model. In addition, we
extended the method to apply to silicate glasses by considering electrostatic interaction. We would also
demonstrate fracture simulation of the oxide glasses by coupling the method with Multiscale Cohesive
Zone Model.

MMM 2018

281

(E12 oral)
Computational generation of the yield surfaces using stress basedloading*mayank chouksey1, Sumit Basu1
1. Indian Institute of Technology, Kanpur

Ductile fracture in metals proceeds through nucleation, growth and coalescence of microvoids.
Determination of the yield surface of a ductile porous material is necessary to simulate onset andevolution of ductile damage in a metal. Since the early work by Gurson where yield function for concentric
spherical void inside a spherical RVE was considered, several extensions have emerged. It has beenshown that the void shape and anisotropy of the bulk matrix modify the yield function significantly. All
these predictions of yield rely on limit analysis of the RVE based on the upper bound theorem and
Hill-Mandal homogenization. These models have not accounted for onset of yield through localization ofplastic strain.
Recently the competition between localization and uniform yield via the Gurson model has been
compared. We have extended this work by performing stress controlled computational homogenization
over sub-spaces of the principal stress space, to probe the yield surface of perfectly plastic materials withdifferent void shapes. A special four-noded user element is developed in ABAQUS, that, is tied to a RVE
with periodic boundary condition, restrained rigid body rotation and subjected to either macroscopic
deformation gradient or Cauchy stress. The macroscopic plastic dissipation rate is monitored to detect
yield. As a result, yield due to both uniform plastic deformation and localization are captured. We havecompared the results of our computationally determined yield surfaces with the theoretical upper boundestimates. We show that over a significant section of the principal stress sub-space, localization modifies
the yield surface. For prolate voids, localization hastens yield while for oblate it is delayed over estimatesprovided by models based on the upper bound theorem. The comparison gives us a rich insight into thecompetition between macroscopic yield through uniform proliferation of plasticity and localization.

282

MMM 2018

(E13 invited)
3D Aspects of Fracture in Crack –Obstacle Interactions and Effects of
Crack Front Curvature*Erik Bitzek1, Polina Baranova1, Johannes J. Möller2
1. FAU Erlangen-Nuernberg, 2. Fraunhofer IWM

Traditionally, fracture mechanics mainly considers 2D problems in which the crack front is approximatedby a straight line. At the nanoscale, cracks can, however, be strongly curved, e.g., in the case of crack
nuclei or as a result of the interaction of a propagating crack with obstacles. Nanoscale crack front
curvature can significantly influence the fracture behavior of semi-brittle materials that show abrittle-to-ductile transition with increasing temperature, like refractory metals, intermetallics orsemiconductors. Their fracture behavior is ultimately determined by the competition between the
dynamics of the atomic bond-breaking processes and dislocation activity in the direct vicinity of the crack.
The relative orientation of the local crack front to possible dislocation slip systems can therefore have amajor impact on crack tip processes.
Here we present the results of our atomistic simulations of straight cracks in W, Fe and NiAl interactingwith various obstacles. Cracks interacting with individual pre-existing lattice dislocations showedstimulated dislocation nucleation and new crack tip blunting mechanisms. Voids were shown to efficiently
stop propagating cracks by impeding the re-nucleation of a sharp crack and by facilitating dislocation
emission. Fully-3D simulations of penny-shaped cracks revealed an increased tendency for crack tip
plasticity compared to straight cracks due to the availability of more slip systems and the resulting
dislocation –crack interactions. The results are discussed in the context of the development of predictive
multiscale models for fracture toughness.

MMM 2018

283

(E13 oral)
Vortex instabilities in the deformation of Cu|Au nanolaminates
Adrien Gola1,2, Peter Gumbsch2,3, *Lars Pastewka1,3,2
1. University of Freiburg, 2. Karlsruhe Institute of Technology, 3. Fraunhofer IWM

Vortex-instabilities are typically associated with fluid-type flow patterns, such as the well-known
Kelvin-Helmholtz instability that can be directly observed in cloud patterns. Recently, Vortex-typeinstabilities have been observed during the deformation of multilayered materials, both with nano- [1] andmicrometer-sized [2] layers. We here use molecular dynamics simulation to investigate Vortex formation in
Cu|Au nanolaminates. Our simulations reveal an instability that leads to corotation of neighboring regionsof the nanolaminate, giving rise to deformation pattern reminiscent of Kelvin-Helmholtz instabilities.
Contrary to the latter instabilities, the plastic Vortex formation occurs at strains below 2 which appears
compatible with strains experienced during frictional loading [1]. We compare our molecular dynamics
results with deformation patterns generated through ideal plastic and fluid mechanical models.
[1] Luo, Z.-P., Zhang, G.-P. & Schwaiger, R. Microstructural vortex formation during cyclic sliding of Cu/Au
multilayers. Scr. Mater. 107, 67–70 (2015).
[2] Pouryazdan, M., Kaus, B. J. P., Rack, A., Ershov, A. & Hahn, H. Mixing instabilities during shearing of
metals. Nat. Commun. 8, 1611 (2017).

284

MMM 2018

(E13 oral)
Molecular dynamics simulation of the interaction between grainboundary and point defects*Liang Zhang1, Yasushi Shibuta1, Cheng Lu2
1. The University of Tokyo, 2. University of Wollongong

Molecular dynamics simulations were carried out to investigate the dynamic interaction between different
grain boundaries (GBs) and two types of point defects, namely the stacking fault tetrahedron (SFT) andthe void. The GB can migrate itself under the shear strain and can serve as a sink to remove SFT and void.
The sink efficiency of GBs is sensitive to their structural characteristics, the size of point defects, andtemperature. The high-angle GBs can show a great ability to remove the point defects even at a lowtemperature, while the increase of temperature can facilitate the annihilation of the point defects at the
low-angle GBs. The simulation reveals a new possible GB-mediated damage healing mechanism ofirradiated materials. In particular, the nanotwinned metals are generally anticipated to be less effective in
the alleviation of radiation damage because they contain mostly coherent twin boundaries, which are
low-energy boundaries and are inefficient defect sinks in irradiated metallic materials. However, recent insitu studies have indicated that some nanotwinned metals exhibit unprecedented radiation tolerance, andthe unexpected self-healing of twin boundaries in response to radiation was observed. In this work, we
proposed two possible self-healing mechanisms of twin boundaries by considering the defective coherent
twin boundary structures which contained incoherent twin segments or self-interstitial atoms. The
mechanisms were confirmed and atomistic evidence was provided by carrying out the long-time
molecular dynamics simulations.

MMM 2018

285

(E13 oral)
Deformation mode analysis by the eigenvectors of the atomic elastic
stiffness*Kisaragi Yashiro1
1. Gifu University

Inspired by the Wang’s stability criteria based on the “elastic stiffness” coefficients, we have proposedto evaluate the local stability by the atomic elastic stiffness matrix and discuss the onset of local
deformation such as dislocation nucleation, crack propagation, etc, with the positive definiteness of thematrix. The 6x6 matrix of the elastic stiffness correlates the stress and strain in the 6 dimensional strain
space; thus the loss of the positiveness means the emergence of the unstable deformation path. The
stiffness matrix is not identical to the “elastic coefficients” or usually named as “elastic constants”
except in the no-load equilibrium or in the linear elasticity. In the thermodynamics of crystal lattices, thestress and the elastic coefficients are defined as the 1st and 2nd order derivatives of the internal energy
per unit volume, respectively, and the stiffness in the nonlinear region is expressed with the elastic
coefficients plus the stress contribution. In the atomic simulations based on the interatomic potentialfunction, the internal energy is the sum of each atom contribution so that we can define the atomic stressand atomic elastic coefficients, then evaluate 6x6 stiffness matrix and its eigenvalue for each atom. The
negative eigenvalue can be found on the surface and various lattice defects as already reported, then weadvanced our instability analysis with the corresponding eigenvector of the atoms with negative
eigenvalue. That is, the principal strain axes of the 3x3 strain tensor are evaluated as unstable
deformation path for the atoms with negative eigenvalue and corresponding eigenvector of 6 strain
components. In the case of crack initiation from the surface at the equator line of circular hole in thebcc-Fe under [110] tension, the deformation mode at the crack tip is normal to the (010) cleavage plane
although the resulting crack opening occurs in the (110) planes. Another example is the dislocation
emission from same circular hole under [111] tension, the deformation modes at the dislocation core are
not only parallel to the [112] slip plane but also large normal mode corresponding to the Peierls barriers.
In the presentation, we demonstrate various application of our deformation analysis to bcc-Fe, hcp-Mg,
and 3C-SiC etc.

286

MMM 2018

(E14 oral)
Stability Controlled Crack Evolution in Staggered Laminate
Bio-material*Yi Yan1, Akihiro Nakatani1
1. Dept. of Adaptive Machine Systems, Osaka University

Nacre that is a bio-material with a staggered laminate micro-structure of hard and soft constituents showsa good performance of strength, stiffness, and toughness. The toughness of nacre with 95% hard andbrittle phase and less than 5% soft collagen phase remarkably increases with cracking. Recent researchshows that cracking in the bio-material is a stable process in macroscopic point of view, in which thefracture process zone widely spreads and in space, rather than the unstable process in many
brittle/quasi-brittle materials, and that the high performance of the material is achieved by a shear
ligament toughening mechanism. In this study, we solve a problem on cracking process under tensile testof model specimen with staggered micro-structure by using FEM and a simplified model, and discuss therelationship between the macroscopic behaviour and the fundamental properties of micro-structure. We
find that the crack initiation in such staggered micro-structure originates from the loss of macroscopic
stability and the following strain concentration. Coupling the normal debonding and shear sliding at
interface of hard phases, a stable behaviour is achieved globally before a globally unstable point instress-strain space, in which locally unstable domains do not contribute to the macroscopic instability.
This mechanism can be considered as a stability enhancement, i.e. a kind of toughening. The cracking
behaviour of our model shows size effect. The macroscopic stable stage is significantly affected by boththe model and grain size of hard phase. Our research offers a different angle of view in exploring the
microscopic mechanism of cracking process of staggered laminate bio-material.

MMM 2018

287

(E14 oral)
A brittle to ductile transition modeling for liquid metal embrittlement*Thierry Auger1
1. CNRS

Liquid metal embrittlement is a surface adsorption induced fracture phenomenon that is currently noteasily amenable to modeling. A strategy framed in terms of a competition between dislocation emission
and Griffith fracture at the crack tip will be presented within the standard brittle to ductile transition
realm. The goal is to be able to qualitatively predict the sensitivity to LME by coupling atomic scalemodeling with a semi-continuous approach. The brittle to ductile transition description of an elasticallyloaded crack tip has been adapted to LME that relies on solid-liquid surface energy modeling at theatomic scale by AbInitio molecular dynamics and generalized stacking faults modeled at the atomic scale
within the Peirls-Nabarro framework. We will present preliminary results on a model system for LME and acomparison with experimental results in terms of sensitivity.

288

MMM 2018

(E14 oral)
Buckling delamination of ductile thin films on rigid substrates*Nadia Ben Dahmane1, Guillaume Parry1, Rafael Estevez1
1. Univ. Grenoble Alpes, SIMAP, F-38000 Grenoble

Thin film coatings submitted to high compressive stresses may experience a simultaneous buckling anddelamination phenomenon called "blistering". The mechanism of formation and propagation of blisters in
the form of straight wrinkles and circular blisters has been extensively studied in the literature consideringa linear elastic behaviour of the film [1,2] . The recent developments in numerical calculations allowed abetter understanding of the mechanism of formation and propagation of complex buckling geometries
including wavy buckles. In particular, the relationship between the mode mixity dependent interfacial
toughness and the morphology of the wavy buckles has been elucidated [3].
However, up-to-date, the response of ductile thin films deposited on rigid substrates remains an open
issue. For instance, it has been evidenced experimentally [4] that circular blisters in ductile thin films
exhibit larger folding angles at their base compared to the elastic model predictions. In addition, recent
experimental observations of 400nm gold films deposited on silicon wafers showed straight buckles withhigher deflections compared to the elastic model. These differences in morphology are thought tooriginate from the elastic-plastic response of the film but the governing features need to be clarified.
In this work, we are interested in the observation and characterization of buckling structures observed ongold films deposited on silicon substrates. In this context, we carried out a Finite Elements simulations
with a model that accounts for isotropic yielding and the non-linearity of the film. A mode mixity
dependent cohesive zone model is used to describe the thin film/substrate interface. This model allowed
us to highlight the effect of plasticity on the equilibrium profiles resulting from elastic-plastic blistering, forboth straight and circular blisters morphologies. In particular, a stabilizing effect of the circular blister
form, which has been observed experimentally, has been demonstrated through calculation.

[1] Hutchinson et al., Adv. in Appl. Mech. 29 (1992) 63.
[2] Hutchinson et al. Acta Metallurgica Materialia, 40 (1992) 295[3] Faou et al., J. Mech. Phys. Sol., 75 (2015) 93.
[4] Coupeau et al., Thin Solid Films 469 (2004) 221.

MMM 2018

289

(E14 oral)
An analytical model of the peeling forces at edges of multilayers
subjected to temperature variations*Chengyin Zhang2, Neng-Hui Zhang1,2
1. Department of Mechanics, College of Sciences, Shanghai University, 2. Shanghai Key Laboratory of Mechanics inEnergy Engineering, Shanghai Institute of Applied Mathematics and Mechanics, Shanghai University

Due to different mechanical properties of various materials in a multilayered beam or plate, there is athermal mismatch between layers when the structural temperature changes. The interfacial peelingstresses and interfacial shear stresses caused by the thermal mismatch can result in a delamination
between layers, especially in the edge regions, and this can lead to the damage of a layered structures.
Because of the complexity of the boundary stresses, it is difficult to obtain exact a closed-form solution forthese interfacial thermal stresses in the edge regions. Therefore, instead of the precise stress field, we
divert to the interfacial peeling moment integrated by the localized interfacial normal stress and theinterfacial shear force integrated by the localized interfacial shear stresses. The in-plane stresses in termsof Zhang’s two-variables are integrated across the cross section in the far-field region in order toformulate the interfacial peeling moment and interfacial shear force in the edge regions for themultilayered beam system. To verify the applicability of the analytical model of the peeling forces(including interfacial peeling moment and shear force), an aluminum-silicon bimaterial beam in electronic
devices is taken as a calculating sample. The influence of the thickness and Young’s modulus of the filmon the peeling forces are discussed. And the analytical predictions have a good agreement with theresults from a finite-element analysis.

290

MMM 2018

(E14 oral)
Characterization and multi-scale modeling of the mechanical responseof the human humerus under dynamic loading*Jamila Rahmoun1, Hakim Naceur1, Pascal Drazetic2, Christian Fontaine3
1. LAMIH-ENSIAME, Valenciennes University, 2. LAMIH-IUT, Valenciennes University, 3. Lab. d'anatomie, Lille
University

The relevance and biofidelity level of the human numerical models are key issues in car accidents related
trauma research. To limit the risk of injuries of upper extremities and plan the preventive intervention, thehumerus biomechanical properties must be correctly assessed. However, the constitutive laws used arelargely derived from experimental characterizations carried out at the macroscopic scale without taking
into account the bone architecture. A multi-scale approach coupled with nanoindentation experiments
revealed to be more appropriate when the robustness of computation and accuracy of results are ofinterest.
In this study, we propose a multi-scale approach for the accurate characterization and modeling of themechanical behavior of the human humerus under dynamic loading. The present model is based on the
coupling between the Mori-Tanaka homogenization scheme, and an isotropic elastic damage model in thethermodynamic framework. In order to consider the strain rate effect on the humerus behavior, thestandard model of Johnson-Cook is adopted. The obtained model is implemented using a User Material
routine within the code LS-DYNA. The validity of the resulting FE model has been validated by comparing
numerical predictions with experimental observations from characterization tests at different
length-scales. To this end, a first experimental campaign was undertaken by means of nanoindentation
tests on prismatic samples extracted directly from the same humerus diaphysis in order to determine its
microscopic elastic properties. Then, local measurements of the damage effects were performed using
several tension/compression and bending tests on small humerus specimens. Once the humerus material
parameters were determined, a set of global validation bending impact tests composed of nine humeri
were carried out using a drop tower for the determination of the mechanical response and the damage
growth until complete humerus fracture.
The outcome of the proposed multi-scale model appears to correctly predict the general trends observed
experimentally via the good estimation of the humerus ultimate impact load. The fracture patterns
predicted by the proposed damage model are consistent with the physical humerus rupture even if thismodel is limited only to the estimation of the failure initiation.

MMM 2018

291

(P1-21)Large-scale molecular dynamics simulations: coupling with dislocation
dynamics*Pavel A. Pokatashkin1, Denis K. Ilnitsky1, Alexei V. Yanilkin1
1. Dukhov Research Institute of Automatics (VNIIA)

Various dislocation-related mechanisms: phonon drag, forest-harderning, thermal activated processes(climb, cross-slip) contribute to material properties e.g. strength. Until recently, studying of deformation
via molecular dynamics (MD) considered simulations of only extremely high strain-rates. However
contribution of various mechanisms might change significantly while proceeding to lower strain rates.
Therefore the accuracy of dislocation dynamics (DD) models used for large strain rate range is an open
question due to extrapolation.
Recent advances in computational powers increased both spatial and temporal scales available foratomistic modeling. Therefore it is possible to make coulping between MD and DD for strain rates ~107 s-1
and higher. We consider such materials as: iron, molybdenum, and uranium.

292

MMM 2018

(P1-22)
Hydrogen embrittlement controlled by reaction of dislocation withgrain boundary in alpha-iron polycrystals*Liang Wan1,2, Wen Tong Geng2,3, Akio Ishii2, Jun-Ping Du2,4, Nobuyuki Ishikawa5, Hajime
Kimizuka2, Shigenobu Ogata2,4
1. Wuhan University, 2. Osaka University, 3. University of Science and Technology Beijing, 4. Kyoto University, 5. JFE
Steel Corporation

Hydrogen atoms absorbed by metals in the hydrogen-containing environments can lead to the premature
fracture of the metal components used in load-bearing conditions. Since metals used in practice aremostly polycrystalline, grain boundaries (GBs) can play an important role in hydrogen embrittlement ofmetals. Here we show that the reaction of GB with lattice dislocations is a key component in hydrogen
embrittlement mechanism for polycrystalline metals. We use atomistic modeling methods to investigate
the mechanical response of GBs in alpha-iron with various hydrogen concentrations. Analysis indicatesthat dislocations impingement and emission on the GB cause the GB to locally transform into an activatedstate with a more disordered atomistic structure, and introduce a local stress concentration. The
activation of the GB segregated with hydrogen atoms can greatly facilitate decohesion of the GB. We showthat the hydrogen embrittlement model proposed here can give better explanation of many experimental
observations.

MMM 2018

293

(P1-23)
Study of solute effect on the yield strength of Fe-based dilute alloy
using atomistically informed kinetic Monte Carlo method*Shuhei Shinzato1, Masato Wakeda2, Shigenobu Ogata1
1. Dept. of Mechanical Science and Bioengineering, Osaka Univ., 2. National Institute for Materials Science

Solid solution hardening/softening is one of the ways to control mechanical properties of alloys. The trendof change in mechanical properties of solid solution alloys due to solute addition is generally non-linear
against a solute element, concentration, strain rate, and temperature. In order to develop the advanced
alloy, prediction of mechanical strength is important to reduce the cost and development time.
It is well known that the solute atoms in crystalline metals affect mechanical strength by interacting otherlattice defects such as dislocation. Although many researchers have studied the interaction between
dislocation and solute atoms to understand solid solution hardening/softening, detail of solute effect on
dislocation motion is not fully clarified yet.
In this work, we investigate the solute effect on the yield strength of body-centered cubic (BCC) alloy byanalyzing dislocation motion using kinetic Monte Carlo (kMC) model based on atomistic understanding.
We focus on dilute BCC Fe-based alloy including substitutional solute Si atom.
First, we analyze the solute effect on screw dislocation motion, which mainly dominates plastic
deformation of BCC metals, based on atomistic modeling. In order to estimate the solute effect on screw
dislocation motion, we performed nudged elastic band calculation to obtain activation energy of
dislocation motion via kink mechanism which includes a kink-pair nucleation and kink migration
processes.
Then, obtained activation enthalpy of each process and solute effect on them is introduced to kMC model
to calculate activation rate of dislocation motion. By performing kMC simulation, we estimate dislocation
velocity in different concentration, temperature, and stress conditions, and evaluate the solute effect on
dislocation velocity.
Finally, we predict yield strength from estimated dislocation velocity and discuss the effect of soluteatoms.

294

MMM 2018

(P1-24)
Molecular Dynamics Simulations of Low-cycle Fatigue Behavior inSingle Layer Molybdenum Disulfide
Yu-Chieh Lo1, *Yu-Cheng Su1, Ming-Chen Chung2, Alice Hu3
1. National Chiao Tung University, 2. National Cheng Kung University, 3. City University of Hong Kong

Graphene-like two-dimensional transition metal dichalcogenides (TMDs) have attracted much interest in
the last decade because of it astonishing properties. Molybdenum disulfide (MoS2) is one of TMDs. It has
direct band gap of 1.8 eV in monolayer but its bandgap can change with the number of layers. Suchproperties make it industrial important. In order to ensure the reliability of nano-devices made by MoS2,
much attention has been focused on their mechanical properties including elastic modulus, stiffness andbreaking strength. However, only limited studies have been done on cyclic deformation and fracture
behavior. As a result, we report on molecular dynamics simulations of low-cycle fatigue behavior in single
layer molybdenum disulfide. We choose the Reactive Empirical Bond Order (REBO) potential to describe
the interatomic interactions. The REBO potential is used to estimate the failure strain and the tensile
strength. The simulations and subsequent analysis suggest that the tensile stress will make the vacancies
penetrate the whole plane, and induce the fracture for incremental brittle crack growth to occur duringnear-threshold fatigue. We want to clarify whether the plastic-strain-controlled fatigue tests would show
the Coffin–Manson relation in fatigue life. Such power-law form originates from plastic-strain-dependent
microscopic damage accumulation. Lastly, the effect of a crack on low-cycle fatigue of monolayer MoS2 interms of failure mode and fatigue life is also discussed.

MMM 2018

295

(P1-25)
Fracture behavior of multi-walled carbon nanotube under biaxial
loading condition*Masaomi Nishimura1, Naoki Kazami2, Daiki Kato2
1. Dept. of Mechanical Systems Engineering, Shinshu Univ., 2. Graduate School of Science and Technology, Shinshu
Univ.

One-dimensional carbon materials such as Carbon Fibers (CFs) and Carbon nanotubes (CNTs) areapplied in a wide range of areas. For example, carbon fiber reinforced plastics are put to practical use forbodyworks of cars and airplanes. Though carbon materials in composites are subjected to complex
deformation by multiaxial loading, the fracture mechanism of them is not clarified under multiaxial stressconditions.
In this study, we have performed deformation simulations on multi-walled CNTs under biaxial loadings inorder to clarify the fracture criterion of one-dimensional carbon materials from nanometer scale
viewpoints. CNTs in simulation cell are compressed in a radial direction, and extended in a longitudinal
direction by molecular dynamics simulation using the adaptive intermolecular reactive empirical bondorder (AIREBO) potential. Fracture of CNTs originates from a bond breaking by the tensile loading forlongitudinal direction, under small compressive stresses for the radial direction. On the other hand, highcompressive stresses for the radial direction trigger a collapse of six-membered ring structures before thebond breaking by the longitudinal tension. The collapse occurs in the neighborhood of the innermost
layer in highly compressed CNTs.
In addition, we have also performed biaxial deformations of composite models which are made by filling
amorphous polyethylene structure in the space between MWCNTs. As a result, fracture behavior varies
with stress conditions, as in the case of models without polyethylene. A collapse of six-membered ring
structures under high compressive stresses occurs not from the innermost layer but from the outermost
layer by the interaction between polyethylene and CNTs.

296

MMM 2018

(P1-26)
On the role of amorphous shells on mechanical properties of fcc Ni
nanoparticles under compression*Alexandra Goryaeva1,2, Claudio Fusco2, Matthieu Bugnet2, Jonathan Amodeo2
1. DEN-Service de Recherches de Métallurgie Physique, CEA, Université Paris-Saclay, 91191 Gif-sur-Yvette, 2.
INSA-Lyon, Université de Lyon, MATEIS, 69621 VilleurbanneNano-objects, such as nanoparticles, nanowires, nanopillars etc., are characterized by the large surface tovolume ratio that, to a certain degree, defines their exceptional physical properties, significantly different
from those in their bulk counterparts. In particular, nanocrystals are known for their promising mechanicalproperties: a size-dependent elastic regime, large values of yield strength and ductility, that are mainly
linked to the dislocation nucleation from surface.
In this context, the various surface states of a sample are expected to lead to significant differences interms of mechanical behavior. So far, in situ nano-compression experiments in the TEM occasionallyreport the presence of amorphous overlay at the top of nano-objects [1], while theoretical atomic scalestudies are mainly focused on perfect crystalline systems [2, 3, 4].
Here we present a Molecular Dynamics (MD) study that aims to investigate the influence of amorphousshells on mechanical response of Ni nanospheres upon compression. In order to avoid complex effects ofchemistry on the onset of plasticity, we focus on pure Ni compound, without adding alloying elements tothe amorphous overlay. Based on multiple EAM potentials and various sample elaboration tests, a uniquemethodology that provides a reasonably slow “crystallization” rate of the amorphous Ni on fcc substrateis proposed. Then, mechanical properties of the designed 20 nm nanospheres with different shell
thickness are investigated under uniaxial compression. The mechanical response of the composite
systems is compared with that of purely crystalline and amorphous particles, with a particular focus on
dislocation-based deformation processes.

References[1] W. Han et al., Advanced Materials 27, 3385 (2015)[2] J.-J. Bian & G.-F. Wang, J Comput Theor Nanosci 10, 2299 (2013)[3] D. Mordehai et al., Acta Materialia 59, 5202 (2011)[4] S. Bel Haj Salah et al., Comput Mater Sci 129, 273 (2017)

MMM 2018

297

(P1-27)
Molecular dynamics analysis of hydrogen diffusion behavior inalpha-Fe bi-crystal under stress gradient*Ken-ichi Saitoh1, Haruka Koga2, Tomohiro Sato1, Masanori Takuma1, Yoshimasa Takahashi1
1. Kansai Univ., 2. Kobe Steel, Ltd.

Delayed fracture in high-strength steel wires produced by drawing is an important issue. The major causeof the delayed fracture is supposed hydrogen embrittlement (HE), and some researches so far show that
HE phenomena is very sensitive to the amount of plastic deformation in drawing process. The
hydrogen(H)-atom diffusion is affected largely by ambient thermal and mechanical conditions, such as,
stress, pressure and temperature. Besides, effect of stress gradient (SD) on atomic diffusion is supposed tobe crucial, but is still unclear. Most of metallic materials which have been provided plastic deformation,
like drawn pearlitic steel wires, have a enormous residual stresses particularly in surface and interface
region, which shows strong SD. In this study, we investigate the behavior of H-atoms diffusing in pure iron
(α-Fe) in the condition accompanied by SD. Since the behavior is observable just in atomic-scale,molecular dynamics (MD) simulation using EAM potentials for Fe and H atomic system is conducted.
There are two types of SD condition: one is the gradient for overall specimen, which can be reproduced
by bending deformation of specimen. Another is an atomic-scale gradient in the interface region, e.g. one
provided by grain boundary (GB) structure. Thus, we build a bi-crystal model including GB structure, and
it is applied bending deformation. For a moderate flexure, the bending stress distributes in a linear fashion
along the lateral cross-section of the specimen. Diffusion coefficient of H-atoms in bulk region increases
with increase of the SD value. Besides, it is clearly observed that the direction of diffusion depends on the
distribution of SD. It is found that H-atom diffusion increases with the decrease of cohesive energy
evaluated around the H-atom. From these MD results, we realize that the increase of H-atom diffusionshows exactly exponential relation to SD values. So, we can successfully obtain an expression between
diffusion coefficient and SD value. We also understand that, by setting temperature effects aside, theincrease of SD will lead to substantial change of entropy effect for diffusion.

298

MMM 2018

(P1-28)
Understanding Interactions of Dislocations with Interfaces in
Nickle-based Superalloys: Insights from Molecular Dynamics
Simulations*Jian Huang1, Yunjiang Wang2
1. Shanghai Institute of Ceramics Chinese Academy of Sciences, 2. Institute of Mechanics, Chinese Academy of
Sciences

Nickel-base superalloys, as the favorite material for the turbine blade of the engines, have attracted a lotof attention due to their excellent creep properties and good microstructure stability. The evolution of
dislocations during creep procedure and core structures of dislocations play an important role in thestrengthening mechanism of nickel-base superalloys. Especially, the dislocation reactions and theirinteraction with γ/γ’ interfaces of nickel-base superalloys contribute to a stable stage of creep. Here,based on developed mechanistic framework, we study the interactions of dislocations with interfaces tobetter understand the origin of ductility of nickel-base superalloys. Using atomistic reaction pathway
calculations based nudged elastic band (NEB) method, we elucidate the slip transfer reaction mediatedby interfaces of nickel-base superalloys. The findings offer new idea on the possible means to optimize
the ductility and strength through interfacial engineering for nickel-base superalloys.

MMM 2018

299

(P1-29)
The influence of nano-sized Ti3Al particles on the mechanicalproperties of α-titanium alloys*Yan He1,2,3, Wang Hao1, Dongsheng Xu1, Yang Rui1
1. IMR, CAS, 2. Coll. of Physics Science and Technology, SYNU, 3. Univ. of chinese Academy of Sciences

In α-titanium alloys, Ti3Al (α2 phase) precipitates under a perceivable aluminum content. The size anddistribution of α2 particles has significant influence on the mechanical property of α-Ti alloys.
Experimentally, it is found that nanometric α2 particles drastically decrease the toughness of α-Ti alloys
after certain thermal treatment. However, the strength and the ductility do not vary linearly with the size of
α2 particles and the atomic details of hardening and fracture remain unclarified. Therefore, we employedmolecular dynamic simulation with the embedded-atom potential to systematically study the deformation
process of α-Ti with different size and distribution of α2 particles. The result shows that 1) the α/α2interface is coherent; 2) in an αgrain with an α2 particle and incoming dislocations, the existence of anα2 particle blocks the dislocations and the strength increases with the size of the α2 particle; andsubsequently cracks nucleate at the impacting site on the gain boundary with the crack stress decreasing
with the size of the α2 particle; 3) the resulting strength and toughness vary with the size of the α2particle in a parabola manner. The present simulation result quantitatively agrees with experiments andhelps identifying the critical α2 particle size for the design of structural titanium alloys.

300

MMM 2018

(P1-30)
The atomic study of tensile property for nickel nanowires with helium
bubble*Hengfeng Gong1, Rui Li1, Tong Liu1
1. CGN

Tensile deformation behavior of nickel nanowires with helium bubble has been investigated using
molecular dynamics (MD) simulation. MD simulations were performed at 1K to 300K employing a strain
9 -1

10 -1

rate of 2×10 s and 2×10 s for the nanowires with cross section width(S)ranging from 8a0 to 12a0(a0=3.5157Å). The diameters were set from 1.2 nm to 1.6nm for helium bubbles, and the ratio of
helium-to-vacancy inside bubble for 1:1, 2:1, 4:1 and 1:0. With the strain rate increasing, the elastic
modulus and yield stress are also increased. However, they present the declined trend at the hightemperature. As the helium bubble size increases or the cross section of nanowires decreases, the elastic
modulus and yield stress increases, while the yield strain become smaller. In addition, the mechanism ofyielding is discussed in details based on the snapshots of defects evolution. As the radio of
helium-vacancy increases, the elastic modulus and yield stress also decrease. The helium bubble could
accelerate the fracture to some extent. This study on the plastic properties of metal nanowires will be
helpful to further understanding of the mechanical properties of nanomaterials.

MMM 2018

301

(P1-31)
Componets of fracture response of alkali-activated slag mortars withsteel fibers*Hana Simonova1, Petr Frantik1, Zbynek Kersner1, Pavel Schmid1, Pavel Rovnanik1
1. Brno University of Technology, Faculty of Civil Engineering

Knowledge of mechanical fracture parameters of composites based on brittle matrix is essential for thequantification of their resistance against crack initiation and propagation as well as for the definition ofmaterial models used to simulate the quasi-brittle behaviour of the structures or their parts made fromthis type of composites. The variability of results experimentally obtained from fracture tests ofcomposites with different kind of fibres is much higher in comparison with composite without fibres dueto the natural heterogeneity of composite containing fibres. Therefore, the main objective of this paper isto quantify the contribution of the matrix of alkali-activated slag mortars with steel fibres on their fracture
response.
First alkali-activated slag material was a reference without fibres; the other composites contain steel fibres
in amount 5, 10, 15 and 20 % of weight of slag, respectively. The mechanical fracture parameters were
determined using evaluation of fracture tests carried out on 40 ×40 ×160 mm beam specimens with an
initial central edge notch. The load vs. displacement (deflection in the middle of span length) and load vs.
crack mouth opening displacement diagrams were recorded during the fracture tests. Each diagram wasprocessed in order to obtain the component that corresponds to the structural response of the matrix ofthe composite consisting from alkali-activated slag and steel fibres reinforcing that matrix. The values offracture parameters were determined using work-of-fracture method and double-K fracture model.
This outcome has been achieved with the financial support of the Czech Science Foundation, project No.
16-00567S and the Ministry of Education, Youth and Sports of the Czech Republic under the “National
Sustainability Programme I” (project No. LO1408 AdMaS UP).

302

MMM 2018

(P1-32)
Molecular Dynamics Simulation of Crack Growth Behavior of Single
Crystal γ-TiAl Alloy Under Different Nb Substitution Mode
*Yuxi Feng1,2, Zhiyuan Rui1,2, Hui Cao1,2, Ruicheng Feng1,2, Xiaocui Fan1,2, Xing Yang1,2
1. Mechanical and Electronical Engineering College, Lanzhou University of Technology, 2. Key Laboratory of Digital
Manufacturing Technology and Application, the Ministry of Education, Lanzhou University of Technology

Because of the low plasticity at room temperature, high rate of crack propagation of TiAl alloy, its
application in the aerospace has been severely restricted. The addition of Nb contributes to solving thisproblem.The Ti-Al-Nb ternary system has become the main trend of development of TiAl alloys, and theinfluence of alloying elements Nb on the properties of TiAl alloys has become the focus of extensive
attention and research in the field of materials engineering.In this paper,the effect of different substitution
modes of Nb on the crack propagation of γ-TiAl alloy was studied from the microscopic scale bymolecular dynamics method.The effect of cracks on the performance of γ-TiAl alloys without Nb, Nb
substitutional Al systems, and Nb substitutional Ti systems was analyzed.The results show that theinteraction between Nb and the neighboring matrix atoms is stronger than the interaction between Ti-Al
atoms before the substitution, and the doped Nb is enhanced compared to the γ-TiAl alloy without Nb.
The bonding and bonding strengths between the atoms in the matrix and the atoms in the TiAl alloy
increase the bond strength of the unit cell, increase the bonding force and cohesion between thesurrounding atoms, and make the atoms in the crack tip region bond tightly, making it difficult to breakbonds and become crack propagation. An obstacle is the passivation of the crack tip, the slower rate ofcrack propagation, and the change in the crack propagation path, which increases the tensile and fracture
toughness of the alloy.The substitutional Ti system has a higher yield strength than the substitutional Al
system, Nb occupies a sublattice of Ti, and the short-range interaction between dislocations and Nb
atoms results in solid solution strengthening, which enhances the strength of the alloy.The lattice
distortion of the substitutional Al system is more serious than that of the substitutional Ti system, and thedislocation density is higher and the fracture toughness is higher. The difference in this phenomenon ismore pronounced at a high Nb concentration of 6%.

MMM 2018

303

(P2-34)
Dynamics of a solidification front made by invasion of fluid with adifferent temperature*So Kitsunezaki1, Chika Yamanaka1
1. Nara Women's Univ.

When fluid flows into another hotter or colder fluid, solidification often occurs in narrow regions at thevicinity of the interface. Such interfaces exhibit peculiar patterns in the growth process because ofaccompanying solidification fronts. Similar phenomena are observed in soft membranes created bychemical reactions of two fluid, as reported by H.~Wagatsuma et al.(Physical Review E, 2017). In
geological scales, we could also see examples of such phenomena in pillow lava and growth of a volcanicisland.
We carried out experiments by pouring ice-cold water into paraffin melt in a Hele-Shaw cell and foundthat solidification of paraffin causes fingering patterns with large meandering. The melting temperature ofparaffin we used is about 56-58 degrees C, but the rheological measurements indicated that paraffinbehaves as a soft viscoelastic material under the temperature. We infer that precipitous increase of theviscosity of paraffin is mainly responsible for large meandering of fingering growth.
A simple two-dimensional mathematical model is considered to find an interface dynamics in suchphenomena theoretically. Although a standard method of the center-manifold reduction can not be used
for solidification fronts growing in time, we develop a similar systematic method to derive the equations ofinterface motion.

304

MMM 2018

(P2-35)
Strengthening through solid solution in W1-xTaxB system*lijuan liu1, Wandong Xing2, Fanyan Meng1, Rong Yu2
1. Dept. of Applied Physics, University of Science and Technology Beijing, 2. School of Materials Science andEngineering, Tsinghua University

The elastic properties, electronic structures and the energy barriers in slip processes of W1-xTaxB systemhave been studied using first-principles calculations. It was found that the (110) plane is the easiest slip
plane in tungsten monoboride. By substituting tungsten with tantalum, slipping on the (110) plane can behindered through dislocation pinning, resulting in the increase of overall hardness of tungsten
monoboride Strengthening of the easiest planes is an effective approach to creating new hard materials in
more metallic materials.

MMM 2018305

(P2-36)
Desiccation crack patterns based on phase-field modeling and theirstatistical properties*Shin-ichi Ito1, Satoshi Yukawa2
1. The Univ. of Tokyo, 2. Osaka Univ.

We investigate morphological properties on desiccation crack patterns through numerical simulations of aphase-field (PF) model. Since our PF model does not require any assumptions related to crack nucleations
and numerical lattice configurations, we can investigate the pattern formations that purely depend on
material/external parameters. Our PF model showed us various pattern formations depending on a drying
speed and material constants. We discovered, in particular, the difference of the drying speed provides asignificantly qualitative difference in the pattern formations. Cellular patterns resulting from sequential
fragmentations of straight cracks can be observed when using a slow drying speed, while random network
patterns resulting from connections of micro cracks that appear simultaneously can be observed whenusing a rapid drying speed. We quantify the difference of the pattern formations statistically, and explain
the origin of the difference on the basis of a simple continuum theory of a thin layer of viscoelastic
material.

306

MMM 2018

(P2-37)
Ce-terminated (111) surface of CeO2
*YaNan Zhao1, Wandong Xing2, Fanyan Meng1, Rong Yu2
1. Dept. of Applied Physics, University of Science and Technology Beijing, 2. School of Materials Science andEngineering, Tsinghua University

Surface structure and properties of ceria are closely related to the applications of ceria in catalysis. Here,the atomic structures of the (111) surface of CeO2 nanoparticles have been studied combining
aberration-corrected transmission electron microscopy and first-principles calculations. Besides theoxygen termination that have reported extensively previously, the cerium termination has also been
revealed by direct atomic imaging, which can be viewed as the simultaneous loss of surface andsubsurface oxygen. The stabilization mechanism, electronic structure and magnetism of the surface, andthe behavior of oxygen vacancies have been discussed.

MMM 2018307

(P2-39)
Numerical analysis of elasto-plastic behavior of metallic architectured
materials*Filip Siska1, Ivo Dlouhy1, Jan Cizek2, Hanus Seiner3
1. Institute of Physics of Materials, Czech Academy of Sciences, 2. Institute of Materials Science and Engineering,
Brno University of Technology, 3. Institute of Thermomechanics, Czech Academy of Sciences

Architectured materials is a class of materials that is characteristic by the ordering of constituents inspecific geometrical manner. Such geometry provides an extra degree of freedom which allows reachingcombinations of properties that cannot be obtained by standard materials. Metallic architectured
materials are especially attractive because metals are important structural materials and adding internalarchitecture can enhance their performance in particular applications.
Our study is focused on a numerical investigation of an elasto-plastic response of different planararchitectured patterns under the basic types of loading (tension-compression, bending). These patterns
are made by different combinations of basic metals (for example: Al, Fe, Ti). The objective is to find therelation between the geometry and the resulting properties like stiffness, strength, hardening, ductility,
buckling resistance. These relations will help to find optimal internal structure geometries for given
materials combinations and loadings. The results from FE simulations will be further used for the
production of the real structures using cold spray technology which is very well suited for a fabrication of
structures and materials made of metals with different mechanical and physical properties.

308

MMM 2018

(P2-40)
Automatic analysis and numerical prediction of flow stress curves foraluminium alloys*Evgeniya Kabliman1, Johannes Kronsteiner1, Ana-Helena Kolody1
1. Light Metals Technologies Ranshofen, Center fo Low-Emission Transport, Austrian Institute of Technology

In present work, we develop a toolkit for automatic analysis of experimental flow stress curves as well for
their numerical prediction by a combined approach of physics based and data driven modelling. By using
a single environment it is possible to filter measured raw data, account for temperature increase during a
deformation process, extract the mechanical properties such as yield and ultimate strength, obtain theprocessing maps for the optimization of deformation conditions, as well as to predict the flow stresscurves by using a dislocation density based model in combination with algorithms of machine learning. It
is possible to account for processes such as work hardening and recovery due to spontaneousannihilation of dislocations and their climb. For testing purposes we choose a conventional AA6082 alloyand perform a series of hot compression tests by using a deformation and quenching dilatometer
DIL805A/D of TA Instruments. In order to choose the most suitable algorithm of machine learning,
different approaches found in literature for the prediction of flow stress curves are compared. A modified
version of a flow stress model is formulated and implemented into a finite element framework, as a result.

MMM 2018

309

(P2-41)
Comparison of different alkali activated mortars with hemp fibres
response during fracture test by acoustic emission method
Libor Topolar1, *Hana Simonova1, Barbara Kucharczykova1, Zbynek Kersner1, Jelena Dragas2,
Ivan Ignjatovic2, Miroslav Komljenovic3, Violeta Nikolic3
1. Brno University of Technology, Faculty of Civil Engineering, 2. University of Belgrade, Faculty of Civil Engineering, 3.
University of Belgrade, Institute for Multidisciplinary Research

In last decades, natural fibres are increasingly used as reinforcements for the production of low-cost
composites in civil engineering. The benefits of natural fibers include non-abrasive nature, high specificproperties, and biodegradability. However, their disadvantages are the bad moisture absorption, poor
wettability and large scattering in mechanical properties. The aim of this paper is contribute to the better
understanding of mechanical behaviour and failure modes of alkali activated materials reinforced byhemp fibers. Two different mortars based on alkali activated fly ash and slag were investigated. The paper
includes the results of acoustic emission measurement captured during the three-point bending fracture
test of specimens made of mentioned composites. Acoustic emission method is proving useful for thecapability of real-time monitoring of materials over the whole volume and with high sensitivity to any
processes generating stress waves. The effect of different mix composition and amount of hemp fibers on
the acoustic signal features such as the energy, counting and amplitude is including in this research. The
obtained acoustic emission results together with mechanical fracture parameters can serve as input
values of material models used for modelling of structure response.
This outcome has been achieved with the financial support of the Czech Science Foundation, project No.
18-12289Y and the results obtained within the project DS-2016-0060, which belongs to Multilateral
Scientific and Technological Cooperation Project in Danube Region between Technische Universität
Wien, Brno University of Technology and University of Belgrade, are presented in this paper.

310

MMM 2018

Symposium F(F1 invited)
An Integrated Experimental and Computational Approach to
Microstructure-Property Relationships in Structural Materials
Longsheng Feng1, Pengyang Zhao1, Steve Niezgoda1, Mike J Mills1, *Yunzhi Wang1
1. The Ohio State University

Location-specific component design requires modeling capabilities that incorporate specifictransformation and deformation mechanisms operating in alloys having different compositions andmicrostructures under different service conditions. In this presentation, we focus on how to utilize
phase-field modeling techniques at different length scales to address this difficult challenge and develop
mechanism-based and microstructure-sensitive modeling tools. In particular, using creep deformation in
Ni-base superalloys as an example, we demonstrate how to integrate phase-field modeling withexperimental characterization and use phase-field method to bridge ab initio calculations and crystal
plasticity (CP) simulations to (a) identify transformation / deformation mechanisms and quantify activation
pathways, (b) provide “mechanism maps” and microstructure-sensitive constitutive laws for dislocation–microstructure interaction and co-evolution, and (c) develop an integrated phase-field + full-field
FFT-based CP modeling framework for collective behavior of precipitate and dislocation microstructuresduring creep deformation. The work is supported by the National Science Foundation under the DMREF
program.

MMM 2018

311

(F1 oral)
Development of a multiscale simulation system based on
microstructure of fine-grained aluminum*ATSUSHI SAGARA1, YOSHITERU AOYAGI1
1. Dept. of Finemechanics, Tohoku Univ.

In plastic working of metal plates, spring back and generation of cracks and sticks cause defective
products. The finite element method is widely used to predict and control behavior of plastic working.
Since press forming that is a sort of plastic working consists of various processes such as deep drawing,
stretching, and bending, a multiaxial stress state dominates plastic deformation. The von Mises yieldfunction is well known as a theory to express yielding behavior of mechanical isotropic metals under amultiaxial stress state. The mechanical properties for the von Mises yield function are usually identified bya uniaxial tensile test. In the case of ultrafine-grained metals produced by severe plastic deformation, they
have strong orientation in mechanical properties due to a rolling texture. Therefore, the isotropic yieldfunction cannot fully express the yielding behavior of ultrafine-grained metals. Some models such as Hill
yield function, Barlat Yld-2000, etc. are proposed to express the mechanical anisotropy of metals.
However, a large number of material parameters are required for determination of yield function and it isdifficult to determine these parameters only by uniaxial tensile test. Tests reproducing a multiaxial stressstate such as a biaxial tensile test and a compression-torsion test, which requires a tremendous labor, arenecessary to obtain a yield surface identifying the material parameters. Numerical prediction of yieldsurface is anticipated on the basis of multiscale simulation considering information of microstructure ofmetals.
In this study, we aim at seamless bridging of design, development, and practical use of mechanicalanisotropic metals with CAE system. Mechanical anisotropy of severe rolled aluminum is predicted by
crystal plasticity analyses reflecting information of rolling texture. Furthermore, biaxial tensile tests wereconducted and the mechanical anisotropy was evaluated by comparison of the experimental andnumerical results. We developed a user subroutine for a commercial CAE software. Simulation result
calculated by the CAE software was compared with microscopic deformation behavior of practical plastic
working to evaluate the effect of predicted anisotropic yield function.

312

MMM 2018

(F1 oral)
Image-based crystal plasticity analysis on the activities of slip systems
in polycrystal alpha-Ti*Yoshiki Kawano1, Tetsuya Ohashi1, Tsuyoshi Mayama2, Masaki Tanaka3, Yelm Okuyama3,
Michihiro Sato1
1. Kitami Institute of Technolgy, 2. Kumamoto University, 3. Kyushu University

It has been pointed out that the activation of basal slip systems in alpha-Ti leads to fatigue failure under
fatigue loading. Thus, it is important to quantitatively evaluate the activity of basal slip systems in alpha -Ti
for ensuring the safety and reliability and predicting the fatigue lifetime with higher accuracy. However,the deformation mechanisms are still incompletely understood, and the activity of basal slip systems
under the deformation is also not clarified.
When forced displacement is applied to polycrystal alpha-Ti, decreases in the amount of deformation in
crystal grains would lead to those of increases in the other crystal grains because the reduction of thedeformation in a crystal grain must be compensated by increases in those in the other crystal grains. Thatis, there is a possibility that changes in the activity of slip systems of crystal grains influence those of thebasal slip systems of the other crystal grains.
In this study, a crystal orientation map obtained by Electron Back Scatter Diffraction (EBSD) patterns of apure titanium (alpha-Ti) specimen was converted into a geometric model for finite element method using
an interface developed by authors, and Crystal Plasticity Finite Element (CPFE) analysis was conducted.
The crystal orientations in the specimen showed that (0001) planes of almost crystal grains declined in
the direction from ND to RD, and the microstructure has a texture so-called RD-Split. A dislocation density
dependent constitutive equation was employed and unidirectional tensile loading was applied to thegeometric model by the forced displacement. Several sets of initial Critical Resolved Shear Stress (CRSS)
were employed for the simulation. The relationship between the initial CRSS of each slip system and theactivity of basal slip systems was investigated.

MMM 2018

313

(F1 oral)
Micro structure-based Crystal Plasticity Modeling of of Duplex
Titanium Alloy During Hot Deformation*Jun Zhang1, Yang Wang2, Yu Wang2, Peng Wang1
1. Institute of Systems Engineering,China Academy of Engineering Physics, 2. Department of Modern Mechanics, CAS
Key Laboratory of Mechanical Behavior and Design of Materials, University of Science and Technology of China

A crystal plasticity finite element model (CPFE) is presented to describe the hot deformation of titanium
alloy with a duplex microstructure. The model accounts for the individual plasticity slips within bothprimary αgrains and lamellar Widmanstatten grains using a isostress homogenization approach. The finite
element model is initial by a random program. The material parameters considering the scale of singal
crystal are determined by the combination of interpolation optimization method and the experiment data
published in the literature. The numerical results are in good agreement with the experimental tension
stress-strain curve at high temperature, indicating that the numerical model is suitable to describe the hot
deformation of duplex titanium alloy. The corresponding results give a deep insight into the relationshipbetween the microstructure and macro-mechanical behavior, and provide richer clues for the
optimization design of titanium alloy.

314

MMM 2018

(F1 oral)
Nonlocal multiscale modeling of deformation behavior ofpolycrystalline copper by second-order homogenization method*makoto uchida1, Akito Taniguchi1, yosihisa kaneko1
1. Osaka city Univ.

The macroscopic mechanical property of metal materials relates to the microscopic polycrystalline
structure. In the present study, micro- to macroscopic nonuniform deformation of the polycrystalline purecopper is evaluated by experimental and numerical studies. The uniaxial tension of the specimen having
curved gage section was performed. The effects of the curvature and the grain size on the deformation
localization in the specimen were experimentally evaluated by digital image correlation method. The
strain localization around the curvature is modulated for the specimen with larger crystal grain. The
computational simulations of the tensile tests were then performed using the second-orderhomogenization method. A series of the numerical simulation clarified the size-depended strain
localization in the macroscopic scale.

MMM 2018315

(F1 oral)
Residual stress prediction for turning of Ti-6Al-4V considering the
microstructure evolution
Zhipeng Pan2, *Donald S Shih1, Hamid Garmestani3, Elham Mirkoohi2, Steven Y Liang2
1. Magnesium Research Center, Kumamoto University, 2. George W. Woodruff School of Mechanical Engineering,
Georgia Institute of Technology, 3. School of Materials Science and Engineering, Georgia Institute of Technology

An analytical model for residual stress prediction considering the effects of material dynamic
recrystallization under process-induced mechanical and thermal stresses is proposed. The effect of
microstructure evolution on residual stress generation during the turning process is considered. The
Johnson–Mehl–Avrami–Kolmogorov model is used to calculate grain size evolution due to thermal
mechanical effects in the machining process. A modified Johnson–Cook flow stress model is developed byintroducing a material grain growth–induced softening term. The classic Oxley’s cutting mechanics
theories are implemented for machining forces calculation. A hybrid algorithm accounting for thermal,
mechanical, and microstructure evolution effects is used to predict the residual stress profile on amachined workpiece surface. The proposed method is implemented for the orthogonal turning of
Ti-6Al-4V material. Comparison is conducted between the model prediction and the measured residual
stress data in the literature. The general trend of the machining-induced residual stress on the machining
surface is accurately captured by the proposed model. Moreover, the parametric study is conducted toinvestigate the effect of rake angle and depth of cut on the residual stress profile.

316

MMM 2018

(F2 invited)
New algorithms for simulating grain boundary motion*Selim Esedoglu1
1. University of Michigan

I will describe the latest developments in a class of algorithms known as threshold dynamics for simulating
the motion of interfaces under curvature and related flows, as appears in many models of grain boundary
motion. These are remarkably elegant algorithms that generate the desired evolution, along with all thecorrect boundary conditions along junctions, by alternating two very simple and efficient procedures:
Convolution with a kernel and thresholding. They allow arbitrarily large time steps, and handle topologicalchanges automatically. The most recent versions allow specifying N-choose-2 anisotropic (normal
dependent) surface tensions and anisotropic (normal dependent) mobilities for a network of N grains. We
will describe simple and explicit formulas that tell us how to bake in the desired set of mobility andsurface tension functions into the algorithm.

MMM 2018

317

(F2 oral)
The Role of Grain Shape in Discrete Element Modeling of Snow
Mechanics*Carolin Willibald1, Thiemo Theile1, Martin Schneebeli1
1. Institute for Snow and Avalanche Research (SLF, ETH)

The macro-mechanical properties of the heterogeneous material snow are determined by its
microstructure. Single ice crystals are connected by sintering and form a complex ice skeleton. In thismicrostructure fracture occurs under high strain rate deformations, which are relevant for applications
such as avalanche risk forecasting or snow mobility. During such a deformation snow transforms from a
porous sintered into a granular material. Both states can be represented by the discrete element method,which is well suited for the mechanical description of snow. However, a convenient but strongly simplified
assumption is a spherical shape of the snow particles. The grain shape is important in both structuralstates: it affects the granular dynamics and controls the number and size of inter-particle contacts, which
are crucial for the strength and fracturing behavior of the structure. Yet, a detailed reconstruction of thesnow grains with discrete elements is neither preferable for the computational efforts nor necessary formost applications. In laboratory experiments, we investigate the mechanical properties of ice beads andtwo natural snow types, differing in crystal shape and size. The ice beads allow one-to-one comparison
with the DEM simulations and are used as a well-defined intermediate step to later adapt the model to thenatural snow types. Compression tests were performed under variation of the sintering time, which isrelated to the bond size and therefore critical for the strength of the snow microstructure. By means of 3D
computed tomography images, the number and size of bonds between the ice beads are quantified andsimulations can be performed with the same bead configuration as in the experiments. The relation of thesintering state and the macroscopic strength of the sample is examined in experiments and simulations.
Once the simulations perform well in the bead case, the model is modified in terms of bond number andsize, to match the mechanical behavior of the natural snow types. Accounting for the grain shape in our
DEM snow model remarkably improves the simulations of snow micro- as well as macro-mechanicalprocesses.

318

MMM 2018

(F2 oral)
Experimental-Computational Analysis of Primary Static
Recrystallizazion in DC04 Steel*Martin Diehl1, Markus Kühbach1, Dierk Raabe1
1. Max-Planck-Institut fuer Eisenforschung GmbH

Low-alloyed steels with body-centered cubic crystal structure (bcc, ferrite) are a material class which is
widely used in automotive sheet metal forming applications. When produced with an adequate
crystallographic texture, the mechanical behavior of steels for forming applications is characterized by anisotropic in-plane flow behavior in combination with a low yield strength. To obtain these beneficial
mechanical properties, an adequate cold rolling strategy in terms of the number of passes, deformation
rates, and total reduction needs to be followed by an annealing procedure with a time-temperature profilethat facilitates primary static recrystallization. The most fundamental connection between cold rolling andheat treatment consists in the reduction of the dislocation-related free energy stored during deformationby the formation of new grains with a very small dislocation content. Hence, the local variation in
crystallographic orientation and defect population lead to very inhomogeneous grain boundary migration
velocities.
We present here results of a coupled experimental-computational approach for studying microstructure
evolution in industrially cold rolled DC04 steel under quasi-isothermal conditions. For the experimental
characterization, quasi in-situ experiments consisting of interrupted isothermal holding at 600°C andsubsequent Electron Backscatter Diffraction (EBSD) orientation imaging were conducted. The results
reveal a strong correlation between local dislocation density, quantified via the Kernel Average
Misorientation (KAM), and the local increase of recrystallized volume fraction. To complement theexperimental findings and gain further insights into the influence of the sub-surface microstructure, aCellular Automata (CA) model was utilized. The employed three-dimensional microstructure model was
directly built from the experimentally characterized deformation microstructure. Different approaches ofscaling the KAM into dislocation density values, i.e. the driving force for recrystallization, are employed.
Similarities and deviations between experimental and computational results are discussed with the aim atincreasing the understanding the mechanisms of static primary recrystallization.

MMM 2018

319

(F2 oral)
Large scale phase-field simulations of solid state sintering*Johannes Hoetzer1,2, Marco Seiz2, Michael Kellner2, Wolfgang Rheinheimer3, Hernik Hierl1,
Britta Nestler2,1
1. Karlsruhe University of Applied Sciences, 2. Karlsruhe Institute of Technology, 3. Purdue University

Ceramic materials are of high interest for technical applications due to their excellent material properties
regarding hardness, strength, density, wear resistance or corrosion resistance.
Due to the dependence of the material properties on the microstructure, a deep understanding of thesintering process is needed to produce advanced ceramics with tailored properties.
During the solid state sintering process the initially loose powder of green body densifies and particles
coarsen.
This process is driven by the reduction of the interfacial energy from the surface as well as the grainboundaries.
Depending on the different mechanisms, volume, surface and grain boundary diffusion, the densificationand grain growth rate in the microstructure can be influenced.
However, due to the complex interplay of the material and process parameters, it is challenging to predictthe microstructure evolution.
In this talk, a phase-field model is presented to investigate the microstructure evolution during solid statesintering.
The model is based on the grand potential approach and considers the different diffusion mechanisms
which can vary by multiple magnitudes.
To resolve realistic green bodies with multiple thousand particles and different partial size distributions,large scale domains are required.
To efficiently investigate such systems, the model is implemented in a highly optimized manner in themassive parallel phase-field solver framework PACE3D.
Therefore the solver is optimized on various levels and the kernels are explicitly vectorized using
intrinsics.
In the first part of this talk, the influence of the different diffusion mechanisms on the microstructure
evolution for two and four particle settings are validated.
In the second part, the densification and grain growth depending on the active diffusion mechanisms areinvestigated using realistic green bodies with multiple thousand grains.
Also the effect of different initial densities and partial size distributions are investigated.

320

MMM 2018

(F2 oral)
Phase-field simulations of coupled dendritic-eutectic growth*Marco Seiz1, Michael Kellner1,2, Johannes Hötzer1,2, Philipp Steinmetz3, Britta Nestler1,2
1. Institute of Applied Materials - Computational Materials Science, Karlsruhe Institute of Techology, 2. Institute of
Digital Materials Science, Karlsruhe University of Applied Sciences, 3. Department of Materials Science andEngineering, University of Alabama at Birmingham

The solidification of alloys shows a large variety of different microstructures depending on the material
system and processing conditions. Since material properties such as tensile strength are dependent on
the microstructure, its prediction is a topic of high interest in order to produce materials with tailoredproperties. Whereas theory is capable of investigating simple geometries, simulations are necessary inorder to ascertain the influence of complex evolving geometries. An example of this is the coupled growthof dendrites and eutectics, which typically grow at different length scales.
One way to simulate such problems is the phase-field method which has been established as a versatile
tool to investigate microstructural evolution. The used phase-field model is based on a grand potential
approach with parabolic free energies approximating thermodynamic CALPHAD data of the system Al-Cu.
Additionally, an ad-hoc nucleation mechanism is implemented.
Validation is done by comparison to analytical theories of pure dendritic and eutectic growth. Following
the validation, the coupled growth of coarse dendrites and fine eutectics during directional solidification
is investigated in two as well as three dimensions. Depending on the process parameters, observations
include closely-spaced dendrites turning into cells, stable coupled growth of dendrites and eutectics,
nucleation of eutectic on dendritic sidebranches as well as transitions to a completely eutectic state.
Based on these results a tentative microstructure map is established.

MMM 2018

321

(F2 oral)
The use of thermodynamic tensor models for phase-field simulationsof spinodal decomposition in quaternary alloys coupled with
CALPHAD data.
*Yuri Amorim Coutinho1, Nico Vervliet2, Lieven De Lathauwer2,3, Nele Moelans1
1. Dept. of Materials Engineering, KU Leuven, 2. Dept. of Electrical Engineering, KU Leuven, 3. Group Science,
Engineering and Technology, KU Leuven - Kulak

The successful coupling of the phase-field and CALPHAD methods for a thermodynamic consistentdescription of the system free energy in a phase-field model is challenging for multicomponent alloys. The
many coupling schemes presented in the literature all tend to suffer from inefficiencies and limitations
when applied to higher-order systems. When collecting calculated thermodynamic data in a multiway
array or a tensor, we can observe that the number of entries grows exponentially as a function of thenumber of components and thus the direct use of tensors to provide thermodynamic information to aphase-field simulation is not feasible for quaternary or higher order systems. However, when a canonicalpolyadic decomposition is applied, we can represent the data contained in the tensor with a small
number of coefficients, which grows only linearly if new components are included. This approach allows
the construction of thermodynamic tensor models, which can be efficiently used to approximate
individual entries of the original tensor with good accuracy. Furthermore, the gains in data reduction
obtained with the tensor decomposition technique increases when more elements are considered in thesimulations. The efficiency of this novel coupling scheme is verified with spinodal decomposition
simulations of quaternary alloys.

322

MMM 2018

(F3 invited)
Biomineralization in molluscan shells: From ideal to hierarchical grain
growth*Dana Zoellner1, Igor Zlotnikov1
1. B CUBE - Center for Molecular Bioengineering, TU Dresden

The microstructure of grain networks as they can be found in many polycrystalline materials like metals,
alloys, and bionic composite structures have an immense impact on their physical properties. Any change
in the structure in terms of the size and shape of the grains leads to a change in the properties. Hence,
understanding structural changes is of enormous importance for materials development, processing, andapplication.
In the present work, we investigate the evolution of the prismatic ultrastructures in molluscan shells inform of a comparative study bringing together large numerical and experimental data sets of molluscan
shells from different families. To that aim, a framework has been developed for a quantitative study of theprocess of shell morphogenesis. The method is based on Monte Carlo Potts model simulations of grainboundary motion that, classically, were developed to study coarsening of polycrystalline metals. By
employing this approach, we fully reconstruct the growth process of the different molluscan shells: While
the prismatic ultrastructure of Atrina vexillum is an archetype of ideal grain growth fulfilling the classicalgrowth theories, shells like Atrina rigida and Pinna nobilis show retarded growth comparable with triple
junction controlled grain growth in nanocrystalline metals, and, finally, the mollusc Pinctada nigra ischaracterized by a two-level hierarchical prismatic microstructure, which can be represented in the Potts
model by introducing sub-grain boundaries.
The proposed framework is a fundamental approach to study the structural regulation duringbiomineralization.

MMM 2018

323

(F3 oral)
Mesoscale modeling of cement: texture, mechanics and durability*Katerina Ioannidou1,2,3, Emanuela Del Gado4, Franz J. Ulm1, Roland J.-M. Pellenq1,2,3
1. Dept. of Civil and Enviromental Engineering, Massachusetts Institute of Technology, 2. MultiScale Material Sciencefor Energy and Environment, Massachusetts Institute of Technology–CNRS/MIT/AMU Joint Lab. at MIT, 3. MIT Energy
Initiative, 4. Dept. of Physics and Institute for Soft Matter Synthesis and Metrology, Georgetown Univ.

Cement is a multiscale porous material, widely produced, more than any other synthetic material on
Earth. In this talk, I will present a multiscale bottom-up approach for cement and specifically for
calcium-silicate hydrate (C-S-H) that is the most abundant phase of cement. During cement hydration
C-S-H nano-scale particles precipitate in the pore solution and form a cohesive gel that is the mainbinding agent in cement and concrete, crucial for the strength and the long-term evolution of thematerial. Even more than the molecular structure of C-S-H particles, the C–S–H mesoscale texture overhundreds of nanometers plays a crucial role for material properties. We use a statistical physics framework
for aggregating nanoparticles and numerical simulations to obtain a first, to our knowledge, quantitativemodel for such a complex amorphous material. Our approach is based on precipitation of colloidal
particles interacting with effective potentials associated to the chemical environment. The effective
potential can be calculated from atomistic models of C-S-H and are corroborated by experiments. This
multiscale informed modelling approach generates realistic micron scale textures in terms of pore size
distributions and solid volume fractions and allows to calculate mechanical properties. The extensive
comparison with experiments ranging from small-angle neutron scattering, EM imaging,
adsorption/desorption of N2, and water to nano-indentation provides new fundamental insights into themicroscopic origin of the cement properties measured. Our results provide a quantitative insight into how
the heterogeneities developed during the early stages of hydration persist in the structure of C-S-H and
impact the mechanical performance of the hardened cement paste. Moreover, this approach allowed toaddress durability issues on cement such as freeze-thaw and alkali-silica reaction damage leading to theformation of cracks and fractures.

324

MMM 2018

(F3 oral)
Actuation in Metal-Polymer Nanocomposites:
Chemoelectromechanical Coupling on Interfaces*Jana Wilmers1, Swantje Bargmann1
1. Chair of Solid Mechanics, University of Wuppertal

Nanoporous metals have recently garnered interest as actuator materials because of their uniquemicrostructure. The characteristic interconnected pore network and resulting high interface-to-volume
ratio allows them to react sensitively to electric signals. By coating the metal backbone withionically-activated polymers, increased actuation strains are achieved while still retaining the metal’s
superior mechanical properties.
In understanding and modelling the chemoelectromechanically coupled behaviour of suchnanocomposite actuators, one has to account for charge carrier transport and coupled effects in thepolymer as well as effects arising from build-up of charges in and at the metal-electrolyte interface. We
present an interface-extended continuum model that couples large deformations with electrostatics andcharge carrier transport for the bulk and the interface. The developed framework utilises the concept ofinterface stresses to model the different coupling mechanisms occurring on the interface, that is, interface
charging and electroadsorption, in addition to the coupled behaviour in the bulk material arising frommass transport in the pore space. This allows to study the different phenomena and their interaction witheach other, giving insight into the underlying physical mechanisms. Simulations reveal that both, thenanocomposite’s structure and the ions’ mobilities, strongly affect the actuator’s response, providing
the means to tailor the actuator’s behaviour to specialised applications.

References[1] Wilmers, McBride, Bargmann, Interface Elasticity Effects in Polymer-Filled Nanoporous Metals, JMPS,
163-177, 2017.
[2] Wilmers, Bargmann, Functionalisation of Metal-Polymer-Nanocomposites: Chemoelectromechanical
Coupling and Charge Carrier Transport, EML, accepted, 2018[3] McBride, Javili, Steinmann, Bargmann, Geometrically nonlinear continuum thermomechanics withsurface energies coupled to diffusion, JMPS 59, 2116-2133, 2011[4] Soyarslan, Bargmann, Pradas, Weissmüller, 3d stochastic bicontinuous microstructures: generation,
topology and elasticity, Acta Mater., 326-340, 2018
[5] Bargmann et al., Generation of 3d representative volume elements (RVEs) for heterogeneous materials:
a review, Prog. Mater. Sci., accepted, 2018

MMM 2018

325

(F3 oral)
Mechanics of Cilia Beating –A Relationship Between Metachronal
Wavelength and Fluid Flow Rate
*Jon Hall1, Nigel Clarke1
1. The University of Sheffield

Cilia driven fluid flow is a complex fluid mechanical system that is dependent on several physicalparameters, including mucus rheology, cilia density, and beat coordination. We have developed acomputational model of an array of beating cilia using a hybrid immersed boundary lattice Boltzmann
algorithm in order to study the how the flow behaviour is affected by these key parameters. Our main
focus is on quantifying the relationship between metachronal wavelength and the rate of fluid flow which
we find exhibits two maxima at particular wavelengths. We attempt to rationalise this observation byconsidering how the metachronal wavelength relates to the volume of fluid that is accelerated by eachcilium during an active stroke, and thus the momentum of the fluid region as a whole. We also observethat the rate of fluid flow decreases significantly when cilia in close proximity are synchronised, and that
this behaviour becomes more dominant as cilia density increases.

326

MMM 2018

(F4 invited)
Showdown! Pitting computer simulations against time-resolved
experimental measurements of grain growth in 3D
Mingyan Wang1, Nan Wang2, Jules Dake1, Søren Schmidt3, *Carl Krill1
1. Institute of Micro and Nanomaterials, Ulm University, 2. Dept. of Physics, McGill University, 3. Dept. of Physics,
Technical University of Denmark

Thanks to dramatic improvements in computational speed, it is now feasible to simulate the coarsening of
3D polycrystalline microstructures containing tens of thousands of grains. Indeed, such calculations havebecome so powerful that the simulation cells rival—and, in some cases, exceed—sample volumes that canbe probed experimentally! But do the computational algorithms underlying these simulations properly
capture the physics of microstructural evolution as it occurs in real materials? We have followed a
two-pronged strategy to address this question, using (i) three-dimensional x-ray diffraction (3DXRD)
microscopy to map the 3D network of grain boundaries in a polycrystalline specimen over the course of
stepwise isothermal annealing treatments, and (ii) a phase field model to simulate 3D grain growthstarting from the same initial configuration as in experiment. A grain-by-grain showdown between tactic (i)and tactic (ii) offers fresh insights into the phenomenon of grain growth—gleaned not only fromdiscrepancies between measured and simulated size trajectories and grain shapes, but also frominstances of agreement.

MMM 2018

327

(F4 oral)
Coarsening and grain-growth in an SOFC-anode under surface andgrain boundary self-diffusion: A Multiphase-field approach*Paul Hoffrogge1, Daniel Schneider1,2, Britta Nestler1,2, Patricia Haremski2,3, Matthias Wieler3,
Piero Lupetin3
1. Karlsruhe University of Applied Sciences, 2. Karlsruhe Institute of Technology, 3. Robert Bosch GmbH

Solide oxide fuel cells (SOFCs) convert chemical energy stored in form of a gaseous fuel into electricalenergy. Due to the typically high energy-conversion efficiencies achieved, SOFCs are a very promisingtechnology to reduce carbon emission. One of the critical parameters to be economically competitive isthe lifetime of an SOFC device. In order to improve the SOFCs long-term performance it is essential toobtain a better understanding of the underlying physical processes leading to a decrease in SOFC
performance during operation.
The present study focuses on the evolution of a polycrystalline and porous 3D microstructure of an
Ni-YSZ (yttria-stabilized zirconia) SOFC-anode during isothermal coarsening. We employ amultiphase-field model which incorporates self-diffusion of nickel by a grand-chemical formulation tosimulate grain-boundary and surface diffusion coupled with simultaneous growth of the inherent
Ni-grains. We validate the model by 2D thermal-grooving simulations under surface-diffusion and by aquantitative comparison with analytical results. A comprehensive study of several input parameters suchas grain-size on the coarsening behavior of the SOFC-anode is performed. The detailed investigation
includes an analysis of the microstructure as a function of particle size, tortuosity and triple-phaseboundary length.

328

MMM 2018

(F4 oral)
Solidification Simulation in Additive Manufacturing Process of Ti-Alloyby ICME approach*Yusuke SHIMONO1, Mototeru Oba1, Sukeharu Nomoto1
1. Itochu Techno-Solutions Corp.

Multi-phase field method (MPFM) coupled with thermodynamics database of calculation of phase
diagrams (CALPHAD) is a powerful tool for simulating solidification microstructure evolutions inengineering casting processes. As MPFM is based on local (quasi-)equilibrium assumption in solidification
theory, applying MPFM to solidification of additive manufacturing (AM) processes is considered to bedifficult because of extremely large cooling rate and temperature gradient.
On the other hand, some researchers have recently reported experimental observations of thecolumnar-to-equiaxed transition in the solidification of the AM processes. These observations suggested
that the local (quasi-)equilibrium assumption can be applied to the solidification of the AM processes.
In this study, AM processes of Ti-alloys are simulated. By using temperature distributions obtained bythermal analyses in finite element method simulations, solidification microstructures are calculated by
MPFM. It is confirmed that the microstructures have the columnar-to-equiaxed transitions. The results are
summarized in a solidification map for the AM process conditions.

MMM 2018

329

(F4 oral)
Phase Field Model of Microstructural Evolution in Metal Alloy for
Designing Mechanical Property
Wooju Lee1, *Dongchoul Kim1
1. Dept. of Mechanical Engineering, Sogang Univ.

Precipitation hardening has been widely employed to improve the mechanical properties of metal alloys.
During the precipitation hardening, the supersaturated alloying elements in a solid solution phase form
secondary precipitate particles. To design the mechanical properties of metal alloy, the relationshipbetween precipitation conditions and morphology of microstructure of secondary phase should be fully
understood. The morphology of microstructure in metal alloy is determined by the composition of thealloy and their intermetallic compounds. Here, we investigate the the morphological evolution of
microstructures in metal alloy during precipitation. To simulate the temporal evolution of microstructures,a phase field model is employed. By simulations, the effects of anisotropic interfacial energy and latticemismatch on the morphology of the microstructure are investigated.

330

MMM 2018

(F5 invited)
Solidification microstructure formation by phase-field simulation withmulti-GPU acceleration*Tomohiro Takaki1, Shinji Sakane1, Munekazu Ohno2, Yasushi Shibuta3, Takayuki Aoki4
1. Kyoto Institute of Technology, 2. Hokkaido University, 3. The University of Tokyo, 4. Tokyo Institute of Technology

Accurate prediction of material microstructures is crucial for the production and design of highperformance material. Phase-field method has emerged as a powerful numerical model for predicting theformation process of complicated material microstructure. Now, it is well accepted as the most accurate
and promising model for microstructure formation in the mesoscale. On the other hand, a large
computational cost due to the diffuse interface is a major drawback, and usually the phase-field
simulation is limited to the small domain. To overcome this drawback, we need a high-performancecomputing for the phase-field simulation.
In this talk, we introduce our recent progresses in the high-performance phase-field simulation using
multi-GPU acceleration. Here, we focus on the dendritic solidification where the microstructure is formed
through the competitive growth among multiple dendrites. We introduce the phase-field simulations ofdendrite competitive growth during directional solidification of a binary alloy in single crystal [1], bicrystal[2], and polycrystal [3]. In addition, the liquid flow drastically changes the solidification microstructure. We
also introduce the large-scale phase-field simulations of dendrite growth with liquid flow [4-6]. The above
all simulations have been performed by the GPU supercomputer TSUBAME at the Tokyo Institute ofTechnology.
[1] T. Takaki, S. Sakane, M. Ohno, Y. Shibuta, T. Shimokawabe, T. Aoki, Acta Materialia 118 (2016)230-243.
[2] T. Takaki, S. Sakane, M. Ohno, Y. Shibuta, T. Shimokawabe, T. Aoki, ISIJ Int. 56(8) (2016) 1427-1435.
[3] T. Takaki, M. Ohno, Y. Shibuta, S. Sakane, T. Shimokawabe, T. Aoki, J. Crystal Growth 442 (2016)14-24.
[4] S. Sakane, T. Takaki, R. Rojas, M. Ohno, Y. Shibuta, T. Shimokawabe, T. Aoki, J. Crystal Growth 474(2017) 154-159.
[5] T. Takaki, R. Rojas, S. Sakane, M. Ohno, Y. Shibuta, T. Shimokawabe, T. Aoki, J. Crystal Growth 474(2017) 146-153.
[6] T. Takaki, R. Sato, R. Rojas, M. Ohno, Y. Shibuta, Comp. Mater. Sci. 147 (2018) 124–131.

MMM 2018

331

(F5 oral)
Phase-field study of eutectic colony formation in NiAl-34Cr*Michael Kellner1,2, Johannes Hötzer1,2, Markus Linnenberg1, Marco Seiz1, Britta Nestler1,2
1. Institute of Applied Materials –Computational Materials Science (IAM-CMS), Karlsruhe Institute of Technology, 2.
Institute of Digital Materials Science (IDM), Karlsruhe University of Applied Science

The properties of a material are mainly defined by their chemical composition and by their underlying
microstructure. Therefore, the development of tailored microstructures is crucial for the applicability inhigh-performance materials with specified properties. Depending on the material properties and process
conditions, multi-scale microstructures evolve during the directional solidification of eutectics. Beside themicroscopic eutectic lamellar and/or fibrous structures, macroscopic structures like eutectic colonies
occur at certain process conditions. Eutectic colonies are mainly observed in ternary systems when thecomposition of the alloy is in the vicinity of a binary eutectic reaction. The formation of colonies is driven
by microscopic instabilities in a macroscopic planar solidification front, due to the impurities by the third
component that diffuse from the two solidifying phases into the liquid.
To simultaneously investigate the formation processes of micro- and macroscopic structures in theircomplex spatial arrangement, two- and three-dimensional large-scale phase-field simulations based on a
Grand potential formalism are conducted for the high-performance material NiAl-34Cr. By systematic
variations of the solidification velocity beyond the stability range the underlying mechanisms leading to
the formation of eutectic colonies are studied. Further the interactions and the coherences between the
eutectic rod-like structure of NiAl-34Cr and the arrangement of the eutectic colony are investigated.
Especially the contact zones between the colonies are of interest, as these are indicated to be the weak
points for the applicability. In additional studies the stability of the colonies for different temperature
gradients and off-eutectic melt compositions is observed.

332

MMM 2018

(F5 oral)
Solidification analysis by non-equilibrium phase field model using
thermodynamics data estimated by machine learning*Sukeharu Nomoto1, Hiroshi Wakameda1, Masahito Segawa1, Toshiyuki Koyama2, Akinori
Yamanaka3
1. ITOCHU techno-Solutions Coorporation, 2. Materials Design Innovation Engineering, Nagoya Univ., 3. Departmentof Mechnical Systems Engneering, Tokyo University of Agriculture and Technology

A multi-phase field (MPF) model coupled with finite interface dissipation proposed by Steinbach et al. isapplied to simulate the dendritic solidification in Fe–Cr–Ni–Mo–C steel. Thermodynamic calculation using
CALPHAD database is replaced by machine learning prediction procedure in this MPF program.
Solidification calculations are performed in a case of quinary system: Fe–16Cr–2Mo–10Ni–0.08C. We
confirm that the microstructure evaluated by using machine learning parameter is good agreement with
one directly coupled with CALPHAD database. Furthermore, this calculation is also confirmed to beapproximately five times faster than the direct CALPHAD calculation method.

MMM 2018

333

(F5 oral)
Phase field modeling of deformation twinning in β-metastabletitanium alloys*Juba Hamma1, Benoît Appolaire2, Yann Le Bouar1, Alphonse Finel1
1. LEM, UMR 104 CNRS-ONERA, Université Paris Saclay F-92322 Châtillon, 2. IJL, UMR 7198 CNRS-Université deLorraine, 54000 Nancy

Recently, new β-metastable titanium alloys, so-called ''TRIP/TWIP titanium alloys'' (TRIP for
TRansformation Induced Plasticity and TWIP for TWinning Induced Plasticity), have been developed to
exhibit improved mechanical properties at ambient temperature. These properties are attributed to very
complex microstructures engendered by the mechanical destabilization of the initial bcc phase (β) in thecourse of the deformation. Indeed, the microstructure features numerous twins following the peculiar{332}<113> twinning mode of the βphase, specific to β-titanium alloys, as well as the orthorhombic (α'') phase ensued from concomitant displacive transformations. Moreover, experimental observations
revealed the possible activation of secondary deformation mechanisms, i.e. the formation of secondary
twins and/or α'' inside primary twins of the βphase.
To get a better understanding of the formation of the complex microstructures described above, we
propose a numerical model using the phase field method. This method provides a thermodynamicallyconsistent framework to couple the mechanisms at the origin of the microstructure evolution. As a firststep of modeling the evolution of the β-metastable titanium alloys microstructure upon deformation, we
focus on the {332}<113> twinning mode of the bcc phase β. In this work, we propose a phase-fieldmodel relying on (i) a finite strain formalism; and (ii) the possibility of taking into account the activation of
secondary twinning inside primary twins. We will show with simple calculations the capabilities of themodel to describe the {332}<113> twin growth. A comparison with a model formulated in a small strain
formalism will also be presented to show the influence of the geometrical non-linearities introduced in thefinite strain framework.

334

MMM 2018

(F6 invited)
Atomistically informed full-field simulation of tempered martensite:
Quenching, tempering and mechanical characterization*Ingo Steinbach1
1. Ruhr-University Bochum

A scale bridging modelling and simulation strategy of virtual production and virtual testing is presented
which starts with parametrization of continuum constitutive relations by quantum mechanical calculations
at the atomistic scale. These models are integrated into a full-field framework at the scale ofmicrostructures in technical materials. The phase-field method is employed to simulate the microstructure
evolution during processing: virtual production. Finally micromechanical simulations of deformation anddamage evolution till catastrophic failure are performed utilizing the full information about the
microstructure, composition and residual stresses resulting from the production route: virtual testing. The
approach is applied to tempered martensite, describing the evolution of the microstructure underquenching, tempering and testing.
The quenching simulation reproduces the martensite transformation under consideration of theKurdjumov-Sachs relationship. Based on these results, tempering is simulated. Diffusion is enabled atelevated temperatures and cementite carbides are nucleated, which decreases carbon composition in themartensite, which also decreases the tetragonal distortion. A virtual tension test provides then information
about strain hardening, damage as well as the macroscopic response of the material to the loading.
Different microstructures, as resulting from different alloy composition and different tempering conditions
are compared regarding their mechanical performance.

[1] Atomistically Informed Extended Gibbs Energy Description for Phase-Field Simulation of Tempering ofMartensitic Steel, Oleg Shchyglo, Thomas Hammerschmidt, Miroslav Čak, Ralf Drautz, Ingo Steinbach,Materials 9 (8), 669[2] Microstructure Design of Tempered Martensite by Atomistically Informed Full-Field Simulation: From
Quenching to Fracture, Efim Borukhovich, Guanxing Du, Matthias Stratmann, Martin Boeff, Oleg Shchyglo,
Alexander Hartmaier, Ingo Steinbach, Materials 9 (8), 673

MMM 2018

335

(F6 oral)
Characterisation and modelling of drawn martensite*Marius Gintalas1, Miguel A. Santajuana 2, Carlos Garcia-Mateo2, David San-Martin2, Jose A. Jimé
nez2, Wim Van Haver 3, Pedro E. J. Rivera-Diaz-del-Castillo4
1. The University of Cambridge, Department of Materials Science & Metallurgy, 27 Charles Babbage Road, Cambridge
CB3 0FS, 2. Materalia research group, Department of Physical Metallurgy, Spanish National Centre for MetallurgicalResearch (CENIM-CSIC), Avda. Gregorio del Amo 8, Madrid E-28040, 3. NV Bekaert SA, Bekaertstraat 2, Zwevegem
B-8550, 4. The University of Lancaster, Department of Engineering, Engineering Building, Lancaster University, Gillow
Ave, Bailrigg, Lancaster LA1 4YW

There has been an extensive work on microstructural refinement via severe plastic deformation, whichleads to improved strength in metallic materials. However, due to the complexity of the microstructuralchanges involved, the mechanisms responsible for the strength enhancement of the cold deformed
marteniste have not been completely revealed. This work summarises advanced experimental andmodelling activities carried out on heavily drawn martensite. These include slow heating high resolution
dilatometry, high resolution transmission electron microscopy and high resolution X-ray diffraction
conducted with both synchrotron and lab x-ray sources. The purpose of employing various
characterisation techniques is to collect data to gain a comprehensive understanding on the
microstructural evolution with respect to applied strain. The modelling work aims to propose a novel
martensite strength model to describe the relationship between microstructure and mechanicalproperties, including the effects of applied strain upon severe plastic deformation. The strengthening
model is validated using stress-strain data of martensite drawn at various strain levels.

336

MMM 2018

(F6 oral)
3D modeling of microstructure evolution in Ni-based superalloys
under creep loading*Maeva Cottura1, Benoît Appolaire1, Alphonse Finel2, Yann Le Bouar2
1. Institut Jean Lamour , 2. LEM - CNRS/Onera

Many materials used in everyday life have a complex internal structure organized at different scales with atexture of grains, and a complex pattern of thermodynamic phases at lower scale. This microstructurestrongly impacts the movement of crystalline defects in the materials and therefore controls themechanical behaviour. The phase field method has emerged as a well-suited method for tackling
microstructure evolutions during phase transformations when elastic coherency stresses are generated. In
addition, in many materials and especially at high temperature, the microstructure evolution is coupledwith plasticity, and there is currently a great research effort to extend the phase field method to take thiscoupling into account.
In this context, we will show how to couple a classical phase field model (for the description of the phasetransformation) with a crystal plasticity model based on dislocation densities. The latter model uses a
storage-recovery law for the dislocation density of each glide system and a hardening matrix to accountfor the short-range interactions between dislocations. The proposed model will be applied to study raftingof ordered precipitates observed in Ni-based superalloys. We will present 3D calculations of
microstructure evolution under [100], [110] and [111] creep loading as well as different shear creep tests.

MMM 2018

337

(F6 oral)
Phase field study of the effect of coherency strains and applied load inmaterial couples*Sourav Chatterjee1, Nele Moelans1
1. Department of Materials Engineering, KU Leuven

We investigate the role of coherency strains and applied load on phase growth and morphologicalevolution in binary diffusion couples using the phase field method. We employ the Kim-Kim-Suzuki modelfor the chemical contribution in the free energy. The elastic energy contribution to the free energy isevaluated using a suitable interpolation scheme considering the elastic heterogeneity between the
different phases. When elastic stresses are considered, we observe bending and splitting of the phaselayers for mechanically constrained, partially constrained and relaxed systems. The interface remains
planar in the absence of elastic stresses. In addition, we study the role of volume fraction of the phases
and finite size of the system on the microstructure evolution.

338

MMM 2018

(F7 invited)
A Cosserat crystal plasticity and phase field theory for grain boundary
migration*Benoit Appolaire1,2, Anna Ask3, Samuel Forest3, Kais Ammar3
1. Institut Jean Lamour, Univ. de Lorraine, 2. LEM, CNRS-Onera, 3. Centre des Materiaux, Mines ParisTech

Elaborating a unified field framework to model concurrent viscoplastic deformation and recrystallizationand grain growth in metal polycrystals has remained a formidable issue. In this work, we will present sucha unified framework relying on the enhancement of a Cosserat crystal plasticity model with an orderparameter to account for diffuse, mobile grain boundaries in analogy with phase field models. The
Cosserat directors are taken to represent the lattice orientation of the grains. In order to introduce anevolution law for reorientation during grain boundary migration, the skew-symmetric part of the Cosserat
deformation tensor is associated with a dissipative stress. The formulation in terms of the Cosserat
deformation provides a natural way to couple reorientation due to deformation and reorientation due tograin boundary migration. In the absence of displacements and for a particular choice of free energyfunction the model can be considered a generalization to three dimensions of the
Kobayashi–Warren–Carter (KWC) orientation phase field model (2000, 2003).
The proposed 3D anisotropic constitutive framework couples the changing orientation at a material pointdue to migrating grain boundaries (which is essential to the KWC model) to the lattice reorientation due
to displacements and plastic slip. Due to the coupling of the Cosserat directors and the elastic
reorientation, the bulk rotation of the grains which is inherent to the KWC model (and considered an
artefact here although it can nevertheless be desirable in certain cases) can be controlled and evensuppressed.
In this contribution, we will explain the main features of this framework and we will illustrate its
capabilities with several cases that could not be handled convincingly by other models.

MMM 2018

339

(F7 oral)
Phase-field modeling of precipitation growth and ripening during
heat-treatment conditions in Ni-base superalloys*Michael Fleck1, Felix Schleifer1, Markus Holzinger1, Yueh-Yu Lin1, Uwe Glatzel1
1. Metals and Alloys, University Bayreuth

We develop a phase-field model for the simulation of chemical diffusion limited microstructure evolution,with a special focus on precipitation growth and ripening in multi-component alloys. Further, the modelaccounts for elastic effects, which result from the lattice-misfit between the precipitate particles and theparent matrix-phase. To be able to simulate particle growth and ripening in one dimension, we introduce
an extra optional driving-force term, which mimics the effect of the curved particle/matrix interface in one
dimension. As a case study, we consider the gamma'-precipitation growth and ripening under the
influence of realistic heat treatment time-scales in the multi-component Ni-base single crystalline castalloy CMSX-4. The respectively required temperature-dependent thermodynamic and kinetic input
parameters are obtained from CALPHAD calculations using the commercial software-package
ThermoCalc. The required temperature-dependent elastic parameters are measured in-house at the chairof Metals and Alloys, using resonance ultrasound spectroscopy for the anisotropic and inhomogeneous
stiffness-tensor and high temperature X-ray defraction for the lattice misfit.
We study the kinetics of growth and ripening of statistically many interacting precipitated particles asfunction of the temperature and time in one and two dimensions, with the explicit consideration of up to
8 independent chemical components. Furthermore, the model is applied to calculate the detailed
shape-evolution of a few interacting gamma'-particles in two and three dimensions with periodic
boundary conditions. Relations to the shapes of gamma'-particles in respectively heat treatedexperimental microstructures are discussed.

340

MMM 2018

(F7 oral)
Two-dimensional Simulation of Cyclic Phase Transformation in
Fe-C-Mn-Si Alloy using Non-equilibrium Multi-Phase-Field Model*Masahito Segawa1, Nomoto Sukeharu1, Akinori Yamanaka2
1. ITOCHU Techno-Solutions Corporation, 2. Division of Advanced Mechanical Systems Engineering, Institute ofEngineering, Tokyo University of Agriculture and Technology

Multi-phase-field (MPF) method incorporating CALPHAD database has attracted much attention as a
powerful numerical tool for simulating microstructure evolutions in various metallic materials.
Transformation modes and diffusion behavior of substitutional solute atoms strongly depend on
temperature and chemical compositions of the alloy especially in multi-component steels, i.e. an Fe-C-Mn
ternary alloy. Therefore, we need to use a robust MPF model which can simulate the transition oftransformation mode and the complex diffusion behavior. Recently, the MPF model with finite interface
dissipation has been proposed (Hereafter, the MPF model is called as the non-equilibrium MPF (NEMPF)
model). The attractive advantages of the NEMPF model are its computational efficiency and flexibility for
simulating microstructure evolutions under strong non-equilibrium interface condition without the equal
chemical potential assumption. In the previous study, we investigated the cyclic γ→αand α→γtransformations in Fe-C-Mn-Si alloys using the NEMPF model coupled with CALPHAD database. The
one-dimensional simulations of the cyclic phase transformation revealed that the switching of thepolarities of Mn and Si spikes formed at the α/γ interface caused the stagnant stage where the phasetransformation was suppressed. In this study, the two-dimensional simulation of cyclic phasetransformation in Fe-C-Mn-Si alloy is performed using the NEMPF model in order to gain a deeper
understanding of the mechanism of the stagnant stage and the transition of transformation modes.

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341

(F7 oral)
Prediction of the microstructure evolution in Electron Beam Melting
Alloy 718 through phase field modelling*Chamara Kumara1, Dunyong Deng2, Johan Moverare2, Per Nylén1
1. Department of Engineering Science, University West, 2. Department of Management and Engineering, Linköping
University

Electron Beam Melting (EBM) is a powder bed additive manufacturing process where powder material isbeing melted selectively by a layer-by-layer approach using an electron beam. It has some unique features
when it comes to manufacturing of components in high-performance superalloys such as alloy 718 thatare commonly used in gas turbines. EBM has high deposition rate due to high beam energy and speed,comparatively low residual stresses and limited problems with oxidation. However, due to thelayer-by-layer melting approach and high powder bed temperature, the resultant microstructure ofas-built EBM Alloy 718 is observed to have a microstructure gradient starting from the top of the sample.
The aim of this study was to use modelling to create a deeper understanding of microstructuredevelopment during EBM and the homogenization that occurs during manufacturing in Alloy 718. A
multi-component phase field modelling approach combined with thermodynamic modelling was used to
predict the experimentally observed microstructure gradient. Of particular interest was to study theelement segregation during the solidification and the subsequent “in-situ” homogenization heat
treatment that occur due to the elevated powder bed temperature. The predicted element composition
was then used in thermodynamic modelling to predict the changes in the CCT and TTT diagrams for Alloy
718. This helps to explain the observed phase evolution within the microstructure. The results indicatethat the approach can be a valuable tool both for creating process understanding and for processdevelopment including subsequent homogenization and heat treatment.

342

MMM 2018

(F7 oral)
Phase-field simulation of solidification morphology in laser powder
deposition of Fe–B alloys*Xiaoxia Li1, Xiangge Qin1
1. School of Materials Science and Engineering, Jiamusi Univ.

Laser melting deposition is a complicated physical and chemical metallurgical process. In this paper, thephase field model of alloy solidification is coupled to laser melting deposition process, and a finite
element model of heat transfer is established. By studying the variation of the spacing of dendrite Fe2B of
Fe-B alloy powder under the conditions of laser melt deposition, the robustness and accuracy of thecoupled model were verified. Experiments show that under different temperature gradient conditions, thesize and spacing of the dendrite Fe2B of the cross-section morphology of the Fe-B alloy deposited by laser
melting deposition have changed significantly. The quantitative phase field simulation of the heat transfer
finite element model under local steady state conditions confirms this behavior. This work demonstrates
the potential of the phase-field method to simulate laser melting deposition techniques and analyze thepotential of alloy solidification under complex heat transfer conditions.

MMM 2018

343

(F8 invited)
From state parameter-based microstructure modeling to temperatureand strain rate-dependent yield stress*Ernst Kozeschnik1
1. Institute of Materials Science and Technology, TU Wien

In the past 20 years, significant progress has been made in the development of state parameter-basedmodeling approaches for the simulation of microstructure evolution, in particular, with respect toprecipitation kinetics, recrystallization, recovery, grain growth, dislocation density evolution in the courseof thermo-mechanical treatment, as well as the mutual interactions of all mechanisms. Once, the relevant
state parameters and their evolution are known, it is straightforward, however not trivial, to deduct fromthese the stress strain response of the microstructure as a function of temperature and strain rate. In thispresentation, an overview of these models is given as well as some implementation details in thethermokinetic software package MatCalc. Finally, an example is discussed, where the precipitation
microstructure and its interaction with deformation in the course of compression testing is simulated andcompared to corresponding experimental data.

344

MMM 2018

(F8 oral)
Modelling of grain boundary segregation and precipitation in
multi-component Al alloys subjected to heat treatment*Dongdong Zhao1, Sylvain Gouttebroze2, Jesper Friis3, Yanjun Li1
1. Norwegian University of Science and Technology (NTNU), 7491, Trondheim , 2. SINTEF Materials and Chemistry,
0314 Oslo, 3. SINTEF Materials and Chemistry, 7491 Trondheim

A newly developed modelling framework which accounts for grain boundary (GB) segregation andprecipitation in multi-component Al alloys subjected to cooling and subsequent heat treatments ispresented. Within this framework, both the equilibrium and non-equilibrium segregation mechanisms are
considered to predict the solute and vacancy segregations towards GBs. In the model of non-equilibrium
segregation, GBs are considered as ideal sinks for vacancy annihilation. The CALPHAD (CALculations pf
PHAse Diagrams) approach is implemented to supply the important thermodynamic and kinetic data ofmulti-component Al alloys as input for the model. The present framework is expected to predict the soluteand vacancy segregations at GBs in multi-component Al alloys during typical heat treatment (e.g.
quenching, ageing) processes. It is shown that the variation of vacancy and solute segregation behavior at
GBs as a function of heat treatment or microstructure variables, i.e. solution/ageing temperature, coolingrate, grain size, can be well described within the present model. Based on the segregation of solute
elements, preliminary precipitation behavior at GBs and bulk are predicted using the Kampmann-Wagner
numerical approach. The simulation results have been compared with experimental observations. This
work is supposed to provide a preliminary theoretical understanding towards the GB segregation andprecipitation in multi-component Al alloys subjected to various cooling and subsequent heat treatment
processes.

MMM 2018

345

(F8 oral)
Modeling the microstructure and electrical conductivity evolution
during aging of Al-Mg-Si alloys*Yijiang Xu1, Yanjun Li1
1. Norwegian University of Science and Technology

Aging is commonly performed after forming process to obtain better mechanical properties and electricalconductivity of Al-Mg-Si alloys (AA6xxx series) applied in the electrical wire. In this work, the nucleation,
growth, coarsening and dissolution of different precipitate phases (MgxSiy) during aging treatment issimulated based on a Kampmann-Wagner Numerical (KWN) type precipitation model. By coupling with
the CALPHAD, the thermodynamics of the metastable precipitate phases is well treated. Furthermore, theprecipitation model is linked to an electrical conductivity model to predict the evolution of electricalconductivity based on the volume fraction, size and spatial distribution of different precipitates, solid
solution level, etc. The modeling results are compared to the transmission electron microscopy (TEM)
experimental results and electrical conductivity measurements. It is revealed that the present model canwell address the influence of alloy composition, aging temperature and aging time on the microstructureand electrical conductivity of Al-Mg-Si alloys.

346

MMM 2018

(F8 oral)
Thermo-kinetic modeling of long-term precipitate evolution inheat-resistant alloys*Jae-Hyeok Shim1, Magdalena Speicher2, Mahesh Chandran3, Woo-Sang Jung1
1. Korea Institute of Science and Technology, 2. Materials Testing Institute, 3. Indo-Korea Science and Technology
CentreThere is an urgent need to improve the thermal efficiency of fossil fuel power plants by raising theoperation temperature and pressure in order to reduce CO2 emission as well as the cost of fuel. Plant
operation at higher temperatures inevitably requires the development of heat-resistant alloys with ahigher creep strength. There has recently been an increasing interest in applying advanced heat-resistantalloys to fossil fuel power plants in order to raise their operation temperature. The important role ofprecipitation in the achievement of good creep properties of heat-resistant alloys has long beenrecognized. One of the most effective ways for improving the creep properties is to uniformly distribute
fine precipitates with a good long-term stability at elevated temperatures. The experimental investigationof the long-term precipitate evolution behavior in heat-resistant alloys have not often been performed,
although it is important in understanding the creep properties. In addition to experimental approaches,
there have recently been a few attempts to simulate the precipitation kinetics in heat-resistant alloys. The
purpose of this study is to simulate the long-term precipitate evolution in various heat-resistant alloys
such as ferritic/martensitic and austenitic steels and nickel alloys using the MatCalc software, which
operates based on the classical nucleation theory and evolution equations derived from thethermodynamic extremum principle that maximizes the dissipation rate of the total Gibbs energy of thesystem. The simulation results, such as the precipitation sequence and the precipitate size, will becompared with experimental observations.

MMM 2018

347

(F8 oral)
Simultaneous Transformation Kinetics Model for Additive
Manufacturing*Narendran Raghavan1, Srdjan Simunovic1, John Turner1
1. Oak Ridge National Laboratory

Metal additive manufacturing is disrupting the traditional approaches in the manufacturing sector.
However, there are uncertainties associated with the qualification of fabricated components owed to thedifficulty in predicting the process-structure relationship in the PSPP linkage. Understanding theprocess-structure linkage includes understanding the (a) liquid-solid phase transformation and (b)solid-solid phase transformation. Previously solidified layers are affected by the thermal cycles during themelting of the subsequent layers. During processing in AM, the conditions are highly non-isothermal,
non-equilibrium and understanding this highly transient condition is crucial to understand theprecipitation kinetics. The temperature of the substrate can also be varied in the machine. This affects thesolid-state phase transformation of a given alloy system. Multiple phases can simultaneously precipitatefrom the product phase as a function of thermal cycle. In this work, we develop a model based on
simultaneous transformation kinetics theory and the developed phenomenological model is then coupledwith the numerical thermal model to predict the solid-state phase transformation and volume fraction ofmultiple phases as a function of thermal cycles and processing conditions. Numerical thermal model isused to predict the thermal cycles as a function of input processing conditions. The thermal cycle is thendiscretized into set of small isothermal steps. The extent of simultaneous precipitation/dissolution of thephases will be calculated at each of the discretized time step. Volume fraction of each of the phases isthen updated and similar procedure is repeated for rest of the discretized isothermal time steps.
Appropriate thermodynamic framework, database will be used to calculate the thermodynamic properties
as a function of alloy system and is used as the input to the model. It is a semi-empirical model and the
input from experimental results are used to calibrate the model. Some of the other calibration parameters
include nucleation site densities and surface energies. Advantages and limitations of this approach will be
discussed in detail.

348

MMM 2018

(F9 invited)
Programmable Materials - tuning effective materials response*Peter Gumbsch1,2, Chris Eberl1,3, Claudio Findeisen1,2, Alexander Butz1, Jan Pagenkopf1,2
1. Fraunhofer IWM, 2. Karlsruhe Institute of Technology (KIT), 3. Albert-Ludwigs-Universität Freiburg

The design of materials and materials microstructures to specific materials properties requires
mechanistic materials modelling over multiple length scales to avoid tedious trial-and-error procedures
and excessive experimentation. The feasibility of such multiscale modelling has now been demonstrated
in several cases including the explicit modelling of the microstructural evolution during processing. As an
example, I will explain the manufacturing process of a steel component, for which the local microstructure
and the resulting material properties after deep drawing can be predicted.
On the component level, locally different processing conditions result in locally different materials
properties which are often dealt with as a nuisance. I would like to take this further and ask whether weare able to design materials such that their locally varying properties are used for component
functionality. Conceptually this can be regarded as locally programming a material to achieve component
functionality. It will require large-scale structural optimization, predictive materials models for materials
with significantly varying properties. I will give examples how this can be achieved. The approach canmost easily be demonstrated using additively built metamaterials. With such metamaterials one can evengo further and design them such that their effective materials properties can still be changed after
manufacturing to achieve intrinsically programmable materials.

MMM 2018

349

(F9 oral)
Equilibrium crystal shape of GaAs by ab-initio thermodynamics
In Won Yeu1,2, Gyuseung Han1,2, Cheol Seong Hwang2, *Jung-hae Choi1
1. Center for Electronic Materials, Korea Institute of Science and Technology, 2. Department of Materials Science andEngineering, and Inter-university Semiconductor Research Center, Seoul National University

The growth of the III-V compound semiconductors on Si is crucial to adapt their superior electronic
properties to the silicon-based CMOS technology. In order to achieve the high-quality hetero-epitaxy
preventing defects such as dislocation, crack, and anti-phase boundary, the selective area growth (SAG)
method has been developed. To optimize this process, the atomic scale understanding is essential interms of both the thermodynamics and kinetics. In this study, we establish a thorough methodology to
calculate the surface energy as a function of temperature (T) and pressure (P) for the III-V compoundmaterials in the combined framework of density functional theory (DFT) calculations and thermodynamics[1], which is competitive compared the previous DFT studies providing the surface energy as a function ofchemical potential. Then, we construct the equilibrium crystal shape of GaAs as a function of T and P,
including the effects of surface vibration. The calculated equilibrium shapes in various T and P conditions
are compared with the experimentally grown shapes and explained in the viewpoint of the surface
vibration, twin formation and kinetic factors.
[1] In Won Yeu et al., Sci. Rep. 7, 10691 (2017).

350

MMM 2018

(F9 oral)
Elastic field of lattice defects in low-dimensional nano-carbon
materials*Xiao-Wen LEI1, Akihiro NAKATANI2, Tomoya TANIGUCHI1
1. Dept. of Mechanical Engineering, Univ. of Fukui, 2. Dept. of Adaptive Machine Systems, Osaka Univ.

Carbon atoms build themselves into different materials such as fullerenes, diamond nano-thread, carbon
nanotube (CNT), and graphene sheet (GS) in nanometer-scale. The materials are formally classified into
point, line, and surface materials, which correspond to zero-, one- and two-dimensional materials,
respectively. Many literatures report that the lattice defects affected mechanical properties and electronic
properties. Lattice defects in such low-dimensional nano-carbon are limited into special atomic structures
rather than bulk (three-dimensional) materials, but the properties of these low-dimensional materials
depend on the lattice defects as well as bulk materials. Continuum theory, such as dislocation theory, hasbeen developed sophisticatedly for very long years. In low-dimensional materials, however, theoretical
approaches for lattice defects are just developing. In this study we discuss the elastic field of latticedefects in low-dimensional nano-carbon, e.g. Stone-Wales (SW) defects on single-walled carbon
nanotubes (SWCNTs) and GS. The various equilibrium configurations of atomistic models with defects are
obtained using LAMMPS. The site potential energy and stress distribution of each atom are evaluated in
the vicinity of defects. We also measure a curvature and bond length to identify the out-of-plane
deformation and in-plane deformation. We also study the dependence of elastic field on the chirality of
CNT and the interaction of two SW defects in details. Formation energy significantly depends on chirality,
i.e. diameter, of nanotube as well as the orientation and position of SW defect in SWCNT and GS. The
results obtained for atomistic models are compared with an classical analytical solutions of plane stresswhich is established ideally with assumption of only in-of-plane deformation, and the effect ofout-of-plane deformation of low-dimensional nano-carbon is discussed.

MMM 2018

351

(F10 invited)
Data Analytics for Mining Process-Structure-Property Linkages for
Hierarchical Materials*Surya Raju Kalidindi1
1. Georgia Tech

A majority of the materials employed in advanced technologies exhibit hierarchical internal structureswith rich details at multiple length and/or structure scales (spanning from atomic to macroscale).
Collectively, these features of the material internal structure are here referred to as the material structure,and constitute the central consideration in the development of new/improved hierarchical materials.
Although the core connections between the material’s structure, its evolution through various
manufacturing processes, and its macroscale properties (or performance characteristics) in service arewidely acknowledged to exist, establishing this fundamental knowledge base has proven effort-intensive,
slow, and very expensive for most material systems being explored for advanced technology applications.
The main impediment arises from lack of a broadly accepted framework for a rigorous quantification of
the material’s structure, and objective (automated) identification of the salient features that control theproperties of interest. This presentation focuses on the development of data science algorithms andcomputationally efficient protocols capable of mining the essential linkages from large ensembles ofmaterials datasets (both experimental and modeling), and building robust knowledge systems that can bereadily accessed, searched, and shared by the broader community. The methods employed in this novel
framework are based on digital representation of material’s hierarchical internal structure, rigorous
quantification of the material structure using n-point spatial correlations, objective (data-driven)
dimensionality reduction of the material structure representation using data science approaches (e.g.,principal component analyses), and formulation of reliable and robust process-structure-property linkages
using various regression techniques. This new framework is illustrated through a number of case studies.

352

MMM 2018

(F10 oral)
Maximization of strengthening effect of microscopic morphology induplex elastoplastic solids*Ikumu Watanabe1, Gaku Nakamura2, Kohei Yuge2
1. National Institute for Materials Science, 2. Seikei University

An inverse analysis method based on nonlinear finite element analysis is developed to find an optimized
morphology of periodic microstructure for improving the macroscopic mechanical properties in duplex
elastoplastic solids. Here a gradient-based computational optimization method and two types ofhomogenization methods are employed. In this study, the optimization problem is defined as the
maximization of the sum of macroscopic external works for several macroscopic deformation modes,
enabling us to obtain a high strength material. The morphologic strengthening effect is discussed througha comparison with experiments and classical theories.
In a homogenization method based on finite element analysis, a representative volume element of an
objective microstructure is modeled with finite elements and the deformation analysis is conducted underperiodic boundary condition in control of macroscopic stress or strain. As the numerical results, thedeformation state of microstructure is obtained along with the corresponding macroscopic material
response. By coupling with this computational homogenization method and a mathematical optimization
method, the microstructure corresponding to a required performance can be found efficiently.
In this study, a computational optimization method for microstructure is applied to maximize the strengthof a dual-component elastoplastic solid. A standard metal plasticity is employed to describe the material
responses of each components and the material constants are determined from the macroscopic
experiments of the corresponding single-component materials. For optimization calculations, thedistributions of two densities are considered with node-based discretization under a constraint condition
to finally segregate two components. And here simple strain-constant analytical homogenization method,so-called mixture rule, is applied to handle the mixture state during the optimization process.

MMM 2018

353

(F10 oral)
Assessment of formability limit diagram prediction by crystal plasticity
finite element method*Duancheng Ma1
1. Leichtmetallkompetenzzentrum Ranshofen GmbHThis study is inspired by the recent development of “virtual material testing laboratory” in which themain equipment are full field crystal plasticity models. Ample examples have demonstrated its
applications to sheet forming operations. In those applications, the mechanical anisotropy originated fromthe crystallographic texture can be adequately described, such as r-values and earing. Formability is also
another very important property in sheet metal forming, which yet have not been equipped in these virtuallaboratories. Though theoretical models for formability can be dated back to 1800s, all popular models at
the moment suffer respective limitations. In this study, we make an attempt to the prediction of formability
limit diagram by crystal plasticity finite element method, with the objective of overcoming the limitations
posed by the current theoretical models. The focus is placed on the texture dependence of the formability
limit diagram354

MMM 2018

(F10 oral)
Analysis of gradient microstructures using crystal plasticity*Balaji Selvarajou1, Jerry Quek Siu Sin1
1. IHPC, Singapore

Gradients in microstructural features such as grain size, initial dislocation density and crystallographic
texture occur in many engineering applications such as shot peened regions and heat affected zone ofwelds. While such microstructures may be desirable to improve mechanical properties, modelling of suchmicrostructures require integration of appropriate length scales into the constitutive model. Crystal
plasticity finite element method (CPFEM) offers an appropriate route to integrate the crystallographic andgrain shape/size effects into the same descriptor for deformation; thus it allows for the probing of thedeformation behaviour of complex gradient microstructures. In this work, we implement a CPFEM modelthat accounts for the effect of the grain boundaries and thus bringing in a length scale effect. The model isused to perform mesoscale simulations of realistic microstructures. Emphasis is on the role of the
distribution of grain size, shape and crystallographic texture on the deformation behaviour. Corroboration
with the fracture behaviour of such microstructures are also made.

MMM 2018

355

(F11 invited)
Mesoscale plasticity models of polycrystalline materials for efficientcomputation of microstructure/property relationships*Ricardo Lebensohn1
1. Los Alamos National Laboratory

Models based on crystal plasticity are increasingly used in engineering applications to obtainmicrostructure-sensitive mechanical response of polycrystalline materials. Three key elements of thesemodels are: a proper consideration of the single crystal plastic deformation mechanisms, a representativedescription of the microstructure, and an appropriate scheme to connect the microstates with themacroscopic response. The latter can be based on homogenization (e.g. self-consistent methods [1]),which relies on a statistical description of the microstructure, or be full-field solutions, which requires a
spatial description of the microstructure (e.g. spectral methods [2]). Full-field models are numericallyintensive, making their direct embedding in multiscale calculations computationally demanding. On theother hand, they can be used to generate reference solutions for assessment of homogenization-based
approaches. In this talk we will review our recent efforts to develop material models based on polycrystal
plasticity to capture anisotropic strength, along with their integration with emerging characterization
methods in Experimental Mechanics (e.g. [3]), and their embedding in Finite Elements formulations (e.g.
[4]) to solve problems involving complex geometries and boundary conditions withmicrostructure-sensitive material response.
[1] R.A. Lebensohn, C.N. Tomé and P. Ponte Castañeda: "Self-consistent modeling of the mechanicalbehavior of viscoplastic polycrystals incorporating intragranular field fluctuations". Phil. Mag. 87,
4287-4322 (2007).
[2] R.A. Lebensohn, A.K. Kanjarla and P. Eisenlohr: "An elasto-viscoplastic formulation based on fast
Fourier transforms for the prediction of micromechanical fields in polycrystalline materials". Int. J. Plast.
32-33, 59-69 (2012).
[3] R. Pokharel, J. Lind, A.K Kanjarla, R.A. Lebensohn, S.F. Li, P. Kenesei, R.M. Suter and A.D. Rollett:“Polycrystal plasticity: comparison between grain scale observations of deformation and simulations”.

Ann. Rev. Cond. Matter Phys. 5, pp. 317-346 (2014).
[4] J. Segurado, R.A. Lebensohn, J. Llorca and C.N. Tomé: "Multiscale modeling of plasticity based onembedding the viscoplastic self-consistent formulation in implicit finite elements". Int. J. Plast. 28,
124-140 (2012).

356

MMM 2018

(F11 oral)
Development of a new consistent discrete Green operator for
FFT-based methods to solve heterogeneous problems witheigenstrains*Komlavi Senyo ELOH1,2,3, Alain JACQUES2,1, Stéphane BERBENNI1,3
1. University of Lorraine, 2. Institut Jean Lamour, 3. Laboratoire d'etudes de microstructures et de mécanique desmatériaux

A new expression of the periodized discrete Green operator using the Discrete Fourier Transform method
and consistent with the Fourier grid is derived from the classic ''Continuous Green Operator'' in order toavoid the problem refered to as ''aliasing'' inherent to Discrete Fourier Transform methods. It is shown thatthe easy use of the conventional continous Fourier transform of the modified Green operator for
heterogeneous materials with eigenstrains leads to spurious oscillations when computing the local
responses of composite materials close to materials discontinuities like interfaces, dislocations,edges...We also focus on the calculation of the displacement field and its associated discrete Green
operator which may be useful for materials characterisation methods like diffraction techniques.
The development of these new consistent discrete Green operators in the Fourier space allows toeliminate oscillations while retaining similar convergence capability. For illustration, the new discrete
Green operators are implemented in a fixed-point algorithm for heterogeneous periodic composites
knows as the Moulinec and Suquet (1994,1998) ''basic scheme'' that we extended to consider eigenstrain
fields, as in Anglin, Lebensohn and Rollett(2014). Numerical examples are reported, such as thecomputation of the local stresses and displacement of composite materials with homogeneous orheterogeneous elasticiy combined with dilatational eigenstrain or representing prismatic dislocation
loops. The mechanical fields obtained for cubic-shaped inclusion, spherical Eshelby and inhomogeneity,
are compared with analytical solutions and the discretization method using the classic Green operator.

MMM 2018

357

(F11 oral)
Efficient FFT-based Homogenisation without Linear Reference
Medium*Till Junge1
1. EPFL, Mech. Engineering, Multiscale Mechanics ModelingIn the field of computational homogenisation of periodic representative volume elements (RVE), over thelast two decades, fast Fourier transform (FFT)-based spectral solvers have emerged as a promisingalternative to the finite element method (FE).
Most spectral methods are based on work of Moulinec and Suquet [1] and split an RVE's response intothat of a linear reference medium and a periodic fluctuation due to heterogeneities. The main advantageof this formulation over FE is that it can be both significantly faster and memory-saving. The two main
problems are 1) the choice of the reference medium, which is typically based on heuristics, non-trivial andhas a strong impact on the method's convergence (A bad choice can render the method non-convergent),
and 2) convergence is not uniform. Numerous studies have suggested mitigations to both of these
problems (e.g. [2]), but they have remained substantial disadvantages compared to the more expensive,
but also more robust FE.
Recent work by Zeman et al. [3] proposes a new formulation for spectral solvers which dispenses with thelinear reference problem and converges unconditionally. We present μSpectre, an open implementationof this novel method and use it to show that the new approach is more computationally efficient than its
linear reference medium-based predecessors, converges in the presence of arbitrary phase contrast including porosity - and eliminates or drastically reduces Gibbs ringing.
[1] H. Moulinec and P. Suquet. A numerical method for computing the overall response of nonlinear
composites with complex microstructure. Computer Methods in Applied Mechanics and Engineering,157(1):69–94, 1998
[2] M. Kabel, et al. Efficient fixed point and Newton-Krylov solvers for FFT-based homogenization ofelasticity at large deformations. Computational Mechanics, 54:1497–1514, 2014[3] J. Zeman, et al. A finite element perspective on non-linear FFT-based micromechanical simulations. Int.
J. Num. Meth. Eng., 2016

358

MMM 2018

(F11 oral)
Multiscale modelling of the effective viscoplastic behavior of
constituents of the mantle transition zone (Mg2SiO4 wadsleyite andringwoodite): bridging atomic and polycrystal scales*Olivier Castelnau1, Katell Derrien1, Sebastian Ritterbex2, Philippe Carrez2, Patrick Cordier2
1. Process and Engineering in Mechanics and Materials, CNRS/ENSAM/CNAM, Paris, 2. Unité Matériaux etTransformations, CNRS / Univ. Lille, Lille

The Earth mantle transition zone (an envelope of the Earth’s interior between 410 and 660 km depth) is
constituted by more than half from wadsleyite and ringwoodite. Waldseyite is present up to 525km depth,
whereas ringwoodite appears below. In the transition zone, pressure ranges from 14 GPa to 24GPa, andtemperature lies between 1880°K and 1900°K. Estimation of the viscoplastic behavior of these
constituents, and the link with their microstructure (crystallographic texture), is crucial to better
understand the structure of large scale convection cells in the mantle responsible for plate tectonic.
In this work, the viscoplastic behavior of wadsleyite and ringwoodite polycrystalline aggregates (cm scale)is obtained by bridging several scale transition models, starting from very fine scale (nm) of the dislocationcore structure. This presentation will emphasize the grain-polycrystal scale transition.
Deformation resulting from thermally activated dislocation glide has been modeled in wadsleyite andringwoodite at high pressures, for a wide range of temperatures, and under laboratory and in situ (i.e.
mantle) strain-rates conditions. The model relies on the structure and kink pairs nucleation enthalpies of
the rate controlling screw dislocations which have been modeled using the Peierls–Nabarro–Galerkinmethod and an elastic interaction model. Corresponding single slip critical resolved shear stresses(CRSS), and associated constitutive equations have been deduced from Orowan’s equation in order todescribe the average viscoplastic behavior at the grain scale, for all available slip systems.
These data are then implemented in two grain-polycrystal scale transition models, a statistical one(Second-Order Viscoplatic Self-Consistent scheme) allowing rapid evaluation of the effective viscosity ofthe polycrystalline aggregates, and a full-field method (FFT based method) allowing investigating many
inter- and intra-granular features such as stress and strain localization in a typical microstructure,
heterogeneous activation of slip systems, etc. Calculations have been performed for pressure,temperature, and strain-rates conditions corresponding to laboratory and in situ conditions. The obtained
effective behavior is in very good match with available experimental data.

MMM 2018

359

(F11 oral)
Scalable and Directionally-Sensitive Three-Dimensional Quantifying ofOrientation and Dislocation Density Gradients in Crystal Plasticity
Computer Simulations*Markus Kuehbach1, Franz Roters1
1. Max-Planck-Institut fur Eisenforschung GmbH

In-grain orientation and dislocation density gradients, especially those building up at grain and phaseboundaries, tune to significant extent a variety of microstructure evolution mechanisms: the nucleationand incipient stage of discontinuous dynamic recrystallization, deformation texture evolution, or phasetransformations to name but a few. Despite being additionally also of relevance for stress and strain
partitioning, their comprehensive quantification in three dimensions, though, has received little attention
so far and remains elusive.
In this work we report on higher-order neighbor- and signed-distance-based techniques for quantifying
such gradients; resulting in a tool applicable to point-cloud-based data. These are tracked for instance via
integration point cloud sets in finite-strain-formalism full-field crystal plasticity computer simulations.
Exemplarily, as such an evaluation of dislocation gradients resulting from Crystal Plasticity deformation
simulations using the DAMASK spectral solver [1, 2] will be presented and their relevance fordiscontinuous dynamic recrystallization discussed.

[1] P. Eisenlohr, M. Diehl, R.A. Lebensohn, F. Roters: A spectral method solution to crystal
elasto-viscoplasticity at finite strains, International Journal of Plasticity 46 (2013), 37 –53[2] P. Shanthraj, P. Eisenlohr, M. Diehl, F. Roters: Numerically robust spectral methods for crystal plasticity
simulations of heterogeneous materials, International Journal of Plasticity 66 (2015) 31 - 45

360

MMM 2018

(F12 invited)
Microstructure formation in large-scale molecular dynamics simulation*Yasushi Shibuta1, Shin Okita1, Eisuke Miyoshi2, Shinji Sakane2, Tomohiro Takaki2, Munekazu
Ohno3
1. The University of Tokyo, 2. Kyoto Institute of Technology, 3. Hokkaido University

Microstructure formation is discussed on the basis of large-scale molecular dynamics (MD) simulations of
nucleation, solidification and subsequent grain grow for pure iron [1-3]. After the nucleation andsolidification, the time change of volume of grains is directly estimated for 2D and 3D grain growths [1].
For the 2D grain growth, grains with seven and more neighboring grains generally grow larger, whereas
those with five and less neighboring grains shrink and disappear [2], which basically agrees with von
Neumann-Mullins law. For the 3D grain growth, threshold number of neighboring grains is estimated to beapproximately 14 [2]. Moreover, the physical origin of the deviation from the ideal grain growth isinvestigated by the direct estimation of the reduced mobility (i.e., the product of the intrinsic grainboundary mobility and the grain boundary energy) and the geometric factor [3]. The new insights basedon large-scale MD simulations up to billion atoms are achieved for the first time owing to a multi-graphics
processing unit (GPU) parallel computation on the GPU-rich supercomputer.
[1] Y. Shibuta, et al., Nature Comm. 8 (2017) 10. [2] S. Okita, Y. Shibuta, ISIJ Int. 56 (2016) 2199. [3] S.
Okita et al., submitted.

MMM 2018

361

(F12 oral)
Directed assembly of structured nanoparticles through rapid
micromixing*Arash Nikoubashman1, Nannan Li2, Athanassios Z. Panagiotopoulos2
1. Johannes Gutenberg University of Mainz, 2. Princeton University

Tailored nanoparticles are increasingly sought after for many scientific and technological applications,such as optoelectronic devices and highly selective catalysts. However, both research andcommercialization of these materials has been impeded by the lack of suitable fabrication techniques.
One promising approach for overcoming this hurdle is flash nanoprecipitation, where (soft) nanoparticles
are assembled through rapid micromixing of polymers in solution with a miscible poor solvent. This
continuous process allows for high yields as well as precise control over particle size and morphology. We
employed multiscale computer simulations of this process to understand its underlying mechanisms andto efficiently explore parameter space. We first performed explicit solvent molecular dynamics (MD)simulations of a bead-spring polymer model to study the microscopic properties of the fabrication
process. Then, we fed the MD data into a kinetic Monte Carlo algorithm to reach macroscopic length- andtimescales. We discovered that the nanoparticle size can be reliably tuned through the initial polymer
concentration and the mixing rate. Further, we were able to fabricate a wide variety of structured colloids,
such as Janus and core-shell particles, when polymer blends were used in the feed stream.

362

MMM 2018

(F12 oral)
Various interfaces related to twinning in hexagonal metals*Vaclav Paidar1, Andriy Ostapovets2
1. Institute of Physics AS CR Prague, 2. Institute of Physics of Materials AS CR Brno

The {10-12} twin mode is the one most commonly activated in hexagonal metals. A primary part of thelattice transition from the matrix to the twin orientation is shear deformation that converts the [0001]
direction of the hexagonal axis in the matrix to a direction almost parallel to the [10-10] direction
perpendicular to the <a> axis in the basal plane in the twin. In addition to primary mechanical twinningalso several types of double-twins were observed as, for example, {10-11}-{10-12} or {10-12}-{10-12}.
Surprisingly, the last one can occur in magnesium with the c/a ratio smaller than √3 but also in zinc withthe c/a ratio larger than √3. Various types of symmetrical and asymmetrical interfaces including also
basal/basal or prismatic/prismatic boundaries will be discussed and their impact on materials properties
will be envisaged.

MMM 2018

363

(F12 oral)
Multiscale mean-field modelling of mechanochemical processes inheterogeneous materials for energy storage*Mikhail Poluektov1, Łukasz Figiel1,2
1. International Institute for Nanocomposites Manufacturing, WMG, University of Warwick, 2. Warwick Centre forPredictive Modelling, University of Warwick

Performance enhancement of energy storage devices, e.g. batteries, requires careful selection ofmaterials. The microstructure of such materials can undergo a complex electro-chemo-mechanical cycling
during battery exploitation, which involves extreme volumetric expansion of the active material during thechemical reaction. The expansion is causing mechanical stress, which, in turn, influences the kinetics ofchemical reactions even up to their arrest [3]. Thus, to predict the mechanochemical behaviour of amulti-material battery electrode, both the multi-physics phenomena and microstructure must be taken
into account.
Up to now, the major focus has been on development of coupling models between mechanics, diffusionand chemical reactions, e.g. [4], particularly, chemical reactions, such as lithiation and oxidation, that takeplace at a surface, e.g. [1-2]. When localised reactions are modelled, the thermodynamic consistency has
to be maintained and the velocity of the chemical reaction front should not violate the entropy production
inequality and the balance laws. One such model is based on the chemical affinity tensor [2]. This model
was used to predict the kinetics and the arrest of the reaction front in free-standing Si particles of abattery anode [5].
The model presented in this talk builds on [5] and accounts for the lithiation kinetics of a collection ofparticles inside an effective matrix material. The battery microstructure is modelled using the multiscale
mean-field framework based on the incremental Mori-Tanaka method. This is the first application of amultiscale mean-field technique to modelling lithiation reaction front kinetics in a complex anodemicrostructure within the finite-strain framework, and to linking the intraparticle kinetics with themacroscopic response of the battery.
Acknowledgements
Financial support from the EU H2020 project Sintbat (685716) is gratefully acknowledged.
References[1] ZW Cui, F Gao, JM Qu (2013) J Mech Phys Solids 61:293–310[2] AB Freidin, EN Vilchevskaya, IK Korolev (2014) Int J Eng Sci 83:57–75[3] K van Havenbergh, S Turner, N Marx, G van Tendeloo (2016) Energy Technol 4:1005–1012[4] VI Levitas, H Attariani (2014) J Mech Phys Solids 69:84–111[5] M Poluektov, AB Freidin, L Figiel (2018) Int J Eng Sci accepted364

MMM 2018

(F12 oral)
Impact of local symmetry breaking on the physical properties oftetrahedral liquids*Rui Shi1, Hajime Tanaka1
1. University of Tokyo

Water and silica are the most important materials with local tetrahedral symmetry. They have similar
crystalline polymorphs and exhibit anomalous density maximum in the liquid state. However, water andsilica also show very different characteristics. For instance, the density of water varies much more sharply
than that of liquid silica near the maximum as temperature changes. More notably, silica is a very goodglass-former, but water is an extremely poor one. The physical origins of these similarities and differences
still remain elusive, due to the lack of a microscopic understanding of the structural ordering in these two
important liquids. Here, by accessing microscopic structural information by computer simulations, wereveal that local translational symmetry breaking is responsible for the density anomalies. On the other
hand, the difference in the degree of local orientational symmetry breaking between water and silica,
which originates from the difference in their bonding nature, causes not only the difference in the
sharpness of density anomalies, but also their distinct glass-forming abilities. Our work not only shows the
crucial roles of local translational and orientational symmetry breaking in the physical properties of thetwo extremely important materials, water and silica, but also provides a unified scenario applicable forother tetrahedral liquids such as Si, Ge, C, BeF2, and GeO2.

MMM 2018

365

(F13 invited)
Advanced analysis tools for atomistic microstructure modeling*Alexander Stukowski1
1. Dept. of Materials Science, Darmstadt University of Technology

Direct molecular dynamics (MD) simulations of materials microstructures become feasible thanks to fast
growing computing capacities and can deliver a wealth of important observations on fundamental
mechanisms. Such simulations are cross-scale rather than multi-scale, i.e. simultaneously large enough torepresent meso- and macroscopic structures and processes and yet fully resolved, tracing every "jiggle" ofatomic motion. Our recent MD simulations of dislocation-based crystal plasticity will be presented as an
example [Nature 550, 492–495 (2017)].
To deal with the overwhelming complexity of processes on these scales, we develop advanced
computational methods and tools to recast the massive transient trajectory data generated in large-scale
MD simulations into a human comprehensible and analyzable form using methods of "in-situ
computational microscopy". These algorithms automatically identify grain boundaries, surfaces andinterfaces, dislocations, other defects, structural phases and deformation fields and can track theirevolution. The data reduction and transformation of the underlying MD model into high-level
microstructure representations make insightful visualization, quantitative analyses and linking withhigher-scale materials models possible.

366

MMM 2018

(F13 oral)Studing thermo-oxidative degradation of polyimide in oxygen
enviorment using MD simulations*Ashwani Kumar Sengar1, Sumit Basu1
1. IIT Kanpur

Polyimides are important class of high-temperature polymers. These polyimides finds applications indiverse fields such as aerospace, electronics and automobiles. PMR-15 is one these polyimide which isextensively used as matrices for composite in the aerospace industry due to its high glass transition
temperature, mechanical strength with high modulus of elasticity. However diffusion of oxygen in polymer
and subsequent reaction with polymer leads to formation of oxidized layer. This layer drastically changes
the mechanical properties of the polymer. Since these polyimide’s founds application in aircraft design,
so thermo-oxidative degradation of polymer may prove fatal and catastrophic in oxygen rich environment.
Crack initiation and damage growth also follows due to thermo-oxidative degradation. We will be studying
thermo oxidative degradation of PMR-15 polymer through molecular dynamics(MD) simulation. Molecularsimulations has been become quite popular tool to predict macroscopic properties of polymer throughstatistical study at atomistic level. However getting correct force field parameter to simulate an exact
material is challenging and of utmost importance. We attempt to study PMR-15 polymer with its fullatomistic details intact and predicting it micromechanical properties through MD simulation. Due to somecomputational and forcefield limitations, coarse grained model of polyimide had been previously studied.
In our analysis, we don’t make any pre-assumption of course graining during sample preparation. We
prepare our sample of PMR-15 with correct cross linking strategy such that we get density close toexperimental value of 1.324. We will quantifies mechanical properties such as elastic constant andphenomenogical quantities such as diffusion constant and tries to predict the life of polymer due tothermo-oxidative degradation. We want challenge the time and scale limitations of MD simulation byattempting to simulate full scale atomistic model of the polymer PMR-15. This study will give more insightabout capabilities of MD simulations and also provides reasons where it fails to give us valid results. This
study will provide an alternative path to study the systems where experimental analysis is difficult to setup
or too expensive and cumbersome.

MMM 2018

367

(F13 oral)
In-plane characterization of structural and thermodynamic properties
for steps at faceted chemically heterogeneous solid/liquid interfaces*Hongtao Liang1, Brian B. Laird2, Mark Asta3, Yang Yang1
1. Physics Department and State Key Laboratory of Precision Spectroscopy, School of Physical and Material Science,
East China Normal Universit, 2. Department of Chemistry, University of Kansas, 3. Department of Materials Scienceand Engineering, UC Berkeley

We present a methodology for studying steps at faceted chemically heterogeneous solid/liquid interfaces
based on molecular-dynamics simulations. The methodology is applied to a faceted Al(111)/Pb(liquid)interface yielding a direct calculation of step free energy and extensive atomic-scale characterization for
the interfacial layer containing the step. We characterized the step by calculating the in-plane density,
potential energy, pressure components and stresses profiles. Main findings include: i) Calculated step freeenergy is in good agreement with TEM experimental measurement. ii) The step is connecting the interface
liquid under tension and the interface solid under compression. iii) Fundamental properties of interface
solid and interface liquid show orders of magnitudes difference in comparing with those predicted fromthe bulk Al-Pb alloy phase diagram. It is hoped that the methodology is extendable to the exploration ofcomplexion equilibria/transitions at grain boundaries.

368

MMM 2018

(F13 oral)
Design of neural network for thermodynamics data of non-equilibrium
multiphase field model*Hiroshi Wakameda1, Sukeharu Nomoto1, Akinori Yamanaka3, Toshiyuki Koyama2
1. ITOCHU Techno-Solutions Corporation, 2. Materials Design Innovation Engineering, Nagoya Univ., 3. Departmentof Mechanical Systems Engineering, Tokyo University of Agriculture and Technology

We construct neural network which estimates Gibbs free energy and chemical potential from temperatureand composition of system for non-equilibrium multiphase field model. The mini-batch gradient descentmethod is selected for training of which data is led from calculation of non-equilibrium multiphase fieldmodel using Thermo-Calc thermodynamic database. We achieve highly precision neural network enoughto use in non-equilibrium multiphase field model by introducing minimum and maximum data to themini-batch method.

MMM 2018

369

(F14 invited)
Large-scale multiphase flow simulations on a GPU supercomputer*Takayuki Aoki1
1. Tokyo Institute of Technology

GPU (Graphics Processing Unit) was originally designed for graphics rendering and has been widely used
for supercomputers in the world, since it has high computational performance and wide memory
bandwidth and suitable for scientific computing. We have developed stencil codes written in CUDA andexecuted them on a GPU supercomputer TSUBAME. Several simulations of gas-liquid and gas-solid
two-phase flows are demonstrated with the numerical methods and the algorithms such as AMR (Adaptive
Mesh Refinement) and dynamic load balance for efficient large-scale simulations.

370

MMM 2018

(F14 oral)
Transport Properties Of Fluid Mixtures In Micro- And Mesoporous
Kerogen Membranes*Patrick Alain Bonnaud1, Romain Vermorel1, Julien Collell2, Guillaume Galliéro1
1. University of Pau and Adour Countries, 2. Total E&P

With the exhaustion of conventional resources, the production of gas from organic-rich shales has
encountered a rising interest over the last 15 years. In such resources, gas is trapped in nodules oforganic matter scattered in an inorganic matrix mostly constituted of quartz, clays, and calcite. Nodules
represent only few percentages of the total volume of shales and result from a maturation process during
the burial stage. These nodules of organic matter contain mostly kerogen that acts as a source, but also as
a container of hydrocarbons. Various forms were reported differentiating to each other by their origin,their maturity, and sediment history, which affects, for example, their chemical composition, their density,and the porosity. A common feature of kerogens is their multi-scale porous network with pore sizes
ranging from micropores (<2 nm) to macropores (>50 nm). Structure and connectivity of pores greatly
affect the materials permeability. In porous structures where the flow is limited by micropores (e.g., porethroats), very low permeabilities are observed.
Despite the important development of shales, fluid flow mechanisms within kerogen matrix remains poorly
understood. While experimental characterization remains difficult, molecular dynamics simulations wereproven very useful to characterize transport mechanisms within kerogen membranes. The aforementioned
studies staid limited to microporous systems, while mass transfer originating from small pores andtransiting to larger pores, fractures, and production wells is a multi-scale mechanism. In this work, we
designed and developed molecular simulation tools to capture both microporosity and mesoporosity of
kerogen through the aggregation and the spatial arrangement of smaller basic units in order to generate
representative molecular structures. From those models, we investigated by molecular dynamics
simulations how the nature (i.e., maturity and sediment origin) and the chemical composition influenceboth the porous network and the related physical and transport properties of fluid mixtures (e.g.,
selectivity, phase coexistence, diffusivity) in kerogen.
This method and findings underscore the importance of accounting for both micro- and mesoporosity toaccurately model fluid transport in kerogen.

MMM 2018

371

(F14 oral)
Numerical simulation of ionic transport through deformable porous
media: application to cortical bone tissue modeling*Jana Turjanicova1,2, Eduard Rohan1,2, Vladimír Lukeš2
1. Dept. of Mechanis, Univ. of West Bohemia in Pilsen, 2. NTIS - New technologies for information society

Cortical bone tissue is known to have a strictly hierarchical porous structure on multiple scales. On thecanalicular-lacunar scale, it consist of two phases: deformable collagen-apatite matrix charged by a small
electric charge and bone fluid filling the pore space created by network of small interconnected channels.
Bone fluid is considered an electrolyte solution of two species of charged monovalent ions of opposite
polarizations. Due to the potential differences, the electrical double layer occurs in the proximity of thesolid-fluid interface. Considering its effect, the transport of such electrolyte through a network of small
channels with charged surface is controlled by coupling between the electric field , Stokes flow, themigration-diffusion process and deformation of solid matrix.
In order to describe cortical bone effective properties on the macroscopic scale, the unfolding
homogenization method is applied on the model of ionic transport through deformable porous media.
The microstructure on canalicular-lacunar scale is simplified by assumption of its periodicity and isrepresented by so-called representative periodic cell. The characteristic responses on this cell are used tocompute the effective coefficients describing cortical bone behavior on the macroscopic scale (scale ofone osteon). Macroscopic model behavior was tested on the simple boundary value problem.
Implementation of upscaling process as well as numerical simulation on the macroscopic model was
made in in-house developed python based FEM software SfePy.

372

MMM 2018

(P1-33)
UNDERSTANDING OF DELAYED HYDRIDE CRACKING FAILUREMECHANISM IN E110 ZIRCONIUM BASED FUEL CLADDINGS BY EBSDAND IN-SITU MECHANICAL TESTING*Hygreeva Kiran NAMBURI1, Zbynek Spirit1, Patricie Halodova1, Ondrej Libera1, Jakub krejci2
1. Research Centre REZ, 2. UJP Praha

Zirconium based alloys are commonly used as material for fuel claddings in the light water reactors.
Claddings act as first metallic barriers against loss of fission products during the nuclear power plant
operation, intermittent storage or final dry storage. During the reactor operation, metallic claddings areprone to water side corrosion and subsequent hydrogen pick-up (in 10’s to 100’s of ppm) due tohigher operating temperatures in reactor. Under specific favorable conditions (stress, temperature andhydrogen concentration level) claddings fail by a time dependent mechanism called Delayed Hydride
Cracking (DHC). This results in critical issue for the safe performance of the power plants and storage
used claddings.
In this work we present results from the DHC study on zirconium based E110 metallic fuel claddings. Test
specimen is oxidized in an autoclave to have desired hydrogen content. DHC experiments are performedin scanning electron microscope chamber by using in-situ tensile testing device at high temperatures.
The paper emphases on microstructural studies, evaluation of threshold stress intensity, crack
propagation rate, understanding fialure mechanism and role of crack tip hydrides fracture during the DHC
failure in E110 fuel claddings.
Keywords: Zirconium alloys, nuclear fuel claddings, microstructure, crack tip hydride fracture, delayed
hydride cracking failure.

MMM 2018

373

(P1-34)
Transition-metal alloying of γ‘-Ni3Al: Effects on the ideal uniaxial
compressive strength from first-principles calculations*Minru Wen2,1, Chongyu Wang2
1. Guangdong University of Technology, 2. Tsinghua University

The addition of transition metal (TM) elements into the γ′precipitate phase of a Ni-based single-crystal
superalloy can significantly affect its mechanical properties, including the intrinsic mechanical property ofcompressive strength. Using first-principles density functional calculations, the effects of 3d (Sc–Zn), 4d(Y–Cd) and 5d (Hf–Au) TM alloying elements on the ideal uniaxial compressive strength of γ′-Ni3Al were
investigated. The stress–strain relationships of pure Ni3Al under [100], [110] and [111] compressive loads
and the site occupancy behavior of TM elements in Ni3Al were prior studied using a total-energy method
based on density functional theory. Our results showed that the capacity of TM elements forstrengthening the ideal compressive strength was associated with the d-electron number. The alloying
elements with half-filled d-bands (i.e., Cr, Mo, W, Tc and Re) manifested the greatest efficacy for improving
the ideal strength of Ni3Al under a deformation along the weakest compressive direction. Furthermore,
the charge redistribution of Ni3Al doped with 5d elements were also analyzed to understand thestrengthening mechanisms of TM elements in the γ′-Ni3Al phase.

374

MMM 2018

(P1-35)
The Influence of Deposition Pattern on Stress and Mechanical
Properties in Wire Arc Additive Manufacturing*Changmeng Liu1, Qianru Wu1, Jiping Lu1, Shuyuan Ma1
1. Beijing Institute of Technology

Wire arc additive manufacturing (WAAM) has exhibited great advantages of high deposition rate, large
fly-to-buy ratio and low cost in aerospace applications. However, the deformation caused by internal
stress is still a technical challenge in additive manufacturing, especially during the manufacturing processof the large-scale components. In this work, based on numerical modelling method and physicalexperiments, by investigating the stress and deformation distribution of Ti6Al4V components depositedby five typical patterns, the preferred deposition pattern will be selected to fabricate components withminimum deformation and uniform stress distribution. The morphology, microstructure and mechanicalproperties of the components with different deposition patterns were studied as well. The results illustratethat short S-shape pattern is identified as the optimal one which has uniform stress distribution andminimum deformation. The stress distribution of the components with spiral pattern and subarea patternvaries a lot and the reasons have been discussed based on the thermomechanical behavior in WAAM.

MMM 2018

375

(P1-36)
On the Significance of the Higher-Order Neighbors for Abnormal Grain
Growth and Recrystallization Nucleation*Markus Kuehbach1
1. Max-Planck-Institut fur Eisenforschung GmbH

Successful nucleation of abnormal grain growth and primary static recrystallization constitutes a rare
event which affects in many cases only each thousandth (sub-) grain if at all. Consequently, the making of
more quantitatively substantiated number density predictions can be improved by studying million-scaleinitial grain populations. Thanks to representative volume element (RVE) method development andsoftware parallelization, simulating the evolution of such large grain populations has recently become
possible.
This work details how 2d and 3d resolved RVE computer simulations with such millions of (sub-) grains
within each single domain allow quantifying the process of preferential grain evolution during abnormal
grain growth and nucleation to primary static recrystallization to hitherto unachieved statisticalsignificance. By assessing not only the nearest but additionally higher-order neighbors of each grain more
precise and accurate predictions at which sites nucleation will likely initiate are possible. Albeit, the study
proofs also that to definitely identify which grains succeed --- and as such also the number density --requires assessing their entire topological event sequence surplus quantifying the distribution of theirindividual capillary- and stored elastic energy-induced grain boundary face migration speeds.

376

MMM 2018

(P1-37)
Hydrogen trapping in carbon supersaturated a-iron and its decohesion
effect in martensitic steel*Wen-Tong Geng1,2, Vei Wang1,3, Jin-Xu Li2, Nobuyuki Ishikawa4, Hajime Kimizuka1, Kaneaki
Tsuzaki5,6, Shigenobu Ogata1,6
1. Department of Mechanical Science and Bioengineering, Osaka University, 2. University of Science and Technology
Beijing, 3. Department of Applied Physics, Xi’an University of Technology, 4. Steel Research Laboratory, JFE SteelCorporation, 5. Department of Mechanical Engineering, Kyushu University, 6. Center for Elements Strategy Initiative
for Structural Materials, Kyoto University

It is generally accepted that the martensite in steels is more susceptible to hydrogen embrittlement than
the ferrite. The atomic-scale mechanism underlying this phenomenon, nevertheless, is not fully
understood yet. Our first-principles calculations demonstrate hydrogen is more stable in carbon
supersaturated martensite than in a-iron, due to the carbon-induced tetragonality in martensite lattice.
The trapped hydrogen leads to remarkable decohesion between (110) planes both inside the martensite
and along the martensite/ferrite interface, with the former being more significant than the latter. This
decohesion can explain recent precise observations that in martensite/ferrite dual-phase steels thehydrogen-promoted crack was initiated in the martensite region and that in lath martensite steel it
propagated not on lath boundaries but showed quasi-cleavage feature along (110) planes at very highhydrogen concentration.

MMM 2018

377

(P1-38)
Size Scale Effect on Energy Absorption Property of Aluminum Foam
*zengyou liang1, fudi liang1, dezhi deng1, chunzhuang miao1, mingguang wang1
1. North University of China

Abstract: Foam metal is extensively used in the fields of petrochemical engineering, aerospace andautomobile manufacturing for its excellent properties. As a typical representative of foam metal,Aluminum
foam is an excellent material in energy absorption,that it can produce larger plastic deformation under the
lower flow stress.Relative to the cell dimension,the size of specimen is an important factor which affects
the performance of its energy absorption. Based on the technological process of the infiltration casting
method, a microscopic structure model of aluminum foam is established by Monte Carlo method andGravity Accumulation method ,with the assistance of PFC3D software so as to study the influence of size
scale effect on energy absorption property of aluminum foam. A finite element model of aluminum foam isgenerated according to MATLAB software. Combining with SPH algorithm, numerical simulation to thecompression behavior of the different specimen size of aluminum foam is performed. From numericalsimulation,we can draw the conclusion that the greater the ratio of specimen dimension to cell dimension
is,the better energy absorption property it performs. The results can provide assistance on design buffer
suction device with foam mental.

378

MMM 2018

(P1-39)Development of charge-transfer type interatomic potential for SiC
oxidation*So Takamoto1, Takahiro Yamasaki2, Takahisa Ohno2, Chioko Kaneta3, Asuka Hatano1, Satoshi
Izumi1
1. The Univ. of Tokyo, 2. NIMS, 3. Fujitsu Lab.

Silicon carbide (SiC) is an attractive semiconductor material for applications in power electronic devices
and the fabrication of a high-quality SiC/SiO2 interface is desired. It is well known that there is a great
difference in oxidation rate between the Si-face and C-face, and that the quality of oxide layer on the
Si-face is better than that on the C-face. However, the atomistic mechanism of the thermal oxidation of
SiC remains to be solved. In this work, we developed a new Si-C-O interatomic potential to reproduce thekinetics of the thermal oxidation of SiC. More than 1000000 properties obtained by DFT calculations
were used for the fitting process. Using this interatomic potential, large-scale SiC oxidation simulations
were performed. In order to focus on the reaction of O2 molecules, the oxidation process is realized byinserting O2 molecules into SiO2 region. The results showed that the activation energy of the Si-face ismuch larger than that of the C-face. Also, the numbers of intermediate oxide states of Si atoms are in good
agreement with the experimental result. In the case of the Si-face, a flat and aligned interface structure
including Si

1+

was created. We estimated activation energies of the change of intermediate oxide statesand proposed that the stability of the flat interface structure is the origin of the high activation energy ofthe oxidation of the Si-face. In contrast, in the case of the C-face, it is found that the Si atom at theinterface are pulled up by the O atoms. This process generates the disordered interface and decreases
the activation energy of the oxidation.

MMM 2018

379

(P1-40)Alpha-phase in engineering aluminum alloys: a multiscale modelingapproach to its mechanical behavior*Duancheng Ma1
1. Leichtmetallkompetenzzentrum Ranshofen GmbH

Alpha-phase, i.e., Al(Fe,Mn)Si, is very common in engineering aluminium alloys. During ingot casting,coarse beta-phase forms. In the subsequent homogenization heat treatment, the beta-phase transforms
into coarse alpha-phase. In the following forming and solution annealing processes, the coarsealpha-phase remains in the matrix and never dissolve. Since the size of the coarse alpha-phase is large (afew micrometers), they hardly contribute to the strengthening effect. There are many examples, however,showing that it is one of the potential damage nucleation sites. Despite being common in engineering
aluminium alloys and being the damage nucleation sites, little is known about the mechanical propertiesof the alpha-phase. In this study, we use a multiscale modeling approach, i.e. from electronic tocontinuum scale, to investigate its mechanical behavior in aluminium.

380

MMM 2018

(P1-41)
Multi-phase-field lattice Boltzmann modeling and simulation ofequiaxed structure*Ryotaro Sato1, Tomohiro Takaki1, Shinji Sakane1, Munekazu Ohno2, Yasushi Shibuta3
1. Kyoto Institute of Technology, 2. Hokkaido University, 3. Tokyo University

Equiaxed structure formed during a solidification process of metals and alloys determines the mechanicalproperty of materials. Therefore, it is crucial to accurately predict and control the formation process of theequiaxed structure. However, the formation process of equiaxed structure is a complicated multiphysics
problem including a motion of multiple grains.
In this study, we construct a multi-phase-field-lattice Boltzmann model which can express the growth,
motion, collision, and coalescence of multiple dendrites and following grain growth. In this model, thegrowth and motion of multiple dendrites are expressed by a multi-phase-field method and equations of
motion, respectively, and the liquid flow is computed by lattice Boltzmann method. The collision andcoalescence of multiple dendrites and/or grains are also modeled in a concept of diffuse interface.
Moreover, the simulation is accelerated by employing the active parameter tracking and graphics
processing unit. By using the developed method, the formation simulations of the equiaxed structure aredemonstrated.

MMM 2018

381

(P1-42)Simulation of Extrusion Process of TiAl alloy prepared by Triple VAR*Fan Gao1, Zhenxi Li1
1. AECC BEIJING INSTITUTE OF AERONAUTICAL MATERIALS

TiAl alloy with low density and excellent mechanical properties at the high temperature is one of the mostpotential materials in aerospace industry, however the extrusion of the ingot with industrial scale isdifficult due to the poor ductility. In this study, FEM was employed to describe the extrusion plastic
deformation behavior of this alloy which prepared by triple VAR. Under various extrusion conditions, thestrain distribution and the extrusion load were studied by numerical analysis. The influence of the dieangle and the extrusion ratio were examined. The results showed that the peak extrusion force wasenhanced with the increased die angle with a certain slop coefficient. The die angle with 100° wassuggested to obtain the uniform deformation. With the raising of extrusion ratio, the effective strain,
extrusion load in billet would be increased. Then extrusion experiment of triple VAR TiAl ingot was carriedout successfully with extrusion ratio of 10.

382

MMM 2018

(P1-43)
Experimental-Computational Analysis of Primary Static
Recrystallizazion in DC04 Steel*Martin Diehl1, Markus Kühbach1, Dierk Raabe1
1. Max-Planck-Institut fuer Eisenforschung GmbH

Low-alloyed steels with body-centered cubic crystal structure (bcc, ferrite) are a material class which is
widely used in automotive sheet metal forming applications. When produced with an adequate
crystallographic texture, the mechanical behavior of steels for forming applications is characterized by anisotropic in-plane flow behavior in combination with a low yield strength. To obtain these beneficial
mechanical properties, an adequate cold rolling strategy in terms of the number of passes, deformation
rates, and total reduction needs to be followed by an annealing procedure with a time-temperature profilethat facilitates primary static recrystallization. The most fundamental connection between cold rolling andheat treatment consists in the reduction of the dislocation-related free energy stored during deformationby the formation of new grains with a very small dislocation content. Hence, the local variation in
crystallographic orientation and defect population lead to very inhomogeneous grain boundary migration
velocities.
We present here results of a coupled experimental-computational approach for studying microstructure
evolution in industrially cold rolled DC04 steel under quasi-isothermal conditions. For the experimental
characterization, quasi in-situ experiments consisting of interrupted isothermal holding at 600°C andsubsequent Electron Backscatter Diffraction (EBSD) orientation imaging were conducted. The results
reveal a strong correlation between local dislocation density, quantified via the Kernel Average
Misorientation (KAM), and the local increase of recrystallized volume fraction. To complement theexperimental findings and gain further insights into the influence of the sub-surface microstructure, aCellular Automata (CA) model was utilized. The employed three-dimensional microstructure model was
directly built from the experimentally characterized deformation microstructure. Different approaches ofscaling the KAM into dislocation density values, i.e. the driving force for recrystallization, are employed.
Similarities and deviations between experimental and computational results are discussed with the aim atincreasing the understanding the mechanisms of static primary recrystallization.

MMM 2018

383

(P1-44)
Combining 4D experiments and phase-field modeling to determine
reduced grain boundary mobilities*Jin Zhang1,2, Peter W Voorhees1, Henning F Poulsen2
1. Northwestern University, 2. Technical University of Denmark

Reduced grain boundary mobilities play an essential role in accurate multi-scale modeling of grain growthin polycrystalline materials. The reduced mobility is a function of a large five-dimensional parameter
space. Traditional bi-crystal experiments only determine one point in this space at a time. In this paper,
we present a method to determine the reduced mobilities by comparison between 4D experiments andphase-field simulations. The growth of 1327 grains in a pure iron sample is visualized in 3D using
diffraction contrast tomography (DCT) at a synchrotron source. Using the first time-step from theexperimental microstructure as input, the evolution of the entire grain structure is simulated using aphase-field model. A fitting approach is applied to find the set of reduced mobilities that yield the best
match between the experimental microstructure and the simulated microstructure. An efficient fitting
algorithm is constructed based on a sensitivity analysis. The fitting algorithm converges fast, and morethan 1000 reduced mobilities can be determined simultaneously.

384

MMM 2018

(P1-45)
Fiber-intersectant microstructure of fish scale and biomimetic
research*Bin Chen1, Jinghong Fan2, Miao Li1, Wei Ye1
1. College of Aerospace Engineering, Chongqing University, 2. Division of Mechanical Engineering, Alfred University

The microstructures of the scale of a chub fish are observed with scanning electronic microscope (SEM). It
is shown that the scale is a kind of natural biocomposite consisting of parallel hydroxyapatite fiber layers
and collage matrix. The hydroxyapatite fiber layers consist of long and thin hydroxyapatitefiber fiber
sheets. It is also observed that the fiber sheets in adjacent fiber layers compose a fiber-intersectant
microstructure. Based on the observed result, the fiber-intersectant microstructure is employed as thepattern for the design of man-made fiber-reinforced composite. A kind of fiber-reinforced composite withthe fiber-intersectant structure is biomimetically fabricated. The fracture toughness of the biomimeticalcomposite is tested and compared with that of the conventional composite with fiber-parallel structure. It
is indicated that the fracture toughness of the biomimetical composite is remarkably larger than that of
the composite with the fiber-parallel structure.

MMM 2018

385

(P1-46)
Microstructures of turtle shell and biomimetic fabrication*Bin Chen1, Jinghong Fan2, Wei Ye1, Miao Li1
1. College of Aerospace Engineering, Chongqing University, 2. Division of Mechanical Engineering, Alfred University

Scanning electron microscope (SEM) observation shows that the shell of red-ear turtle is a kind of natural
sandwich composite consisting of exterior cortex and interior cancellus. It is also observed that thehydroxyapatite fibers in the composite continuously surround the holes of the composite forming aparticular fiber-surrounded-hole structure. Based on the result of the observation, a biomimetic
composite with the fiber-surrounded-hole structure is fabricated. The ultimate strength of the biomimetic
composite is tested and compared with that of the conventional composite with thenon-fiber-surrounded-hole structure. It is indicated that the ultimate strength of the biomimetic
composite is remarkably larger than that of the conventional composite.

386

MMM 2018

(P1-47)
Phase-field Simulation of Solidification Process in Welding Pool of
Fe-C Binary Alloy*lan zhan1, Xiangge Qin1
1. Jiamusi Univ.

Abstract: In this paper, the phase field method was used to study the growth process of Fe-C binary alloy
welding pool dendrite. In the phase field model, the characteristics of small volume, fast cooling rate,
large temperature difference and high degree of superheat of the welding pool were considered. Andbase on the model, the influence of undercooling on the crystal morphology of the alloy was predicted.
Firstly, the growth morphology of Fe-C alloy dendrites in welding pool was successfully simulated by using
the phase field model. Meanwhile, The effects of C concentration distribution on dendrite growthmorphology and dendritic spacing were further investigated. The simulation results are consistent withthe melten pool solidification theory.

Keywords: Fe-C alloy;solidification of weld molten pool;phase field method; dendritic morphology

MMM 2018

387

(P1-48)
Phase field simulation of the phase separation in the TiC-ZrC-WC
system*Zelin Luo1, Hong Ma1, Sai Tang1, Yingbiao Peng2, Yong Du1, Zikui Liu3, Qianhui Min1, Yafei Pan4
1. State Key Lab of Powder Metallurgy, Central South University, 2. College of Metallurgy and Materials Engineering,
Hunan University of Technology, 3. Department of Materials Science and Engineering, Pennsylvania State University,
4. School of materials science and engineering, Hefei University of Technology

TiC-ZrC-WC system with high hardness is a promising material being widely used in industries likeprocessing and manufacturing. Understanding the microstructural evolution and the mechanism duringphase separation process is still a formidable challenge nowadays. Microstructural evolution mechanisms
during phase separation process are explained for the first time through the methodology combing our
CALPHAD data and two-dimensional Cahn-Hilliard/elastic strain energy model, and we used theparameters in our database of thermodynamics and dynamics. We investigate the effect of elastic strain
on lamellar structure, agreeing well with previous results in terms of the variation of the periodicity of the
distribution of element composition and the periodicity of regularly lamellar microstructures. It is obviousthat phase field method coupled with thermodynamic database is a useful approach to study the
microstructure evolution of TiC-ZrC-WC materials and in further speed up the research and developmentof new materials of TiC-ZrC-WC system.

388

MMM 2018

(P1-49)
Switching of coordinate transformations of a repetitive bar-and-joint
framework under uniaxial compression*Hiro Tanaka1, Kazutoshi Hamada1, Yoji Shibutani1,2
1. Dept. of Mechanical Engineering, Osaka Univ., 2. Nanotechnology Program, Vietnam Japan Univ.

In recent years, a variety of artificial microstructures with multi-functionality have been extensively
developed by incorporating specific geometric features. The topic appears in a wide range of fields fromgeometry to crystallography to engineering, and indeed the mechanical behaviors of many of thesestructures remain unexplored. Some examples of such behaviors include auxeticity in materials ofnegative Poisson’s ratio, origami-based folding and deployment, and deformability of hierarchicallyarranged structures. In this context, we proposed the compressive structural system switching two typesof kinematic transformations toward diamond- and square-patterns. To clarify the transition mechanism,
we modelled a specific repetitive bar-and-joint framework with the two angular variables specifying therotation and distortion of the linked square components. Numerically exploring the equilibrium paths
then reveals a transition state of the structure at a critical value of the internal stiffness. A simplified
formulation of the model with weak nonlinear terms yields an exact solution of its transition state. We
further investigated the viscoelastic transition feature of the proposed structure to replace thecell-binding springs inside structure with damping components, and revealed the interestingtransformation characteristics, which depend on the compressive load speeds. The main idea is that ourabstract representation might provide an original perspective to nonlinear elastic phenomena in solidstate matter.

MMM 2018

389

(P1-50)
Understanding the effect of Residual Stresses in 3D Printed Metals*Alankar Alankar1, BVSS Bharadwaja1, Ritam Chatterjee1
1. IIT Bombay

We present phase field and atomistically informed simualtions of 3D printed CP-Ti. In the simulations, themicrostructure can be controlled as a function of heat intensity, source velocity and powder size. Also,these process parameters are shown to have direct effect on the preferred crystallographic texture,
porosity fraction and residual stresses in the additively created microstructure. This syntheic
microstructure is then used as input for crystal plasticity model. In the crystal plasticity model the effect ofvoids coupled with crystallographic texture is studied as a function of various boundary conditions.

390

MMM 2018

(P2-43)
Design of spontaneous formation-based 3D plasmonic opticalstructure, using multyphysics modeling*Jihwan Song1, Inhee Choi2, Yonghee Shin3, SoonGweon Hong5, Younggeun Park5, Dongchoul
Kim4, Taewook Kang3, Luke Lee5
1. Dept. of Mechanical Engineering, Hanbat National University, 2. Dept. of Life Science, University of Seoul, 3. Dept.
of Chemical and Biomolecular Engineering, Sogang University, 4. Dept. of Mechanical Engineering, Sogang University,
5. Dept. of Bioengineering and Berkeley Sensor and Actuator Center, University of California, Berkeley

Oil droplets in water or water droplets in oil have been generated fascinating science and utilized inenormous applications from medicine to energy harvesting. However, the creation of integrated
three-dimensional architectures by liquid droplet and immiscible liquid interface is relatively lessinvestigated. Here we report interfacial energy-driven and spontaneous formation of plasmonic cavity atroom temperature without an external force. With the multiphysics approach considering the densities
and interfacial energies of two different liquids, we simulated the spontaneous formation of cavity when aliquid water droplet meets immiscible liquid interface. At the interface, the metal ions in the liquid droplet
are automatically reduced and they form the interfacial plasmonic layer onto the cavity surface. Due tothe both optical cavity and integrated plasmonic structure, the significantly enhanced fluorescence isobtained by 1000 times. We believe our findings could offer a new avenue and advance in a variety ofphotonic and plasmonic materials and devices.

MMM 2018

391

(P2-44)
Characterization of KxNa1-xNbO3 powders and ceramics prepared byhydrothermal synthesis*Jing Yang1, Aifen Tian1, Xuan Xi1, Huiling Du1
1. Dept. of Materials Science and Engineering, Xi An Univ. of Science and Technology

There is an increasing demand to replace Pb(Zr,Ti)O3-based piezoelectric materials with lead-freealternatives, because of the toxicity of lead oxide which is largely used during the production process. Kx
Na1-xNbO3 (KNN) is considered as one of the most promising candidates for lead free piezoelectric
ceramics due to its high Curie temperature and good electrical properties.
KxNa1-xNbO3 ceramics can be used for several applications such as high frequency transducers,
ultra-sonic diagnostics and tunable micro-wave components. However, it is well known that dense andwell-sintered K0.5Na0.5NbO3 ceramics are very difficult to obtain by the ordinary sintering process owing to
the high volatility of alkali elements at high temperatures. The major strategy to overcome this problem issimply to synthetized KNN powders at low temperature. One method of making dense (KxNa1−x)NbO3
ceramics is to use refined powder with improved sintering activity, prepared in the molten salt process,sol-gel routine or hydrothermal process. In this work, (KxNa1−x)NbO3 powders and ceramics were preparedby hydrothermal synthesis. X-ray diffraction and scanning electron microscope were performed toinvestigate the structure and surface morphology of the (KxNa1−x)NbO3 powders and ceramics. The results
showed that all the KNN powders possessed the pervoskite structure and a handful of second phases. The
K0.7Na0.3NbO3 ceramic prepared by the powders exhibits relatively good properties (relative dielectric
constant ε= 416 and piezoelectric coefficient d33=40 pC/N).

392

MMM 2018

(P2-45)
Numerical and experimental investigation of liquid metal dealloying of
Cu-Ni alloy in liquid silver.
*Pierre-Antoine Geslin1,2,3, Takumi Suga2, Takeshi Wada2, Hidemi Kato2
1. INSA Lyon/CNRS, 2. Institute for Materials Research, Tohoku University, 3. Frontier Research Institute forInterdisciplinary Sciences, Tohoku University

Liquid metal dealloying has emerged as a promising technique to produce finely porous structures ofvarious nature (non-noble metals, refractory metals or semi-conductors) presenting a high surface area,
valuable in a numerous applications (catalysis, battery materials, sensors,...). This process consists inemerging a binary precursor alloy (i.e. Cu-Ni) in a liquid metal (Ag) chosen such that only one element of
the precursor alloy (Cu) dissolves into the metallic melt while the other element (Ni) reorganizes into a
porous structure. We investigated the formation of this microstructure based on the ternary phase
diagram of the Ni-Cu-Ag system. First, we developed a quantitative phase-field model to investigate theinitiation of this dealloying process. The phase-field method is particularly adapted to investigate this kindof free-boundary problem and the complex morphogenesis of the structures, but is enable to reach theexperimental time and size-scales. In a multi-scale approach, we use phase-field results and experimental
observations to develop a macroscopic diffusion model able to reproduce the kinetics and thecomposition profiles obtained experimentally. Also, based on this work on the Cu-Ni-Ag model system, wewere able to generalize our findings to other systems and assess the potential of other systems to form
finely porous microstructures upon dealloying.

MMM 2018

393

(P2-46)
Application of DLVO theory to predict dispersion stability of ZrO2
submicron particles in electrolyte solutions*Ming-Hong Chiueh1, Tien-Jung Huang1
1. Industrial Technology Research InstituteIn the manufacturing process of multi-layer ceramic capacitors (MLCC), electronic components often
used in modern mobile phones, dispersion stability of ZrO2 submicron particles can be improved byaltering the pH of electrolyte solution. In this study we used the DLVO theory to predict the energy
barriers of interaction forces between two ZrO2 particles in various electrolyte solutions at different pH.
The electrolyte solutions may be strong basic, weakly acidic, or strong acidic. The distance-dependent
potentials of van der Walls force and electrical double layer force were calculated. The calculation results
show that weakly acidic solution induces larger energy barrier between ZrO2 particles, because ofstronger electrical double layer force. This larger energy barrier can prevent aggregation of ZrO2 particles
and lead to dispersion stability.

394

MMM 2018

(P2-47)
Multi-GPU large-scale phase-field lattice Boltzmann simulation ofdendrite growth with thermal-solutal convection*Shinji Sakane1, Tomohiro Takaki1, Munekazu Ohno2, Yasushi Shibuta3, Takayuki Aoki4
1. Kyoto Institute of Technology, 2. Hokkaido University, 3. The University of Tokyo, 4. Tokyo Institute of Technology

Thermal-solutal convection, that inevitably occurs during terrestrial solidification of an alloy, drasticallychanges the dendrite morphology and microsegregation. Although phase-field method is the mostpowerful computational tool for predicting the dendrite morphology and microsegregation, we need many
computational costs in the phase-field simulation taking the thermal-solutal convection into account. In
this study, we enable a large-scale simulation for phase-field lattice Boltzmann model, which can express
the dendrite growth with the transport of solute and heat and the fluid flow. Here, to reduce thecomputational cost, we employ a multi-level mesh and multi-level time step when solving phase-field
equation, advection-diffusion equations for heat and solute, and lattice Boltzmann equation forcomputing the fluid flow. In addition, to accelerate the large-scale simulation, we implement the parallel
computation using multiple graphics processing units (GPU). By employing the developed scheme, we
perform the dendrite growth simulation during directional solidification of a binary alloy withthermal-solutal convection and investigate the effects of thermal-solutal convection on the dendrite
morphology.

MMM 2018

395

(P2-48)
Stress analysis of 4H-SiC power devices via FEM and Raman
spectroscopy*Hiroki Sakakima1, Asuka Hatano1, Akihiro Goryu2, Kenji Hirohata2, Satoshi Izumi1
1. The Univ. of Tokyo, 2. Toshiba

We developed a scheme to analyze the stress distribution of 4H-SiC power devices by FEM and Raman
spectroscopy. Raman spectroscopy is widely applied as a method for evaluating stress distribution ofsemiconductor devices. However, the relationship between phonon frequency, which is measured by
Raman spectroscopy, and stress tensor is not clarified for 4H-SiC. In addition, it is impossible to evaluate
the distribution of the stress tensor having six components only by Raman spectroscopy since the phonon
frequency is a scalar quantity. To solve these problems, we detected phonon deformation potentials,
which are the relationships between phonon frequency and stress tensor, and developed the analysis
method combining FEM and Raman spectroscopy. Firstly, phonon deformation potentials were detectedby first principle calculation. The phonon frequency of the strained crystal is calculated. All componentsof the phonon deformation potential constants were obtained from the relationship between themagnitude of stress and the phonon frequency shift. The calculated deformation potential constants werevalidated by previous experimental results. Secondly, multi-step thermal-stress FEM analysis which
reproduces actual fabrication process was conducted for a pin diode. Young's modulus, linear expansion
coefficient and intrinsic stress of thin films formed on SiC substrates were measured. The obtained stress
distribution was converted into the distribution of the phonon frequency shift and validated throughcomparison with the result of the micro Raman spectroscopy. The obtained stress distribution and its
origin will be presented. This work was supported by Council for Science, Technology andInnovation(CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Next-generation
power electronics/Consistent R&D of next-generation SiC power electronics” (funding agency: NEDO)396

MMM 2018

(P2-49)
A Functionally Graded Multi-Phase Micromechanical Model for
Carbon Nanotube –Polymer Composites*Vahidullah Tac1,2, Ercan Gurses1
1. Middle East Technical University, 2. Turkish Aerospace Industries

Carbon nanotubes (CNT) are widely known for their superior stiffness as well as strength since theirdiscovery in 1991. While our current level of understanding of carbon nanotubes prevent us from using
them in structural parts per se, embedding them in polymers for strengthening and stiffening purposesshows a great potential. However practical efforts towards designing, manufacturing and employing suchnanocomposite materials have not yet fully culminated largely due to a lack of understanding of thebonding between the nanotube and the polymer.
Latest experimental and molecular mechanical observations of the region around a carbon nanotube
embedded inside a polymer indicate the presence of at least four distinct “phases” in nanocomposites;
the CNT, the thin interfacial gap between the CNT and the polymer, a large portion of polymer around theCNT with linearly varying properties, and the bulk polymer phase.
Hence, to accurately model nanocomposite material the varying nature of polymer in the proximity of the
CNT has to be taken into account, among other things.
We adopt a multi-phase micromechanical model that allows gradual degradation/upgradation of theconstituent phases to study the mechanical properties of CNT-Polymer composites. Using this model themechanical properties of the polymer is gradually enhanced in the vicinity of the CNT. We also study theeffect of the gap between the CNT and the polymer and the role it plays in such nanocomposites. The
results of our analyses are then compared to experimental data and discussed in detail.

MMM 2018

397

(P2-50)
Atomic surface treatment of copper nanowires by electron beamirradiation simulated by first principle calculation*Shih Kuang Lee1
1. National Chiao Tung University

Atomic surface treatment of copper nanowires by electron beam irradiation simulated by first principle
calculation
Alex Lee a,Ting-Yi Lin a, Yu-Chieh Lo a, and Wen-Wei Wu aa

Department of Materials Science and Engineering, National Chiao Tung University 1001 University Road,

Hsinchu 300, Taiwan 300*

Correspondence and requests for materials should be addressed to W.W.W (email:

WWWu@mail.nctu.edu.tw)

Recently, the techniques of atomic surface treatment by electron beam etching has developed vigorously.
However, the improvement and more details should be understood specially in atomic scale. In theexperiment when we applied the electron beam on copper nanowires with copper oxide (111) surface
without heating, it was found the reduction reaction and the following Cu clusters slip on Cu (111)
surface. For further complete the mechanism, we provide the Vienna ab initio simulation package (vasp)
to perform the GGA calculation with PAW pseuodopotentials. For reduction reaction, we compare theenergy between the theoretical structure of Cu with oxide surface and Cu (111) surface to predict thebinding energy of oxygen. For the slip of Cu clusters, we calculate the energy mapping of slip path on Cu(111) surface to find the most probable routine of slip. The calculation data should help us control theintensity of electron beam radiation when we do the surface treatment of material and be the complementof slip observation.

398

MMM 2018

(P2-51)
Molecular-Dynamic Simulation of Rapid Solidification of Dipolar
Molecular Crystal from Its Melt
*Xianqi Xu1, Yang Yang1
1. East China Normal University

We present results of molecular dynamics (MD) simulations of crystal growth from the melt. The work
focuses on a face-centered-cubic molecular crystal consist with molecules modeled by an extended point
dipole model. We will present results of non-equilibrium MD growth simulations as a function oftemperature and molecular dipole moment. An analysis of the interfacial position as the function ofsimulation time was employed to extract the steady-state, and the data of the kinetic coefficients vs.
molecular dipole moments and their anisotropies were calculated and will be presented. Values of the
kinetic coefficient for the (100), (110) interfaces are compared quantitatively to the prediction of
Mikheev-Chernov (MC) theory. Our study suggest that incorporating a second relaxation time due to thedipolar fluctuation beside the relaxation time of density waves, is necessary for extending MC theory to be
applicable for molecular crystals.

MMM 2018

399

(P2-52)
Intrinsic Analysis of Structural Order Parameter at Equilibrium
Crystal-Melt Interfaces*Wenliang Lu1, Hongtao Liang1, Yang Yang1
1. EAST CHINA NORMAL UNIVERSITY

We present atomistic simulations of precisely equilibrated crystal-melt interface under ambient pressure,for pure Ni and Fe. We demonstrate the capillary waves roughen the surface, but the intrinsic interfaces
can be sharply defined. We use different types of local structural order-parameter together with areference lattice to characterize the intrinsic interface. The statistical analysis on the structuralorder-parameters for the interfacial solid and interfacial liquid atoms represents universal scaling
behavior, nearly independent of the order parameter type, crystal structure and interface orientations. We
will discuss the potential application of such intrinsic analysis to the investigations of crystal nucleationand steady-state crystallization from melt.

400

MMM 2018

(P2-53)
Multiscale Modelling of Indirect-to-Direct Band Gap Transition in
Silicon Nanosheets*Byung-Hyun Kim1, Chan-Woo Lee1, Mina Park2, Gyubong Kim2, Kersti Hermansson3, Peter
Broqvist3, Heon-Jin Choi4, Kwang-Ryeol Lee2
1. R&D Platform Center, Korea Institute of Energy Research, 2. Computational Science Research Center, Korea
Institute of Science and Technology, 3. Dept. of Chemistry–Ångström Laboratory, Uppsala University, 4. Dept. ofMaterials Science and Engineering, Yonsei University

Two-dimensional (2D) nanomaterials such as graphene, boron nitride (BN), and MoS2 have attracted great
attention owing to their exceptional and tuneable properties, which are distinguishable from those of theirbulk phases. Recently, Si nanosheets (Si NSs) have been synthesized by various experimental techniques.
Compared to other materials, Si-based nanostructures have great advantages when it comes tocommercialization, as Si is compatible with the conventional device manufacturing processes in the
microelectronics industry.
In experiments, (111) Si NSs showed thickness-dependent light emissions in the visible wavelength
regime, originating from quantum confinement effects. This observation indicates that thin (111) Si NSs
have a direct band gap, whereas bulk Si normally has an indirect band gap. However, the question of thephysical origin behind this nano-effect of Si left unanswered.
The effect of biaxial strain on the band structure of 2D Si NSs with (111), (110), and (001) exposed
surfaces was investigated by means of a multiscale modelling approach combining molecular dynamics
simulations with a reactie force field and the density functional theory. For all the considered Si NSs, anindirect-to-direct band gap transition occurs as the lateral dimensions of Si NSs increase, i.e. increasing
lateral biaxial strain from compressive to tensile always enhances the direct band gap characteristics.
Further analysis revealed the mechanism of the transition which is caused by preferential shifts of the
conduction band edge at a specific k-point due to their bond characteristics. Our results explain aphotoluminescence result of the (111) Si NSs [U. Kim et al., ACS Nano 2011, 5, 2176-2181] in terms of
the plausible tensile strain imposed in the unoxidized inner layer by the surface oxidation.

MMM 2018

401Symposium G(G1 invited)
A systematic method to develop a potential model for harshenvironments*Takuji Oda1, Sehyeok Park1, Woonghee Cho1
1. Dept. of Nuclear Engineering, Seoul National Univ.

Molecular dynamics (MD) calculation is an important computational method for materials science andengineering. To obtain meaningful results from MD simulations for harsh environments, one has toprepare a quality potential model that works appropriately not only in equilibrium states but also in
non-equilibrium states. We have recently developed a systematic method to construct (i) two-bodypotentials [T. Oda, W.J. Weber, H. Tanigawa, Comp. Mater. Sci. 111 (2016) 53] and (ii) embedded-atom
method (EAM) potentials [T. Oda, submitted]. In this method, potential functions are expanded withcosine/sine series in reference to energies, forces and stresses evaluated by first-principles calculations.
In the present study, using this method, we aim to construct potential models of a metal (bcc-Fe) and a
metal oxide (Li2O) for radiation environments. For this aim, results of first-principles molecular dynamics
calculations of several atomic collision events are included in the reference data. MD simulation resultswith the constructed potential models show a reasonable agreement with available experimental data on
fundamental material properties and threshold displacement energies. It is indicated that the method caneffectively construct a potential model for MD simulations of harsh environments.

402

MMM 2018

(G1 oral)
Molecular dynamics study of the bulk cascades in W-Re alloy*Hyunggyu Lee1, Byeongchan Lee2, Keonwook Kang1
1. yonsei university, 2. kyung hee university

Tungsten is widely used as a facing material, because it has a high thermal conductivity and low
sputtering yield. However, under the neutron irradiation, pure tungsten transmutes into the Re and Os.
This causes degradation of material properties due to the formation of defect clusters, such as
precipitates, that caused by irradiation. Therefore, it is important to understand the formation mechanismof defect clusters for preventing the degradation of material properties. In this study, for knowing theformation mechanism of defect clusters, we conducted irradiation simulation of bulk W-Re alloy using
molecular dynamics simulation. To describe the neutron irradiation, we choose the certain atom that is hit
by neutron, which is called PKA (Primary Knock-on Atom) and shot the PKA with certain kinetic energy.
We analyze the defects formation of bulk system as changing the initial energy of PKA.

Acknowledgement
This research was supported by Nuclear Fusion Research Program through the National Research
Foundation of Korea(NRF) (2017M1A7A1A01016221)

MMM 2018

403

(G1 oral)
Defect Energetics in W-Based Transition-Metal Ternary Systems*Younggak Shin1, Byeongchan Lee1
1. KyungHee Univ.

W has been considered as a potential core element for plasma facing material of fusion reactors for ITER
thanks to its outstanding high-temperature properties. However, studies so far on W and W-based binary
alloys show precipitation and defect clustering under neutron irradiation. In this talk, we report the firstprinciples calculations of the defect energetics in ternary W alloys. W-Ta-Re systems are chosen forextensive calculations on interstitial defect structures around solute atoms.
Overall, as in pure W or W binary alloys such as W-Re, the most preferable defect type is found to be thebridge interstitial, followed by <111> interstitials. With negative solute-solute binding energy, Ta and Re
atoms prefer to make a solute pair in the system and the solute pair strongly attracts W self-interstitialatom, forming a solute-interstitial complex. The binding between a Ta-Re solute pair and a W
self-interstitial atom can be stronger in this ternary alloy than in binary systems, and plays a role in slowing
down the W interstitial diffusion as a primary dissociation barrier. The defect energetics in the alloys canbe understood as the combined results of both strain-relief effect and electronic effect, but the former,
which is related with atomic size of elements and local pressure felt by each atom in lattice, seems
dominant.
To summarize, the defect complexes in W-Ta-Re alloys are expected to trap W self-interstitial atoms,
preventing self-interstitial atoms from escaping to the surface or grain boundary. The reduction in thenumber of disappearing self-interstitial atoms increases the odds for Frenkel defects to annihilate,
reducing residual void concentration and hence void formation. It is also expected that, at elevatedtemperature, the rise in configuration entropy increases the interaction energy between solute pair andself-interstitial atom, and hence further slows interstitial migration.

404

MMM 2018

(G1 oral)
A numerical study of channel deformation and fracture in irradiatedstainless steel single crystals*Jean-Michel Scherer1,2, Samuel Forest2, Jacques Besson2, Benoit Tanguy1, Jérémy Hure1
1. DEN-Service d'Etudes des Matériaux Irradiés, CEA, Université Paris-Saclay, F-91191 Gif-sur-Yvette cedex, France,
2. MINES ParisTech, PSL-Research University, Centre des matériaux, CNRS UMR 7633, 63-65 rue Henri Auguste
DesbruèresBP 87 91003 Evry Cedex, France

Stainless steels are widely used as structural materials for nuclear core internals. Due to their proximitywith the nuclear fuel, these materials are subjected to high level of irradiation doses. In the last decades,
irradiation effects on structural materials’ mechanical properties have been heavily investigated in orderto ensure structures’ reliability over nuclear plant lifetime. Irradiation induced defects - such as Frankdislocation loops and nanocavities - can lead to significant mechanical properties changes, as for example
a significant hardening due to pinning of dislocations on defects. Under mechanical loading, gliding
dislocations may incorporate sessile defects lying in their gliding plane, leaving behind a reduced defectdensity path. These zones, typically bands or channels of 10-100 nm width, beeing therefore softened bythe "defects cleaning”, may localize strain and lead to highly heterogeneous deformation at the grain
scale. This process is called dislocation channelling. A peculiar fracture mode called channel fracture is
oberved in austenitic stainless steels irradiated at high temperature to high doses, leading totransgranular facets on fracture surfaces. It has been hypothesized [1] that channel fracture is due tonanovoids coalescence in dislocation channnels. Channel fracture is therefore assessed numerically inthis study using a strain gradient crystal plasticity model that has been developped in order to take into
account irradiation defect densities in the hardening/softening behaviour of the material and that has
been implemented in a Finite Element solver [2]. Such model allows to regularize strain localization, and isused to model dislocation channelling. Single crystal porous unit-cells are subjected to simple shear, andthe effect of void sizes and dislocation channels widths are systematically investigated. Numerical results
are finally used to assess the hypothesis proposed in [1] regarding channel fracture in irradiated stainlesssteels.
[1] Margolin B. et al. (2016) The radiation swelling effect on fracture properties and fracture mechanismsof irradiated austenitic steels. Journal of Nuclear Materials[2] Ling C. et al. (2017) Void growth and coalescence in triaxial stress fields in irradiated fcc single
crystals. Journal of Nuclear Materials

MMM 2018

405

(G2 oral)
Multiscale modeling of strength enhancement of aluminium
honeycombs under combined shear-compression at high strain rate
*Han Henri ZHAO1, Bing Hou2
1. Ecole Normale Paris-Saclay, 2. School of aeronautics, NWPU,

Aluminum honeycombs are widely used as sandwich cores in the protective component in transportindustry. For example, they are involved in the bird strike shielding as well as in the crashworthinessdesign, etc. Under impact loading, testing results showed a significant enhancement of the out-of-planecrushing strength of honeycombs with respect to the quasi-static case, which cannot be explained by therate sensitivity of aluminum alloy.
This paper firstly presents the quasi-static and impact testing results for a series of aluminium
honeycombs of different cell sizes (3003 and 5052 alloys). A significantly different strength enhancementof 3003 honeycombs is found from the 5052 ones. In order to understand and model this strength
enhancement, a comprehensive numerical investigation with rate insensitive constitutive laws is
performed to reproduce the experimental results for different cell size/wall thickness/base materials. In
fact, due to the lateral inertia effect under impact buckling, larger strain under impact loading thanquasi-static case in the element nears intersectional line is observed. Different ratios of the strain
hardening stress over the yield stress of the 3003 alloy from the 5052 alloy lead to the different
honeycomb strength enhancements. Finally, aluminum 5052 and 3003 honeycombs are studied under
combined shear-compression loading. It was focused on the interaction between the additional shear
loading and the lateral inertia effect under the impact loading. It was found that the expansion of thecrush envelopes from the quasi-static loading to the dynamic loading was almost homogeneous for thetwo honeycombs.
Hou B., Zhao H., Pattofatto S., Liu J.G., Li Y.L., 2012, Inertia effects on the progressive crushing ofaluminium honeycombs under impact loading, Int. J. Solids & Struct.,, 49 (19–20), 2754-2762.
Hou B., Pattofatto S., Li Y. L., Zhao H., 2011, Impact behavior of honeycombs under combined
shear-compression, Part II Analysis, Int. J. Solids & Struct.. 48, 698-705

406

MMM 2018

(G2 oral)
Multiscale mechanical analysis of silicon and silicon dioxide as highcapacity anode materials for lithium ion batteries.
*Janghyuk Moon1, Kyeongjae Cho2, Maenghyo Cho3
1. School of Energy System Engineering, Chung-Ang Univ., 2. Department of Materials Science and Engineering,
University of Texas at Dallas, 3. School of Mechanical & Aerospace Engineering, Seoul National University

The mechanical issues on high capacity electrode materials, such as silicon, tin, and silicon oxide,
prevents their usage for lithium ion batteries. So, the oxides (i.e. silicon dioxide) are the prevalentmaterials used for a high-expected capacity and strong mechanical stability during cycles. Althoughtremendous efforts have devoted to the study of the electro-chemo-mechanical behaviors ofhigh-capacity electrode materials, the mechanical behavior of amorphous SiO2 during electro-chemical
reaction remains largely unknown. Here we systematically investigate the inelastic stress evolution, theelectronic structure and the mechanical deformation of lithiated silicon dioxide through first-principles
computation and finite element method. The structural and thermodynamics analysis was performed to
predict electrochemistry characteristics Si and Si-O system. The mechanical strength and brittle behaviorof SiO2 due to strong Si-O bonds are also compared with Si. Although both Si and SiO2 experiencemechanical softening during lithiation, the clearly distinguishable relaxation kinetic of SiO2 inducing
deviatoric inelastic strains due to mechanical constraints is found by density functional theory calculation.
These results would provide fundamental perspectives on the chemo-mechanical behavior of silicon andsilicon oxide electrode for the practical use.

ACKNOWLEDGMENT
This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea
government(MSIT) (No. 2017R1C1B5017837).

MMM 2018

407

(G2 oral)

Finite element analysis of the effect of interfacial bubbles on
performance of epoxy coatings under the alternating hydrostaticpressure*Li Liu1
1. Institute of Metal Research, Chinese Academy of Sciences

Coatings suffer dramatic deterioration and premature damage in the deep-sea environment, in sharp
contrast to the good protective performance in typical marine environments. Hydrostatic pressure (HP),
which varies with ocean depth, has pronounced impact on coating performance when compared withother factors, such as oxygen level and pH value. The idea that loss of wet adhesion is the first step
towards coating failure seems to be raising some controversy, with some researchers suggesting that theloss of adhesion results from the pressure relief process in the experiment rather than from hydrostaticpressure itself. AHP decreased the protective properties of coatings more rapidly than HP via a“push-and-pull” effect, which promotes bubbles formation at the interface between coatings and basemetal.
Compared to other simulation methods, the finite element method (FEM) is capable of providing a multiphysical field to solve complex engineering problems more effectively. The interface is most susceptible
to corrosion and the coating failure can be affirmed when the corrosion products are visible on there.
In this study, we attempt to develop a model of the growth of a bubble and the change of the stress
distribution at the coating/substrate interface under AHP by means of FEM. The results show that the
impacts of alternating hydrostatic pressure and hydrostatic pressure on the interface were compared witha precondition of bubbles inducing the coating failure.
1. Hydrostatic pressure cannot mechanically destroy the coating/metal interface because it acts as acompressive stress on the dry bubbles. If the bubbles turn into wet, the pressure loaded on the bubble
will disappear.
2. Alternating hydrostatic pressure can provide tensile stress on the wet bubbles during every immersion
period, so it accelerates disbonding of the coating.
3. The maximum stress on the bottom of wet bubbles is able to be minimized to less than the stableadhesion value if the lag time is large enough. In this case, alternating hydrostatic pressure cannot
accelerate disbonding any more. The lag time will be different if the period of alternating pressure ischanged. Thus, the lag time of a coating should be considered when it is designed to serve underalternating hydrostatic pressure.

408

MMM 2018

(G2 oral)
Multiscale Modeling and Design of High-Strength and Low-Density
3D-Archecticured Metamaterial Systems*Hussein M Zbib1, Mehdi Hamid1, Rahul Panat2, Sadeq Saleh2
1. Washington State university, 2. Carnegie Mellon University

Designing high energy-efficiency structural materials with superior mechanical properties is a key
challenge for scientists and engineer. Among these properties, strength versus mass density and strength
versus ductility are amongst the most important factors that affect component design, and thus structure
weight. Ideal structural materials are the ones that may retain high strength with low mass density andexcellent ductility. However, current state-of-the-art materials have a drastic tradeoff between theseproperties. In this work, a multiscale modeling approach is developed and used to design a new class ofhigh-strength-ductility material system. It is shown that by integrating the effects of nanoparticle,
nanolaminate and gradient structure on strength and ductility, it is possible to design and manufacture
3D-architectured lightweight metamaterial systems. The system is made of interconnected microscale
metallic ligaments reinforced by nanolaminate metallic layers and have superior mechanical properties(strength-density, strength-ductility) compared to their bulk counterparts.

MMM 2018

409

(G2 oral)
Multiscale-multiphysics simulations of metal nanotips under highelectric field*Mihkel Veske1, Andreas Kyritsakis1, Kyrre Ness Sjobak2, Vahur Zadin3, Flyura Djurabekova1
1. Helsinki Institute of Physics, University of Helsinki, 2. Department of Physics, University of Oslo, 3. Institute ofTechnology, University of Tartu

We propose a method for efficiently coupling the finite element method with atomistic simulations likemolecular dynamics or kinetic Monte Carlo. Our method enables to dynamically build an unstructured
mesh with optimized density that follows the geometry defined by atomistic data. On this mesh,multiphysics problems can be solved to obtain distribution of physical quantities of interest, which canthen be fed back to the atomistic system. The simulation flow is optimized to maximize computational
efficiency while maintaining good accuracy.
We use this method to simulate the evolution of nanostructures under high electric field. By solving
Poisson equation, we obtain the 3D distribution of electric field around the nanostructure. Using the field,
we calculate electron emission currents, surface and space charge and electrostatic forces for surface
atoms. By taking Joule and Nottingham heating into account and solving 3D heat equation, we also obtainatomistic velocity perturbation.
Our method has shown remarkable overlapping with an analytical solution and has proved to be efficientand robust enough to simulate large-scale thermal runaway processes. Using those simulations, wedemonstrated for the first time the disintegration of Cu nanotip in extreme field conditions. This process is
widely believed to lead to the formation of plasma and cause vacuum arcing in high gradient structures.

410

MMM 2018

(G3 invited)
Simulations on severely transient FSI problems associated with shockcompression of matters in extreme conditions*Jai-ick Yoh1
1. Seoul National University

Some special numerical techniques are desired when simulating the behavior of matters in extreme
conditions, and in particular, the chemical-thermal energy release from the combustion of explosives andpropellants must be carefully addressed when capturing the high strain rate deformation of metals andpolymeric binders interacting with the energetic elements of the system. In this talk, selective researchaccomplishments associated with modeling of both energetic and inert matters exposed to high pressure
conditions are discussed. Models are developed for addressing the problems of shock-induced ignition
and multi-scale hot spot initiation of metalized energetic components from the shock compression
system.
To do so, one needs to adapt a stable interface tracking algorithm for severely transient fluid-structureinteraction problems. We have developed a multi-material hydrodynamic shock physics code with ahybrid particle level set method for simulating the shock induced chemical reaction of energetic materials
and volumetric deformation of metals. The experimental measurements are obtained to validate thesimulated results presented in this talk.

MMM 2018

411

(G3 oral)
Modeling and simulation of shock waves in solids using branched
Hugoniots
Jae-Wan Lee1, Jimin Choi1, Keonwook Kang1, Soonho Song1, *Won-Suk Ohm1, Jung Su Park2
1. Yonsei University, 2. Agency for Defense Development

Shock compression is an effective means of probing the mechanical and thermodynamic properties ofsolids. The majority of shock compression studies to date have reported principal Hugoniot curves, forwhich the material ahead of the shock is conveniently chosen to be in the quiescent state (P = 0 and u =0). However, studies of branched Hugoniots (that emanate from non-quiescent states) are relatively rare,
although branched Hugoniots are equally important for accurate description of shock wave physics
involving interaction. In this talk, we discuss a sophisticated modeling scheme for shock hydrodynamics,
accompanied by a full deck of principal and branched Hugoniots obtained with molecular dynamics
computations. The system is a single-crystalline aluminum impacted at velocities ranging from 0.1 to 3.0
km/s, as described in the companion paper. Simulations show that branched Hugoniots should be used
for accuracy in predictions, especially when there are material boundaries causing reflection andinteraction of shocks.

412

MMM 2018

(G3 oral)
Branched Hugoniot curve of aluminum in strong shock using
molecular dynamics*Jimin Choi1, Sanghyuk Yoo1, Soonho Song1, Jung Su Park2, Keonwook Kang1
1. Yonsei Univ., 2. Agency for Defense Development

Two different embedded-atom method(EAM) potentials were used in single crystalline FCC Al : WKG (J.M.
Winey, Alison Kubota, and Y.M. Gupta), and FWYD (Qin-Na Fan, Chong-Yu Wang, Tao Yu, Jun-Ping Du). A
300k single crystalline Al system is inflicted planar shock at an adiabatic condition of 0.1 to 3.0 km/s.
Pressure, volume and temperature profiles are obtained after shock as impact velocity. We extract the
principle Hugoniot curve in averaged value of profiles. Branch points are selected on the principle
Hugoniot curve. We made an isentropic equilibrium system that approximates branch points. The
isentropic equilibrium system is inflicted planar shock to impact at an adiabatic condition of 0.1 to 3.0
km/s to obtain the branched Hugoniot curve. It is observed that either stacking fault formation or phasetransformation from FCC to HCP or FCC to BCC occurs depending on the potential model. This study
shows how the branched Hugoniot curve of single crystalline aluminum differs from the principle
Hugoniot curve in strong shock. In addition, by analyzing the differences according to potential models,
we show that the potential model must be carefully selected in the strong shock simulation.

MMM 2018

413

(G3 oral)
Shear relaxation behind the shock front in <1 1 0> molybdenum*Roman Kositski1, Dominik Steinberger2, Stefan Sandfeld2, Dan Mordehai1
1. Department of Mechanical Engineering, Technion –Israel Institute of Technology, 32000 Haifa, 2. Chair ofMicromechanical Materials Modelling (MiMM), Institute of Mechanics and Fluid Dynamics, TU Bergakademie
Freiberg, 09599 Freiberg

In this work we study shock-induced plasticity in Mo single crystals, using a method to relate informationof dislocations from the atomic to the continuum scale [1]. We use molecular dynamics (MD) simulations
to simulate the shock propagation along the <110> crystal orientation. Shock waves with compressive
axial stresses of about 120 GPa are simulated. These stresses induce homogeneous nucleation of
dislocations but barely lead to a phase transition. Using the Dislocation Extraction Algorithm (DXA) andthe newly developed Discrete to Continuum (D2C) technique, the atomistic information is turned into
continuous dislocation fields. In conjunction, we extract from the MD simulations thermo-dynamical
macroscopic quantities such as macroscopic strain, density, temperature and stress tensor during the
propagation of the shock wave. Correlating the stress evolution and the evolution of the continuous
dislocation fields, we analyze the attenuation of the sharp elastic jump at the front of the shock wave andthe plastic deformation behind it. Analyzing the MD simulations, we show that the initially an elastic
precursor wave overshoots the dislocation nucleation stress, after which dislocations on a specific groupof slip planes (which we denote as out-of-plane) are nucleated, slightly lagging behind the elastic front. As
dislocations are nucleated in the out-of-plane direction, the resolved shear stress on these planes isrelaxed, but the principal lateral stresses increase. This leads to an increase in the shear stresses on aplane parallel to the shock wave (denoted as in-plane), resulting in an additional retarded front of
dislocation nucleation on planes parallel to the shock front. Finally, the two-stage process of plasticity
results in close to isotropic stress state. Interestingly, the DXA-D2C analysis shows that non-zerocontributions in the Kroener-Nye tensor appear only in the first stage of plasticity, right behind the shock
front. The MD simulation results are employed to calculate the dislocation densities on specific slip
planes and the plastic deformation behind the shock, bridging the gap between the information on theatomic scale and the continuum level.
[1] Kositski, et al. Computational Materials Science 149 (2018): 125-133.

414

MMM 2018

(G3 oral)
Hypervelocity shock behavior of graphene-metal nanocomposites via
molecular dynamics simulations*Stefano Signetti1, Seunghwa Ryu1
1. Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology (KAIST)

Graphene and other 2D materials with superior mechanical properties have been widely proposed as
effective reinforcement for stronger and tougher nanocomposites [1] to realize structures capable tosustain extreme mechanical conditions, such impact loads. However current studies are limited toconstitutive properties in the quasi-static regime [1] or to the impact properties of stand-alone 2D
materials armors [2] without considering the behavior of the resulting nanocomposite under real
hypervelocity shock conditions, where second order effect occur, such as elastic-plastic shock wave
propagation or induced phase transformation. We present a molecular dynamics (MD) simulation study on
shock behavior of graphene-reinforced single crystal copper (Cu), investigating the role of graphitic
inclusion on the shock Hugoniot of such nanocomposites. Our results give insights of the failure behaviorof graphene-metal nanocomposite under hypervelocity shock load and the consequent implications totheir engineering applications.

References[1] Y. Kim, J. Lee, M. Yeom, J. Shin, H. Kim, Y. Cui, J.W. Kysar, J. Hone, Y. Jung, S. Jeon, S. Han.
Strengthening effect of single-atomic-layer graphene in metal–graphene nanolayered composites. Nature

Communications 4:2114, 2013.
[2] S. Signetti, S. Taioli, N.M. Pugno. 2D Materials Armors Showing Superior Impact Strength of Few Layers.
ACS Applied Materials & Interfaces 8:40820-40830, 2017.

MMM 2018

415

(G4 oral)
Effect of surface and internal defects on the mechanical properties ofmetallic glasses
Sunghwan Kim1, *Seunghwa Ryu1
1. Dept. of Mechanical Engineering, Korea Advanced Institute of Science and Technology

Despite the significance of surface effects on the deformation behaviors of small-scale metallic glasses,
systematic investigations on surface states are lacking. In this work, by employing atomistic simulations,
we characterize the distributions of local inhomogeneity near surfaces created by casting and cutting,along with internal distributions in pristine and irradiated bulk specimens, and investigate the effects ofinhomogeneity on the mechanical properties. The cast surface shows enhanced yield strength anddegrees of shear localization, while the cut surface shows the opposite effects, although the fraction ofvibrational soft spots, known to indicate low-energy barriers for local rearrangement, is high near both
surfaces. Correspondingly, plastic deformation is initiated near the cut surface, but far from the castsurface. We reveal that improved local orientational symmetry promotes strengthening in cast surfaces
and originates from the effectively lower quenching rate due to faster diffusion near the surface. However,a significant correlation among vibrational soft spots, local symmetries, and the degree of shear
localization is found for the pristine and irradiated bulk materials. Our findings reveal the sensitivity of thesurface state to the surface preparation methods, and indicate that particular care must be taken whenstudying metallic glasses containing free surfaces.

416

MMM 2018

(G4 oral)
Weighted Voronoi Tessellation for metallic glasses by molecular
dynamics and powder packing by discrete element method*Junyoung Park1, Jaehee Lyu1, Shibutani Yoji2
1. Dept. of Mechanical Design Engineering, Kumoh National Institute of Technology, 2. Dept. of MechanicalEngineering, Osaka University

Voronoi tessellation technique, or also called as Voronoi analysis, has been quite widely adopted todescribe the internal structure of materials from molecular level such as short-range order of icosahedra
on metallic glasses to centimeter-order powder level such as occupied volume on powder packing.
Especially, Voronoi tessellation technique is applied to the investigations by molecular dynamics or
discrete element method. In the ordinary Voronoi tessellation technique, a face is placed halfway between
equal-sized atoms or particles. However, as the size of particles or atoms in single material are not same,
bisection cannot depict the exact plane to represent a convex polyhedron including an atom or a particle.
Moreover, packed particles in an vessel usually has a Gaussian particle size distribution for natural
powders and a Rosin-Rammler particle size distribution for pulverized powders. It is not just a problem forsize distribution. It makes a huge difference on estimation of occupied volume and number of faces byatoms or particles. Therefore, we suggest an alternative way, Radical Plane Method, to solve these
problems in this paper. It shows that the error of results could reach up to 60% differences.

MMM 2018

417

(G4 oral)
Potential of lineal-path function as a characterization parameter
related to material properties*Tong-Seok Han1, JI-SU KIM1
1. Yonsei University

The hydrated cement paste has complex microstructures. The pore and solid phase distribution
characteristics within microstructures affect the material responses significantly. Material microstructure
and its responses have been correlated with porosity [1]. However, porosity might not be sufficient todescribe the complex microstructures. In this study, the relation between microstructure characteristics
and material properties is investigated using a statistical method. To extract the microstructuralcharacteristics of cement paste, micro-CT images are used. Pore and solid phase are separated frommicro-CT images using the Powers’ model. A lineal-path function [2], one of the low-order probability
functions, is selected for estimating the microstructure characteristics. Particularly, the area of lineal-path
function [3] is used as a quantitative parameter related to the material property in this study. Virtual
specimens obtained from synchrotron micro-CT are used to evaluate material mechanical properties. The
phase field fracture model is applied to evaluate stiffness and strength of cement paste with complex
microstructures. From this study, it is concluded that the area of lineal-path function has a potential to be
an alternative parameter for correlating material characteristics and properties.

[1] C. Miehe, M. Hofacker, F. Welschinger, A phase field model for rate-independent crack propagation:
Robust algorithmic implementation based on operator splits, Comput. Meth. Appl. Mech. Eng. 199 (2010)
2765–2778.
[2] B. Lu, S. Torquato, Lineal-path function for random heterogeneous materials, Phys. Rev. A 45 (1992)
922–929.
[3] T. S. Han, X. Zhang, J. S. Kim, S. Y. Chung, J. H. Lim, C. Linder, Area of lineal-path function for
describing the pore microstructures of cement paste and their relations to the mechanical properties
simulated from μ-CT microstructures. Cem. Concr. Compos. 89 (2018) 1-17.

418

MMM 2018

(G5 invited)
Metallic materials under extreme pressure: Interplay of plasticity andphase transitions*Nina Gunkelmann1, Hoang-Thien Luu1, Diego R. Tramontina3, Carlos J. Ruestes3, Yudi Rosandi4, Herbert M. Urbassek2
1. Clausthal University of Technology, 2. University of Kaiserslautern, 3. National University of Cuyo, 4. University of
Padjadjaran

Shock waves are ubiquitous in various fields of activity including space science and engineering. They areof key interest for materials engineering, where shock-induced phase transitions may be used to produce
new materials with increased strength which might be stable at ambient conditions.
As prototypical materials, we consider iron and iron-carbon alloys showing a pressure induced phasetransformation from the bcc to the hexagonal close-packed phase at around 13 GPa depending on the
carbon content. We study waves in polycrystalline Fe using an interatomic potential that faithfully
incorporates this phase transition at the desired equilibrium pressure.
Our simulations show that the phase transformation is preceded by plastic activity, leading to theso-called 3-wave structure: An elastic compression wave is followed by a plastic wave which then leads toa phase-transformation front. We show that the phase transformation from bcc to hcp and vice-versa
helps to drive twinning and decreases the probability of multiple spallation and crack formation. In
agreement with experiments, the fracture surface is influenced by the phase transition showing smooth
spall surfaces.
Despite large differences in material properties, shock waves in aluminium nanofoams exhibit, similar topolycrystalline iron, a 3-wave structure indicating three wave regimes: an elastic precursor is pursued byplastic activity in the filaments before eventually the foam structure is crushed and a compact material
results. The collapse of the foam is well described by an analytical compaction model.

MMM 2018

419

(G5 oral)
Dislocation dynamics modeling of fracture behavior with considering
dislocation shielding effect*Akiyuki Takahashi1, Hayato Sugasawa1, Masanori Nagura1
1. Tokyo University of Science

Ductile-brittle transition (DBT) in ferric steels is a critical issue to ensure the structural integrity of nuclear
power plants. The DBT is believed to be caused by the shielding effect of dislocations at the crack tip.
Therefore, in order to make a deep understanding of the fracture behavior and the DBT mechanisms, it isnecessary to develop a computational method that can take into account the dislocation nucleation, thedislocation behavior and the stress of dislocation around the crack tip, and can derive the fracture
toughness as a result of dislocation-crack interactions. This paper presents a dislocation dynamics (DD)simulation technique for the fracture toughness calculation with the consideration of dislocation shielding
effect. In this study, the crack is represented with discrete dislocations, and the crack problem is solved
using the DD method. The dislocation nucleation from the crack tip is simply modeled with a critical shear
stress in the immediate vicinity of the crack tip. The nucleated dislocations move in the material, andproduce the stress at the crack tip. The complex system of crack-dislocation interactions can be solved
only with the DD method. In the DD method, the stress intensity factor at the crack tip can be easily
computed by calculating the Peach-Koehler force, which is normally calculated in the DD simulations,acting at the crack tip dislocation. When the stress intensity factor reaches a critical value, the fracture
toughness is determined by the applied stress. To demonstrate the potential of the developed DD
method, we performed a simulation of dislocation shielding with various dislocation mobility, which
imitates the temperature dependence of dislocation behavior and fracture toughness. The numericalresult clearly shows that the higher dislocation mobility gives higher fracture toughness, which isqualitatively in agreement with experimental results.

420

MMM 2018

(G5 oral)
Dynamic behaviors of dislocations and grain boundaries induced byphonon scattering in nanoscale*Soon Kim1, Sung Youb Kim1
1. Dept. of Mechanical Engineering, UNIST

Conventional dislocation dynamics based on continuum theory is limited in its ability to describe motionof dislocations in nanoscale. This is because discrete characteristics are no longer ignored in nanoscale
unlike in micro- or macroscales. Especially, the discreteness becomes a significant factor to describe coreregion of dislocation so that it contributes to nonlinear properties of the core. In a sense that the core
determines mobility of dislocations and concerns interactions among them, the discrete approach must
be carefully considered in nanoscale dislocation dynamics. To reflect the discrete characteristics innanoscale, computer simulations and discrete lattice dynamics approach have been used to quantify
properties of the dislocation core.
In our work, we carried out molecular dynamics simulation to study motion of the dislocation in nanoscale
and found surprising behavior that internal stress of system where dislocation is inserted is dropped when
the dislocation is in motion by externally applied stress. By using discrete lattice dynamics, we proved that
this behavior occurs due to scattering of the wave emitted from breaking of atomic bond in the core [1].
Furthermore, we extended our approach to grain boundary since it can be assumed as a collection of
dislocations if its misorientation angle is small enough. We simulated the grain boundaries for various
misorentation angles under external stress. As a result, dropping of internal stress is also observed as indislocation case. Not only the stress-drop, but the grain boundaries were curved during their motion andmagnitude of the curvature and speed were inversely proportional to the misorientation angle. We
analyzed these behaviors by using the discrete lattice dynamics and concluded that these are able to beappeared only in nanoscale materials.

References[1] S. Kim, D. T. Ho, K. Kang, and S. Y. Kim, Acta. Mater, 115 (2016), p143-154.

MMM 2018

421

(G5 oral)
Investigation on 1/2<11-1>{112} and 1/2<11-1>{123} mixed
dislocations in BCC iron by classical molecular dynamics*Tomohisa Kumagai1, Akiyoshi Nomoto1, Akiyuki Takahashi2
1. Central Research Institute of Electric Power Industory, 2. Tokyo University of Science

Mixed dislocation motion of 1/2<11-1>{112} and 1/2<11-1>{123} slip systems in pure BCC iron wasobserved by using classical molecular dynamics (MD). Angles between Burgers vector and dislocationline, which is referred as mixed dislocation angle, were between 0 degree and 180 degree (i.e., 0 degreeand 180 degree correspond to screw dislocation and 90 degree corresponds to edge dislocation). As an
initial configuration, one mixed dislocation was introduced in the MD box. Then, shear strain parallel tothe slip plane with constant strain rate was applied to glide the dislocation to calculate Peierls stress, or
shear stress was applied to calculate mobility and friction stress.The directions of dislocation motion wereboth twinning and anti-twinning directions. Dislocation mobility and friction stress at 100K and 300K
were evaluated as well as Peierls stress. High Peierls stress was observed on a mixed dislocation whose
core structure was straight (i.e., it does not contain kinks) since all of the dislocation had to jump to thenext position at the same time. On the other hand, low Peierls stress was observed on a mixed dislocation
whose core structure contains kinks since it could gradually move through kink movement along with thedislocation line. There seemed that the friction stress and the mobility of the mixed dislocations wereaffected by the dislocation core structure at 100K; the friction stress was high and the mobility was low at
the mixed dislocation angles where Peierls stress was high. On the other hand, there seemed to be lesseffect of the core structure on the friction stress nor the mobility at 300K. It is found that directions of
dislocation motion have almost no effects on the Peierls stress, mobility and friction stress as a whole.

422

MMM 2018

(G6 oral)
Repulsive Correction in Tersoff Potential for Irradiated Si*Youhwan Jo1, Taeyeon Kim1, Byeongchan Lee1
1. Kyung Hee University

Engineering applications for nuclear fussion/fission facilities usually require a material with exceptional
properties. Silicon carbide (SiC) and its composites are prospective materials for such usage due to low
degradation of mechanical properties at high temperature as well as high resistance to irradiation. There
are ongoing attempts to employ SiC/SiC composites like flow channel inserts (FCI) in dual coolant leadlithium (DCLL) blankets and fuel claddings for the Generation IV nuclear reactors. SiC/SiC composites are
yet to be examined further before confirmed to be the best option. As an example, irradiation damages in
SiC/SiC composites may cause a serious problem by interacting with various elements in nuclear
operations.

Understanding the microstructural evolution under irradiation is critical to validate SiC/SiC composites
and the resulting safety under irradiation, and Molecular dynamics (MD) is a viable option for that
purpose. Nevertheless the reliability of MD simulations heavily depends on the accuracy of an interatomic
potential. The Tersoff potential is a good candidate, but not particularly optimized for high-energy
environment, resulting in a few pathological behaviors.

In this talk we propose a correction function that minimizes the problems of the Tersoff potential
developed for bulk equilibrium. In particular, the correction function replaces the repulsive part of Tersoff
within a threshold interatomic spacing. The function behaves monotonically and is continuous with theoriginal repulsive part up to the third derivative at the threshold, and moreover, complexity does notincrease significantly because the correction function introduces only two additional free parameters. By
primary knock-on atom (PKA) simulations, the effect of the correction is examined. We evaluate amaximum penetration length and the number of Frenkel pairs and discuss a benefit from the correction.

MMM 2018

423

(G6 oral)
Phase-field modeling of microstructural evolution of Fe-Cr-Al system*Kunok Chang1
1. Kyung Hee Univ.

After Fukushima nuclear power plant accident, the Accident Tolerant Fuel Cladding (ATFC) has attracted
a great deal of attention. The Fe-Cr-Al system has been studied for a long time as a potential candidate for
ATFC due to its excellent high-temperature oxidation resistance. To optimize the microstructure has been
performed for a long period of time in order to enhance the mechanical stability of the Fe-Cr-Al alloy inthe neutron irradiation environment. The spinodal decomposition generally takes place at nuclear power
plants operation temperature, 290 Celsius degree and it is generally have negative effect on theirperformace. Therefore, we performed the phase-field modeling to predict the phase separation behavior
in Fe-Cr-Al system to investigate a role of Cr and Al compositions on decomposition behavior. To obtainlarge-scale 3D microstructure, we applied CUDA parallelizing computing technique. I will also present
comparative analysis results with the microstructure obtained experimentally.

424

MMM 2018

(G6 oral)
Dissolution kinetics of ejecta in hydrogen at megabar pressure*Arslan B. Mazitov1,2, Artem R. Oganov3,1,4, Alexey V. Yanilkin1,2
1. Dukhov Research Institute of Automatics (VNIIA), Moscow, Russian Federation, 2. Moscow Institute of Physics andTechnology, Dolgoprudny, Russian Federation, 3. Skolkovo Institute of Science and Technology, Skolkovo Innovation
Center, Moscow, Russian Federation, 4. International Center for Materials Discovery, Northwestern Polytechnical
University, Xi’an, China

A process of metal particles, ejecta, transport in gases is the subject of recent researches in the field of
nuclear energetics. Practical necessity of the study arises from the processes of inertial thermonuclear
fusion, which are often accompanied by separation of ejecta particles from the interior surface of the fuelcontainer. At the pressure of approximately 1 megabar, saturation of the ejecta with hydrogen adversely
affect a fraction of pure fuel in the system. In this work, we studied a solution process of titanium ejecta inwarm dense hydrogen at megabar pressure. Thermodynamic and kinetic properties of the process were
investigated using classical and quantum molecular dynamics. We estimated such features as a solution
time of ejecta, a degree of saturation of titanium atoms with hydrogen as well as a heat effect of thesolution. It was found that particles with a radius of 1-10 mkm dissolve in hydrogen within a time of 10-1
-10-2 mks, while a mixing process can be described by diffusion law at discussed the conditions.
Presented approach demonstrates the final state of the titanium-hydrogen system as a homogenized fluidwith completely dissolved titanium particles. This result can be generalized to all external conditions
under which titanium and hydrogen are an atomic fluids.

MMM 2018

425

(G6 oral)
MD simulation study of displacement damage in bulk wurtzite GaN byproton irradiation*SangHyuk Yoo1, Yongwoo Kim1, Keonwook Kang1
1. Dept. of Mechanical Engineering, Yonsei Univ.

Gallium nitride (GaN) based optoelectronic devices including LED and HEMT (High Electron Mobility
Transistor) are widely used in aerospace industry because of large, direct band gap, high breakdown
voltage and high electron mobility. Those applications loaded in satellites will be damaged during
operating on the low earth orbit by radiation, which is mostly consisted of protons. Previous experimental
research showed the decrements of the electron mobility and LED light intensity according to the proton
fluence [1, 2]. However, it is difficult to show which kind of point defects are created and affected to thedevice. In this study, authors conduct Primary Knock-on Atom simulations in order to predict
displacement damage on bulk wurtzite GaN by proton irradiation. PKA energy used in the MD simulation
are obtained from GEANT4 simulation and experimental results. And author will show favorable type ofpoint defects induced by displacement damage.
Reference
1. Xinwen, H., et al., Proton-irradiation effects on AlGaN/AlN/GaN high electron mobility transistors. IEEE
Transactions on Nuclear Science, 2003. 50(6): p. 1791-1796.
2. Khanna, S.M., et al., Proton energy dependence of the light output in gallium nitride light-emitting

diodes. IEEE Transactions on Nuclear Science, 2004. 51(5): p. 2729-2735.
Acknowledgement
This research was supported by Basic Science Research Program through the National Research
Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning(2017M1A3A3A02015033).

426

MMM 2018

(P2-54)
Component-wise Effect of Incompatibility Tensor on Misorientation
Development in Lath Block Structure Model based on FTMP
*Yuta Amano1, Tadashi Hasebe1, Yasutaka Matsubara1
1. Dept. of Mechanical Engineering, Kobe Univ.

Modeling complex microstructures, e.g., those like lath martensite structures in high Cr ferritic heat
resistant steels, are one of the critical issues in multiscale modeling of materials, although neither
conventional schemes exist nor effective methodologies have been developed to date. In the present
study, FTMP (Field Theory of Multiscale Plasticity)-based Di-CAP (Deformation-induced
Context-dependent Autonomic Pluripotency) concept is applied to computationally fabricate complex
microstructured samples to be further utilized in various deformation analyses based on, e.g., FEM. Here,
we focus on the process of modeling single lath-block structures, which can be obtained in preliminary
bi-axial compression analyses, provided the corresponding eigenstrain distributions based on the Bainlattice correspondence is initially introduced. One of the keys for the lath-block modeling is thedevelopment of misorientation across the lath boundaries, roughly satisfying K-S variant, together with theattendant internal stress fields. FTMP-based approach exhibits spontaneous evolution of suchmisorientation when substantial contribution of the incompatibility tensor is introduced in the hardening
law. Here we decompose the incompatibility tensor into (a)pure deformation and pure rotation, (b)edge
and screw, and (c)spherical (isotropic) and deviatoric components, respectively, to examine themechanisms for the misorientation developments. Analyses are conducted using two basic models for asingle lath block structure, i.e., vertical and horizontal models, where lath sub-blocks are aligned verticallyand horizontally to the [111] axis, respectively. Demonstrated for (a) is that the pure deformation partshows relatively larger contributions to the misorientation developments, while, for (b), dominant
contributions of the screw component are confirmed. For (c), on the other hand, the weighted sphericalpart is shown to have weak but basically the same contribution.

MMM 2018

427

(P2-55)
Construction of virtual ITZ specimens using extended stochastic
optimization and evaluation of their permeability*Se-Yun Kim1, Tong-Seok Han1
1. Dept. of Civil and Environmental Engineering, Yonsei Univ.

The external substance penetrates into an interfacial transition zone (ITZ) that is located between an
aggregate and bulk cement paste and has a relatively high porosity than the bulk cement paste. However,
it is difficult to confirm the 3-D microstructure of the ITZ with the functionally graded void distribution toevaluate its permeability. In this study, 3-D microstructures of bulk cement pastes with three kinds ofporosities and the void gradient of the ITZ obtained from 2-D SEM image are used to construct the virtual
3-D microstructures of the ITZ. Based on the two information, the phase distribution characteristics of the
ITZ are generated, and they are used for constructing the virtual ITZ specimen using a stochastic
optimization. The stochastic optimization is an appropriate method to construct a random heterogeneousmaterial [1], but the ITZ has the functionally graded void distribution, which depends on the distance fromthe aggregate. To construct the functionally graded microstructure, an extended stochastic optimization
is proposed. In addition, an efficient iteration method for stochastic optimization is proposed and utilized
for construction of the virtual ITZ specimens, which improved the computational cost. The permeability ofthe virtual ITZ specimens are evaluated by a finite element method. The effect of the ITZ from thepenetration of the external substances is confirmed by the permeability analysis using the virtual ITZ
specimen. This study shows that the proposed extended stochastic optimization process is effective forconstructing functionally graded phase distribution, while the real 3-D microstructure is difficult to obtainfrom experimental techniques. This study also confirms that the virtual experiment procedure can besynergistically used with the real experimental approaches.
[1] S. Torquato, Random Heterogeneous Materials: microstructure and macroscopic properties, vol. 16,
Springer Science & Business Media, Berlin, 2013.

428

MMM 2018

(P2-56)
Estimation for probabilistic distribution of material response according
to microstructural characteristics*JI-SU KIM1, Tong-Seok Han1
1. Yonsei Univ.

There is a strong correlation between the material microstructure and its response [1]. It is expected thatthe statistical distribution of material response has a relation with that of the microstructure
characteristics, so that the effect of the microstructure to the response can be investigated. In this study,
the sensitivity of the material responses due to microstructural characteristics is investigated using afirst-order second moment (FOSM) method [2]. The FOSM method is a probabilistic method, which
determines the mean and deviation of a function of responses with random input variables. For applying
the FOSM method, specimens with certain microstructure characteristics might have to be reconstructed.
For this reason, the reconstruction process [3] to generate the target specimens are needed. The area of
lineal-path function and porosity of cement paste specimens are selected as random input variables, andthe stiffness and strength evaluated by phase field fracture model are selected as output variables. The
result of sensitivity analysis from the FOSM method is compared to the simulation results using whole
specimens. From this result, the sensitivity of material response to microstructure is estimated using two
reconstructed specimens, and the FOSM method is confirmed to reduce the time and cost for evaluating
the probabilistic distribution of properties.

[1] Mindess, S., Young, J. F., and Darwin, D. Concrete. 2nd ed. Prentice Hall U.S.A. (2003) 57-80.
[2] Lee, T.-H. Probabilistic Seismic Evaluation of Reinforced Concrete Structural Components andSystems. University of California, Berkeley (2005).
[3] Chung, S.-Y., Han, T.-S., Kim, S.-Y., and Lee, T.-H. Investigation of the permeability of porous concretereconstructed using probabilistic description methods. Construction and Building Materials (2014)
66:760-770.

MMM 2018

429

(P2-57)
Hypervelocity impact and shock behavior of pillared graphene foams*Stefano Signetti1, Seunghwa Ryu1
1. Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology (KAIST)The effectiveness of graphene as material for impact protection has been confirmed both by atomistic
simulations [1] and microscale experiment [2] obtaining unprecedented impact toughness up to ~50
MJ/kg at the nanoscale [1]. However, specific energy absorption could be, in principle, further increasedby tailoring inter-layer interaction [3] via interface structuring or functionalization. In this study we present
a modified graphene nanoarmor concept obtained by the introduction of pillar structures in the form ofcarbon nanotubes [4] of variable spatial density, aspect ratio, and size which allow the realization of stablegraphene multilayers with variable spacing. Impact strength and shock behavior of such structures areinvestigated via molecular dynamics (MD) simulations and the effect of foam geometry on the specificenergy absorption capability is evaluated across different size-scales.

References[1] S. Signetti, S. Taioli, N.M. Pugno. 2D Materials Armors Showing Superior Impact Strength of Few Layers.

ACS Applied Materials & Interfaces 8:40820-40830, 2017.
[2] J. Lee, P.E. Loya, J. Lou, E.L. Thomas. Dynamic mechanical behaviour of multilayer graphene via
supersonic projectile penetration. ACS Applied Materials & Interfaces 346(632):1092-1096, 2014.
[3] S. Signetti, N.M. Pugno. Evidence of optimal interfaces in bio-inspired ceramic-composite panels forsuperior ballistic protection. Journal of the European Ceramic Society 34:2823-2831, 2014.
[4] S. Lee, D. Kang, I. Oh. Multilayered graphene-carbon nanotube-iron oxide three-dimensional
heterostructure for flexible electromagnetic interference shielding film. Carbon 111:248-257, 2017.

430

MMM 2018

Symposium H(H1 invited)
Multiscale modeling of electro-responsive gels
*Masao Doi1
1. Beihang University

Certain kind of ionic gels show electro-mechanical coupling: the gel deforms when electric field isapplied, and, conversely, electric field is created when the gel is deformed. Such phenomena may be used
for actuators and sensors. In this talk, I will discuss the physical modeling of the electro-mechanicalcoupling in ionic gels.
[References] Masao Doi, Fluid transport in gels, Oxford Handbook of Soft Condensed Matter, Oxford

MMM 2018

431

(H1 oral)
Non-monotonic particle size effect on the glass transition inpolymer-particle blends and its application to shape memory polymers*Elias M. Zirdehi1, Fathollah Varnik1
1. Ruhr-University Bochum

Both via experiments and molecular dynamics simulations, it is shown that adding small molecules
enhances the dynamics of structural relaxation in polymer melts and glasses [1]. While this effect is amonotonic function of concentration [1], it is found to be a non-monotonic function of particle size [2]. A
detailed survey of non-Gaussian parameter for small molecules and polymer beads reveals a separation oftime scales for cooperative motion between the above two species. This is indicative of a decoupling of
the fast dynamics of small molecules from the chains’ relaxation dynamics and a resulting weak effect.
The dynamics of large particles, on the other hand, is strongly coupled to that of polymer. However, sincelarge particles are only slightly more mobile than the polymer beads, their enhancing effect is also
relatively weak. For intermediate particle sizes, the particles are sufficiently fast and the coupling topolymer strong enough to induce a large enhancement of the chains' relaxation kinetics. This
non-monotonic effect occurs both for entangled and non-entangled polymers, hinting towards local
packing effects as the main origin of the phenomenon [1]. It is shown that the effect can be explored inshape memory polymers to tune the triggering temperature of the shape recovery process in a
non-monotonic manner.
[1] Elias M. Zirdehi, Axel Marquardt, Gunther Eggeler and Fathollah Varnik, Molecular dynamics
simulations of entangled polymers: The effect of small molecules on the glass transition temperature,
Procedia Computer Science 198C, 265 (2017).
[2] Elias M. Zirdehi and Fathollah Varnik, Non-monotonic particle size effect on the glass transition inpolymer-particle blends (submitted).

432

MMM 2018

(H1 oral)
Thermomechanical behavior of shape-memory polyurethane
copolymer : A coarse-grained molecular dynamics simulation*SUNGWOO PARK1, JUNGHWAN MOON1, BYUNGJO KIM1, MAENGHYO CHO1
1. Seoul National University, South Korea

Shape-memory polymers (SMPs) are smart materials that react to external stimuli to restore their original
shape. The stimuli can be various things such as light, heat, humidity, and so on. Segmented polyurethane
copolymer is a representative thermo-responsive SMP which are synthesized with a polyol and an
isocyanate. The hard-segment which is clustered together helps to memorize the original shape bystabilizing the network, and the soft-segment which can achieve polymer crystallization acts as aswitching-segment which fixes the temporary shape and induces the shape-memory effect. This
dual-segment system is a necessary condition for shape-memory behavior of polyurethane. Full-atom
molecular dynamics (MD) simulation can cover the atomistic structures and shape-memory properties,
but it has limitations on observing mesoscale phenomena such as polymer crystallization, andmicro-phase separation. To overcome this issue, we have developed a coarse-grained (CG) MD modelwith reduced degrees of freedom by treating multiple atoms as a single bead. Full-atom MD simulation
was performed to obtain intra- and inter-bead potential of the CG model via iterative Boltzmann inversion(IBI) method. As a result, we could observe the crystallinity and shape-memory properties of SMPU models
with different hard-segment contents. Then, the effect of microstructure on the mechanical deformationand shape memory behavior were investigated. We expect this study provides an insight to design thesegmented polyurethane copolymer with enhanced shape recovery performance.

MMM 2018

433

(H1 oral)
Investigation of photo-mechanical behavior of azobenzene-based
polymer: A coarse-grained molecular dynamics study*Junghwan Moon1, Byungjo Kim1, Joonmyung Choi2, Maenghyo Cho1,3
1. Divison of Multiscale Mechanical Design, School of Mechanical and Aerospace Engineering, Seoul National
University, 2. Mechatronics R&D center, SAMSUNG ELECTRONICS CO., LTD, 3. Institute of Advanced Machines andDesign, Seoul National University

Photo-active deformable structures have drawn great attention because they can exhibit complex 3D
deformations without any electronic circuit. Among the photo-responsive polymers (PRPs), liquid
crystalline polymers (LCPs) doped with photochromic azobenzene molecules have diverse applications
for soft micro-mechanical actuators because of a reversible photo-isomerization under ultraviolet(UV)/visible light illumination. The micro-scale photo-mechanical behavior of the polymer network can begreatly represented using conventional molecular dynamics (MD) simulations. However, MD simulation
cannot be applied to examine the meso-scale phenomena occurred in the photo-deformable structure
because too much degrees of freedom (DOFs) are required for full-atomistic description. To overcome
this limitation, we propose a multi-scale/multi-physics computational framework based on acoarse-grained (CG) molecular dynamics simulation, which combines molecular switching induced by thephoto-isomerization, and overall shape change of the polymer network. The structure-based iterative
Boltzmann inversion (IBI) method was utilized to systematically represent the photo-chemical
reaction-induced change of the polymeric structure, and effectively expand time- and length-scales of theatomistic simulations. As a result, we could observe the transition between three different liquid
crystalline phases (smectic A - nematic - isotropic) under light irradiation, which cannot be reproduced
using the all-atom MD simulations. Also, we investigated a unique photo-deformation of the smectic LCP
network and its effect on the mechanical properties. We expect the proposed multiscale modeling andsimulations to be a guideline for mechanically designing the photo-responsive smart materials.

434

MMM 2018

(H2 invited)
Hydrogels with Dynamic Sacrificial Bonds –From Toughness toAdhesion to Composites –*Jian Ping Gong1,2
1. Faculty of Advanced Life Science, Hokkaido University, 2. Soft Matter GI-CoRE, Hokkaido University

Invention of the tough double network hydrogels (DN gels), consisting of interpenetrated rigid/brittle
network and soft/stretchable network, shows that the effective energy dissipation by the breaking of thecovalent bond of the brittle network prevents catastrophic crack propagation upon deformation, and thus,
gives the extraordinarily high toughness of the material. Such sacrificial bond effect has been successfully
applied to develop tough double network hydrogels of diverse chemistry and also to double and triple
network elastomer materials. Thus, sacrificial bond concept is proved to be a general approach fordeveloping tough soft materials. As the internal rupture of DN gels is due to the irreversible breaking of
the covalent bonds of the brittle network, the conventional DN gels deteriorate gradually after repeated
deformation. To address this problem, many recent works have replaced the covalent bonds with
non-covalent bonds to allow the sacrificial bonds to be reformed. In this talk, novel hydrogels withreversible sacrificial bonds developed in author’s group are reviewed and their excellent mechanicalbehaviors such as high toughness, self-healing, adhesion to biological tissues, and fast underwater
adhesion are demonstrated. Furthermore, this principle is extended to develop tough composites using
tough hydrogels as energy dissipative soft matrix.

MMM 2018

435

(H2 oral)
Elastic properties and effective interactions of in silico realistic
microgels*Lorenzo Rovigatti1,2, Nicoletta Gnan1,2, Emanuela Zaccarelli1,2
1. Institute for Complex Systems, Uos Sapienza - CNR, 2. Dipartimento di Fisica, Sapienza Università di RomaThe bulk behaviour of colloidal suspensions depends crucially on the microscopic details of theparticle-particle interaction[1]. For polymer-based building blocks, the interactions depend on a large
number of parameters such as the particle microstructure, its composition and related physico-chemical
properties (solvophobicity, charge density, etc.). Among the huge variety of available systems,
stimuli-responsive microparticles built out of polymer networks, so-called microgels, have emerged as oneof the most interesting class of soft particles, for both a theoretical and applicative standpoint[2].
Here we build upon a recently-developed method[3] to generate and simulate realistic in silico microgels.
We first look at the single-particle mechanics by calculating the elastic moduli in the small-deformations
regime. We then use Umbrella Sampling and a generalised Widom insertion method to accurately
estimate the two-body effective interaction.
We show for the first time that the Hertzian theory works well for large separations, and that in this regime
the single-particle elastic moduli can predict the amplitude of V(r) for a wide range of network topologies.
However, for smaller separations the two microgels start to strongly interact and change their shape andV(r) deviates from the predicted Hertzian behaviour.
This work establishes a clear link between the microscopic network properties and the resulting
microgel-microgel interactions, paving the way for a deeper understanding of the bulk behaviour of
microgel suspensions.
[1] C. N. Likos, Phys. Rep., 348, 267 (2001)[2] A. Fernandez-Nieves, H. Wyss, J. Mattsson, D. A. Weitz, Microgel Suspensions: Fundamentals andApplications; John Wiley & Sons (2011)[3] N. Gnan, L. Rovigatti, M. Bergman, E. Zaccarelli, Macromolecules, 50, 8777 (2017)436

MMM 2018

(H2 oral)
Controlling the mechanics of a synthetic hydrogel with motor-likeinternal contractility
Marcos Fernandez-Castano Romera1, Rint P Sijbesma1, *Cornelis Storm1
1. Eindhoven University of Technology

Stiffening due to internal stress generation is common in living materials and regulates many
biomechanical processes. For example, cells stiffen their surrounding matrix by pulling on collagen andfibrin fibers. At the subcellular level, molecular motors prompt fluidisation and stiffening of thecytoskeleton by sliding polar actin filaments in opposite directions. I will present results showing thatsynthetic materials, likewise, are able to change their stiffness in response to internally generated andexternally applied forces. Theoretical and experimental results are presented for a system where chemical
crosslinking of thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) into a fibrous matrix of synthetic
semi-flexible polymers allows for internal stress generation upon induction of coil-to-globule transition,
resulting in a macroscopic stiffening response spanning up to three orders of magnitude. Strikingly, theforces generated by PNIPAM collapse are sufficient to drive a fluid material into a stiff gel within a few
minutes. Rigidified networks dramatically stiffen in response to applied stress featuring power law
rheology with exponents that match those of reconstituted actomyosin networks pre-stressed bymolecular motors. This concept holds potential for the rational design of responsive synthetic materials
that are fluid at room temperature and rapidly rigidify at body temperature to form hydrogels
mechanically compatible to cells or tissues.

MMM 2018

437

(H2 oral)
Study on viscoelastic behavior of natural rubber with multiscale
approach*Byungjo Kim1, Junghwan Moon1, Maenghyo Cho1
1. Seoul National University

In this study, the viscoelastic behavior of natural rubber is investigated with the aid of a coarse-grained(CG) molecular dynamics (MD) simulation. It is challenging to understand the long-term nature ofpolymeric materials using a conventional MD simulation due to the inherent limitation in the time andlength scale. To overcome the drawback in terms of scalability in time and length, the CG model can offer
a way to assess the long-term physical behavior by substantially reducing a degree of freedoms with theimplementation of bead particles which are equivalent to the specific group of atoms. To describe theinter- and intra-interactions between the beads consisting of the system, the iterative Boltzmann inversion(IBI) method is employed. As an important viscoelastic behavior of elastomer, the time-dependent shear
relaxation modulus is calculated using the stress autocorrelation function. The dynamic modulus in afrequency domain is further studied with using the Fourier transformation. The influence of vulcanizationand chain length of natural rubber is taken into account. Plus, the spherical shape of SiO2 nanoparticles
are considered as filler materials, and the behavior of filler-rubber is also studied for varying filler loading
conditions. With the present work, the viscoelastic nature of rubber is understood by employing CG MD
simulations which can enhance the scale of computation in terms of length and time. Further, this work
can be extended to examine more complex polymeric system regarding the long-term physical nature or
thermodynamic property.

438

MMM 2018

(H3 invited)
Polydomain liquid crystal elastomers 1*Kaushik Bhattacharya1
1. California Institute of Technology

Liquid crystal elastomers are rubbery solids with liquid crystal mesogens incorporated into their main
chains. They display an isotropic to nematic phase transformation accompanied by a large spontaneous
deformation. Depending on how these liquid crystal elastomers are synthesized, they can either be amono domain (uniform liquid crystal order) or polydomain (nonuniform liquid crystal order). This talk will
describe various phenomena and modeling questions related to the effective behavior of polydomain
liquid crystal elastomers 1

MMM 2018

439

(H3 oral)
Mechanical behavior of hydrated polymers at nanoscale: fromelasticity to rupture.
*William Goncalves1, Takuya Mabuchi2, Takashi Tokumasu1
1. Institute of Fluid Science, Tohoku University, Sendai, Miyagi 980-8577, 2. Frontier Research Institute forInterdisciplinary Sciences, Tohoku University, Sendai, Miyagi 980-8577

Because of their emissions free of carbon dioxyde, polymer electrolyte fuel cells (PEFCs) are of greatinterest for energy production respectful of the environment. The transport of charges between theelectrodes of the fuel cell is insured by a hydrated polymer membrane such as perfluorosulfonic acid(PFSA) membrane. The durability of the fuel cell is still limited by the chemical and mechanicaldegradation of the polymer membrane after cycles of utilization. This work focuses on the study of themechanical behavior of PFSA membrane at nanoscale. The understanding of the relation between
structure and mechanical properties is the key point to improve the resistance of the membrane against
mechanical degradation.
Molecular Dynamics (MD) simulations are performed using a Coarse-Grained model to generate samples
of PFSA membranes. The results obtained from structural characterization are compared with data fromprevious atomistic models and experiments. After validation of the nanostructures, mechanical tests are
performed on the hydrated polymer membrane. With an increasing level of hydration, the results forelastic properties are in good agreement with experimental values and a similar trend as experiments isobserved for early plastic behavior. The computation of the local mechanical properties of the samples
allows to discuss the void nucleation and the influence of water content on the rupture behavior.

440

MMM 2018

(H3 oral)
Structural properties of mixtures of stars polymers and long chains*Emanuele Locatelli1, Daniele Parisi2,3, Maria Merola2, Domenico Truzzolillo4, Mario Gauthier5,
Christos N. Likos1, Dimitris Vlassopoulos2,3
1. Faculty of Physics, University of Vienna, 2. Institute of Electronic Structure and Laser, FORTH, 3. Department ofMaterials Science and Technology, University of Crete, 4. Laboratoire Charles Coulomb, UMR 5221 CNRS –Universitède Montpellier, 5. Faculty of Science, University of Waterloo

We employ a novel coarse-graining approach[6], to study star-chain mixtures[1-4], in the limit of long
chains. The coarse-grained approach is based on the calculation of the effective interaction between astar polymer and a short chain of length Nc =10 at infinite dilution. A multi-blob-like approach is thenemployed to coarse-grain chains of arbitrary length Nc . We study mixtures of star polymer and linear

chains in the protein limit, i.e. when q = Rhc/Rhs >1 . We find that the addition of linear chains makes thestar fluid more structured, confirming experimental observation obtained through rheologicalmeasurements.
[1] E. Stiakakis, et al. Physical Review Letters, (2002).
[2] E. Stiakakis, et al. Europhysics Letters, (2005).
[3] M. Camargo and C.N. Likos. Physical Review Letters, (2010).
[4] B. Lonetti, et al. Physical Review Letter, (2011).
[5] C. Pierleoni, B. Capone, and J.P. Hansen. Journal of Chemical Physics, (2007).
[6] Locatelli, B.Capone and C.N. Likos. Journal of Chemical Physics (2016)

MMM 2018

441

(H4 invited)
Enhanced Dissipation Behavior of Main-Chain LCE Networks*Thao D Nguyen1
1. Johns Hopkins University

Liquid-crystalline elastomers (LCEs) are soft stimuli-responsive materials that exhibit remarkablemechanical and optical properties. LCEs consist of stiff mesogens, bound in a network of flexible polymer
chains, which can self-organize into crystalline order while retaining liquid-like properties. The directional
ordering of the mesogens changes in response to external stimuli and the director coupling between themesogens and network chains brings about unusual behaviors. These include large reversible actuation in
response to temperature or light and soft-elasticity. LCEs also exhibit enhanced dissipation behaviors,
including a high tan delta 0.5-1.0 for wide range of frequencies and temperatures, spanning the glass andnematic transition temperatures, and large hysteresis in the stress response that increases with the strain
rate [1]. We hypothesize that the enhanced dissipation behavior arises from the relative motions of themesogens within the network and set out to measure the effect of mesogen ordering and networkorientation on the enhanced dissipation behavior of LCEs. Experiments were performed to measure therate-dependent hysteresis and frequency dependent dynamic properties for main-chain LCEs in thenematic state with different network structures, including an unoriented and macroscopically isotropic
polydomain as well as monodomains with different degrees of mesogens and chain alignment. Oriented
monodomains with an aligned or unaligned network can be synthesized by applying a second-stage
crosslinking or exchangeable bond reaction to a stretched LCE sample. In this presentation, I will briefly
describe the experimental methods, then compare the dissipative behavior of the different network
structures, and discuss how the results will be applied to develop a rate-dependent constitutive model forthe main-chain LCE networks.
[1] A. Azoug, V. Vasconcellos, J. Dooling, M. Saed, C. M. Yakacki and T. D. Nguyen (2016) Polymer
98:165-171.

442

MMM 2018

(H4 oral)
Thermo-mechanically coupled model for large strain of ultra-highmolecular weight semi-crystalline polymers*Chrystelle A. Bernard1, Tiana Deplancke4, Olivier Lame4, Kazuhiro Ogawa3, Jean-Yves Cavaillé2
1. Frontier Research Institute for Interdisciplinary Sciences (FRIS), Tohoku Univ., 2. Engineering Science Lyon Tohoku
joint lab for Materials under Extreme Conditions (ELyTMaX) UMI3757, Tohoku Univ., 3. Fracture and Reliability
Research Institute (FRI), Tohoku Univ., 4. Materials Engineering and Science (MATEIS), CNRS, INSA-Lyon, UMR5510
Univ. de Lyon

Polymers are largely used in everyday life; however, their large strain behavior is still not well understoodbecause of the complexity of the mechanisms involved in their deformation. To improve the current
knowledge on these materials, extensive experimental and theoretical researches have been performed
leading to development of constitutive numerical models to predict their large strain behavior. Most ofmodels based on polymer physics have been developed for amorphous polymers. The few ones predicting
the mechanical behavior of semi-crystalline polymers do not consider the important evolution of
microstructure occurring during the plasticity. This is especially true for ultra-high molecular weight
semi-crystalline (UHMWSC) polymers. Because their microstructure exhibits very long macromolecular
chains, one chain belongs at the same time at the crystal network and macromolecular network. During
the plastic deformation of the polymers, fibrillation process occurs leading to the progressive collapse of
the crystalline network and inducing strong variations in the material mechanical properties. Thus, toaccurately predict the mechanical behavior of UHMWSC polymers, the mechanical coupling between
crystallites and fibrils and its evolution during the deformation has to be taken into account. From theexpression of the mechanical coupling for the elastic modulus given by Humbert et al. [1](non-parallel/non-series configuration), Deplancke et al. [2] developed a one-dimensional model for theprediction of the mechanical behavior of Ultra High Molecular Weight PolyEthylene (UHMWPE). However,to extend its possible range of applications, transformation into a three-dimensional model is needed.
Therefore, we have developed a 3D model which take into account the evolutive mechanical coupling,
between crystals and fibrils, as a function of the deformation. The model is based on series configuration
with consideration of the crystal ratio in the definition of the deformation gradients. Good agreement isfound between the experimental results and 3D numerical predictions during loading and unloading.
[1] Humbert et al., Polymer 52 (2011) 4899-4909[2] Deplancke et al., on goingMMM 2018

443

(H4 oral)
Design principles for high modulus and toughness of assembled hairy
nanoparticles
Nitin Krishnamurthy Hansoge1, Tianyu Huang1, Robert Sinko1, Wenjie Xia1, Wei Chen1, *Sinan
Keten1
1. Northwestern University

Matrix-free polymer-grafted nanocrystals, called assembled hairy nanoparticles (aHNPs), can significantly
enhance the thermomechanical performance of nanocomposites by overcoming nanoparticle dispersion
challenges and achieving stronger interfacial interactions through grafted polymer chains. However,effective strategies to improve both the mechanical stiffness and toughness of aHNPs is lacking given thegeneral conflicting nature of these two properties and the large number of molecular parameters involved
in design of aHNPs. Here, we propose a computational framework that combines multi-response Gaussian
process metamodeling and coarse-grained molecular dynamics simulations to establish design strategies
for achieving optimal mechanical properties of aHNPs within a parametric space. Taking poly(methyl
methacrylate) grafted to high-aspect ratio cellulose nanocrystals as a model nanocomposite, ourmulti-objective design optimization framework reveals that the polymer chain length and grafting density
are the main influencing factors governing the mechanical properties of aHNPs, in comparison to thenanoparticle size and the polymer-nanoparticle interfacial interactions. In particular, the Pareto frontiermarking the upper-bound of mechanical properties within the design parameter space can be achieved
when the weight percentage of nanoparticle is above around 60% and the grafted chains exceed thecritical length scale governing transition into the semi-dilute brush regime. We show that theoreticalscaling relationships derived from the Daoud-Cotton model capture the dependence of the critical lengthscale on graft density and nanoparticle size. Our established modeling framework provides valuable
insights into the mechanical behavior of these hairy nanoparticle assemblies at the molecular level andallows us to establish guidelines for nanocomposite design.

444

MMM 2018

(H4 oral)
Effect of Chain Alignment on Entanglements, Diffusion and Polymer
Weld Strength
Marco Galvani1, Thomas C O'Connor1, *Mark Owen Robbins1
1. Dept. Physics and Astronomy, Johns Hopkins Univ.

Extrusion in fused filament fabrication is typically fast enough to produce significant chain alignment that
may change the weld strength of 3D printed objects in several ways. Some studies suggest that alignment
may enhance interdiffusion because of the entropic force driving chains towards unaligned conformations
or by reducing entanglement density. Alignment of strong backbone bonds along the interface may alsochange the mechanical properties of material near the joint. As part of a multiscale study of 3D printing,
we have used molecular dynamics simulations of a generic polymer model to examine the effect ofalignment on the dynamics of welding and evolution of weld strength. Entropy drives chain retraction in
the tube, but this does not speed interdiffusion since the tubes are aligned along the interface. There isalso no indication of accelerated interdiffusion due to entanglement loss. The ambiguities in recent real
space methods for identifying entanglements will be discussed. Alignment does reduce the strength ofbulk material adjacent to the weld. At intermediate times this greatly enhances weld strength by moving
failure away from the interface. The dependence of the bulk strength on alignment is presented for
different thermal histories.

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445

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Microstructural Effects in the Dynamic Response of Random 3D
Structural Polymeric Foams*Axinte Ionita1, Brittany Branch1, Brian M. Patterson1, Brad E. Clements1, Dana M. Dattelbaum1,
Alexander H. Mueller1
1. Los Alamos National Laboratory

Polymeric foams materials can have various applications in engineering. We consider in this work theeffect of the foam microstructure in the homogenized response of a random 3D structural polymeric foam
under high dynamic mechanical loading conditions (shock). In the analysis, the microstructure of the foam
is extracted from 3D computed micro-tomography. The mechanical response of the solid part of the foam
is described in terms of deviatoric and pressure response. Using Direct Numerical Simulations (DNS),
Finite Element Method (FEM) and the concepts of Micromechanics the average foam response to shock is
determined. The results are compared with the 1D-shock equations for the homogenized case todetermine the effect of fluctuating fields of physical quantities due to the randomness of the foam
microstructure.
LA-UR-18-22612

446

MMM 2018

(H5 oral)Monte Carlo simulation predicting generation and growth of
spherulites in thermoplastic polymer*Ryota Osawa1, Yoshiteru Aoyagi2
1. Dept. of Mechanical Engineering, Tohoku Univ., 2. Tohoku Univ.

Crystalline thermoplastic polymers have a complicated hierarchical structure consisting of lamellae of
amorphous phases with a random coil structure and crystalline phases in which molecular chains are
regularly arranged. The lamellae grow radially repetition of secondary nucleation on a surface of growing
lamellae and spherulite structures are generated. Peculiar mechanical characteristics of the crystalline
thermoplastic polymers, such as nonlinear elasticity, strain softening after yielding, propagation ofnecking, and orientation rehardening cannot be seen in the case of metals. Mechanical properties ofcrystalline thermoplastic polymers depend on the complicated microstructures, and the microstructuresvaries depending on molding conditions. A theoretical model predicting the microstructure andreproducing the mechanical properties are expected in the field of industrial CAE to enhance formability
and reliability of the crystalline thermoplastic polymers. Phenomenological models on macroscopic
crystallinity or crystallization rate are popular in terms of industrial availability. Such models cannot
investigate effects of sizes, shapes, and distribution of spherulites. In this study, we construct a model
expressing generation of spherulite structures for prediction of microstructures of crystalline
thermoplastic polymers depending on thermal conditions. A spherulite growth are represented by initial
nucleation in an amorphous phase and secondary nucleation on surfaces of spherulites. Rates of the both
nucleation are identified by experimental observations of spherulites growth under different thermal
conditions. Monte Carlo simulations are performed using the constructed method to reproduce thegeneration and growth of spherulites in an amorphous phase. In addition, we evaluate effects ofcrystallization temperature on the nucleation rate and the growth rate by comparison of experiment andanalysis results.

MMM 2018

447

(H5 oral)
Determination of mechanical properties of polymers from coarse
grained molecular dynamics simulations: a few case studies*Sumit Basu1, Sumit Basu
1. Indian Institute of Technology Kanpur

Polymers, by virtue of being synthetic, microscopically tailorable and light materials, are finding uses inchallenging new technological situations. These new applications, involving high temperatures, harshenvironments, impact, sustained dead loading and others, often demand changes in the macromoleculararchitecture. The ability to predict the effects of an architectural alteration on the ultimate mechanicalbehaviour of a bulk polymer is therefore, useful. Coarse grained molecular dynamics provide a route to
conducting large and long simulations to establish structure property relationships in these materials.
However, a wide variety of coarse graining methodologies exist and their suitability for obtaining
mechanical properties needs to be systematically established before they can become effective predictive
tools. In this talk, we will focus on our experience with coarse grained molecular dynamics simulations onlong chained and crosslinked polymers. In particular, we will discuss, coarse graining strategies that rely
on Boltzmann inversion based techniques for calibrating bonded interactions, and a variety of heuristic
techniques targetting specific physical properties for the non-bonded. We will highlight the problems
associated with simultaneously targetting multiple end properties like density, total energy and atomic
virial. Moreover, the mismatch in time scales between detailed atomistic and coarse grained simulations
and their influence on the rate dependent behaviour of properties like the glass transition temperatureand uniaxial stress-strain response will also be discussed. Finally, we will demonstrate a few case studies
on polystyrene and a group of crosslinked polyimides to show that coarse grained molecular dynamics
simulations of polymers have good predictive capability and sometimes can provide useful insights intomaterial behaviour.

448

MMM 2018

(H5 oral)
Modulating Elastomer Strength and Toughness with Metal Ligand
Cross-linking*Meredith Silberstein Silberstein1, Yuval Vidavsky1, Suwon Bae1
1. Cornell University

Non-covalent cross-linking interactions such as metal ligands can be used to tailor stiffness and structureof linear polymers while enabling interesting features like self-healing, energy dissipation, and extended
stretching capability. Here we present a novel system of copper(II) carboxylate bonds used to crosslink a
low Tg acrylic backbone. These crosslinks behave in a covalent manner to form novel materials withincreased stiffness, strength, and dissolution resistance, and decreased ductility. It was found that
substitution of the copper cation with different ligands imparts a dynamic nature to the crosslinks, thereby
enabling dissolution and regeneration of the initial linear polymer. These ligands also strongly influence
behavior in the solid-state polymer, increasing ductility with minimal stiffness and strength reduction, andenabling self-healing behavior. The polymer was characterized under monotonic uniaxial tensile loading
at different temperatures and strain rates. This experimentally observed mechanical behavior is thenconsidered in the context of topological and mechanochemical elastomer models. The mechanochemical
model highlights how modulation of the dynamic cross-link strength within an otherwise linear elastomer
can modulate bulk mechanical behavior.

MMM 2018

449

(H6 oral)
Coarse-Grained Molecular Dynamics Simulation of Filled Rubber
under Cyclic Tensile Deformation*Takashi Kojima1, Masataka Koishi1
1. The Yokohama Rubber Co.,Ltd.

Filler morphology, radius, and strength of the filler-polymer interaction impact the physical properties offilled rubber. It is crucial to understand relationships between them so as to improve tire performances.
Four large-scale coarse-grained molecular dynamics models were created. The first model is a reference
model in which filler particles are distributed in a lattice pattern. The second model is an aggregatedmodel including a non-homogenous filler distribution. The third model is a small particle size model inwhich small particles are distributed with identical morphology and volume fraction with the reference
model. The forth model is a weak interaction model. The polymer-filler interaction is weaker than thereference model, even though the morphology and the particle size match the reference model.
Comparing stress-strain curves, we confirmed that effects of them which are observed in experimental
results; filler aggregates, small particles, and strong interaction make modulus and hysteresis greater,
were reproduced qualitatively. Measuring a polymer density distribution and a change of polymer chainsbridging fillers, it was found that an increase of polymer density around fillers induced by thepolymer-filler attractive interaction grows the modulus of the polymer phase and irreversible changes ofpolymer chains cause hysteresis. We determined that these changes observed in all models arefundamental mechanisms of filled rubber. Comparing the stress-strain curves of the reference model andthe aggregated model, we found that the differences are attributed to filler stress. Fillers are to becontacted with another filler particle during deformation and filler stress grows when fillers areaggregated. Comparing the reference model and the weak interaction model, we found that the number
and force per a bond of extended chains in the reference model, which are main sources of polymer
stress, are greater than the weak interaction model, even though length of the extended chains are muchthe same. These differences of polymer stress make modulus and hysteresis greater.
Last, a significant increase of the number of extended chains grows polymer stress, when fillers are small
and the volume fraction of filler is identical with the reference model. Thus the modulus and hysteresis
become greater.

450

MMM 2018

(H6 oral)
Modelling of Influence of Fatigue Damage on Oxygen Diffusion in
Rubbers*Jan Heczko1, Radek Kottner1
1. NTIS - New Technologies for the Information Society, Faculty of Applied Sciences, University of West Bohemia

Rubbers are frequently used in vibration isolation, e.g. as the material for engine mounts or tram wheelsuspension segments. These parts are subjected to periodical mechanical loading and to the effects ofthe environment, such as temperature or chemical reactions. Depending on the nature of the structurechanges induced by the environment, physical or chemical ageing may be distinguished (see e.g. [1]).
Chemical ageing is governed by the consumption of oxygen, which in turn depends on its transport bydiffusion.
Both damage and ageing lead to changes in mechanical properties, each of the phenomena, however,takes place at a different length scale. This contribution is focused on the influence of fatigue damage on
the diffusion of oxygen. The central assumption is that the formation and growth of microcracks influence
the paths of the gas molecules, which manifest itself as the coefficient of diffusivity on the macroscopic
scale. We present various examples of crack geometries and the resulting oxygen flux and concentration
computed by the finite element method (FEM). These results will be further used to derive aphenomenological model of coupled fatigue damage and diffusion-limited oxidation (DLO). This model
will be an extension of an existing model [2], in which, however, no explicit relation for the coupling
between fatigue damage and diffusivity is considered.
[1] Herzig, A. , Johlitz, M. and Lion, A. Consumption and diffusion of oxygen during the thermoxidative
ageing process of elastomers. Mat.-wiss. u. Werkstofftech, 47: 376-387, 2016.
[2] Heczko, J., Kottner, R. Modelling of ageing and fatigue under large strains. In Computational

Mechanics - EXTENDED ABSTRACTS, 2017.

MMM 2018

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(H6 oral)
On the modeling and calculation of tensile properties of real rubber
using molecular dynamics simulation*Osamu HIno1
1. TOYO TIRE&RUBBER CO.,LTD.

A molecular dynamics approach to calculate general tensile properties of real rubber is developed toalleviate lengthy experimental procedures for the determination of the strain energy function of therubber material. In this presentation, first, the general framework for the calculation of tensile propertiesof rubber using molecular dynamics simulation will be illustrated. The overview on the molecularmodeling for the real rubber is followed and some pilot calculations are performed to obtain the tensile
properties for carbon black filled styrene-butadiene rubbers.

452

MMM 2018

(H6 oral)
A molecular dynamics study of dissolution of covalent adaptablenetworks in organic solvent*Yaguang Sun1, Hua Yang2, Yafang Guo1
1. Dept. of Mechanics, Beijing Jiaotong Univ., 2. State Key Lab. for Geomechanics and Deep UndergroundEngineering, China Univ. of Mining and Tech.

Thermosets have outstanding mechanical strength and solvent resistance due to their permanently
cross-linked networks. They cannot flow upon heating and cannot be reshaped and recycled. The recently
developed reversible networks provide opportunities to recycle thermoset polymers. For example, thecovalent adaptable networks (CANs; also known as vitrimers) can rearrange their network topology
through reversible bond exchange reactions (BERs). It was recently reported that CANs can be dissolved
in solvents. At high temperature, small solvent molecules can break long polymer chains into shortsegments via BERs. In this study, molecular dynamics simulations are used to investigate the dissolution ofan epoxy CAN in alcohol solvents due to the transesterification type BER. The diffusion of solvent
molecules and the decomposition of epoxy networks are analyzed. It is found that the dissolution
proceeds in a surface erosion mode, which is consistent with experimental observations.

MMM 2018

453

(H6 oral)
A minimal micromechanical model for the viscoelasticity in biophysicalfilamentous networks
Arjan E. Boerma1, Patrick R. Onck1, *Erik Van der Giessen1, Stefanos Papanikolaou2
1. Univ. of Groningen, 2. West Virginia Univ.

The static elastic behaviour of biophysical networks comprising cross-linked semi-flexible filaments is wellcharacterised due to extensive theoretical, computational and experimental studies of in-vitro materials.
The dynamic response of these networks, however, still holds several mysteries, including the peculiar
variation of the loss modulus with frequency, G’’(ω). For example, entropic effects can explain theincrease of G’’ with frequency at high frequencies, but not the minimum in G’’(ω) at intermediate
frequencies. The latter feature has been suggested to be governed by the cross-linker dynamics, but theexperimental trends are yet to be predicted by theory in a phenomenologically consistent manner. Here,
we present a minimal micromechanical description, consisting of two semi-flexible filaments coupled bycross-linkers, whose dynamics is described through a Grand Canonical Monte Carlo scheme.

454

MMM 2018

(H7 invited)
Computational modeling approach for the rational design of DNA
nanostructures*Do-Nyun Kim1, Jae Young Lee1, Jae Gyung Lee1, Young-Joo Kim1, Kyung Soo Kim1, Chanseok
Lee1
1. Department of Mechanical and Aerospace Engineering, Seoul National University

Structural DNA nanotechnology has recently been advanced significantly enabling the construction of
nanometer-scale structures with controllable three-dimensional shapes and properties. While overallmechanical properties of single and double DNA helices have been relatively well characterized bothexperimentally and computationally, those of various DNA motifs essential to build a DNA bundle
structure are still hardly known due to experimental limits despite their importance for the rational designof DNA nanostructures. In this presentation, we introduce our computational modeling approach toestablishing an efficient design process in order to achieve fine control over the geometrical shape andmechanical properties of self-assembling DNA nanostructures. Molecular dynamics simulations are highly
utilized to extract the unknown mechanical properties of local DNA motifs that are fed into a morecoarse-grained DNA model built on the finite element method. Then, this finite-element-based model of
DNA nanostructure is used to predict its three-dimensional shape and mechanical properties, which
provides designers structural feedback as quick as possible offering a chance to revise the design before
actual synthesis. We expect our multiscale analysis framework facilitates the rational design of DNA
nanostructures with a target shape and mechanical (or other derived) properties with precision.
This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korea
government (Ministry of Science and ICT) (NRF-2016R1C1B2011098, NRF-2017M3D1A1039422, andNRF-2014M3A6B3063711).

MMM 2018455

(H7 oral)
Quantitative Multiscale Modelling of Bionano Interface*Vladimir Lobaskin1, Stefano Poggio1, David Power1, Hender Lopez2
1. School of Physics, University College Dublin, 2. Institute Laue-Langevin, Grenoble

In biomedical and food technologies, the functionality of the engineered materials and potential hazards
are triggered and controlled by molecular-level interactions at the biointerface, a nanoscale layer where
biological fluids meet the foreign substances. The main concerns are related but not limited to theemerging risks for human health. The questions of biocompatibility of materials arise naturally in respect
to medical appliances such as stents, dental and cardiac implants, or prosthetic joints as they can cause
immediate hazard upon introduction into the human body. These interactions are often non-specific andunintended. Quantitative understanding of biointerface structure is therefore crucial for achieving a better
control over the surface activity biomaterials and reducing the associated health risks.
In this work, we propose a systematic multiscale bottom-up method to coarse-grain the interactions offoreign materials with biological fluids of arbitrary composition. Biomolecules (lipids, proteins andcarbohydrates) are coarse-grained by mapping their main chemical fragments onto single beads, and theirinteraction with the substrate surface is characterised by potentials of mean force (PMF) from atomistic
simulations [1]. The substrate is represented by a two-layer model where the surface interacts with themolecule beads via those PMFs, while the core interacts with via van der Waals forces calculated using
Lifshitz theory. The united-atom model for biomaterial-biomolecule segment interaction is used toevaluate the adsorption free energy of arbitrary biomolecules on a specified foreign surface. This
mesoscale representation is used to build a united-block model, which can address competitive
adsorption of entire proteins and lipids onto the adsorbent and predict the content of biomolecularcorona.
The main outcome of our work is a framework for comparative characterisation of nano- and biomaterials
in terms of descriptors of bionano interface such as protein binding affinity and content of the corona,which forms a basis for construction of nanoinformatic data-driven models for predicting material
functionality.
Funding: EU H2020 grant SmartNanotox, contract 686098, EU H2020 project NanoCommons, contract
731032.
[1] H. Lopez, V. Lobaskin, J. Chem. Phys. 143, 243138 (2015)456

MMM 2018

(H7 oral)
Multiscale modelling of intrinsically-disordered proteins
Ankur Mishra1, Henry De Vries1, Hamid Jafarinia1, Erik Van der Giessen1, *Patrick R. Onck1
1. University of Groningen

Proteins are often called the work-horse molecules of life because they are involved in almost any
structure and activity in living systems. It has long been thought that proteins inherit their function fromthe secondary and tertiary structure in which they are folded. However, more and more evidence isappearing that show that also unfolded, intrinsically-disordered proteins (IDPs) play an importantbiological role, in both health and disease. In this work we focus on two of these biological roles, i.e., (i) inmediating transport through the nuclear pore complex (NPC) and (ii) in the formation of pathologicalprotein aggregates in neurodegenerative diseases. IDPs are long chains of amino acids with
conformations that are primarily dictated by the non-covalent interactions between the individual aminoacids. Due to the large size of the protein complexes of interest, high-resolution (all-atom)
molecular-dynamics simulations are restricted to study only a few IDPs. Here, we use a coarse-grained,
one-bead-per-amino-acid model that is fine enough to capture the exact amino-acid sequence, butcoarse enough to account for the collective interaction of hundreds of IDPs. In this presentation, recent
results are presented on the collective behavior of IDPs in forming the selective permeability barrier of the
NPC and on the phase-separation of pathological IDPs in the neurodegenerative disease ALS. For bothstudies the relation between amino-acid sequence and collective/aggregation behavior will beemphasized and discussed.

MMM 2018

457

(H7 oral)
Tracing the interplay of polymer topology and hydrodynamics*Lisa B. Weiss1, Arash Nikoubashman2, Christos N. Likos1
1. University of Vienna, 2. Johannes Gutenberg University Mainz

Polymers exhibit a variety of topologies ranging from linear to cyclic and various knot types, which areof importance in biology, rheology and material science. Macromolecules of distinct topology consist ofthesame type and number of monomers making it challenging to separate them using chemical approaches.
Nevertheless, topology influences strongly their response to flow fields in concentrated [1] and dilute
solutions [2]. Exploiting these differences for future separation strategies requires a detailed
understandingof the interplay of hydrodynamic interactions and topology. Due to the inherent separation of time scalesof macromolecules and solvent, here the Multi-Particle Collision Dynamics algorithm is employed tocouple an explicit coarse-grained solvent to polymers treated with Molecular Dynamics.
We investigated the transport of topologically-distinct polymers in microfluidic slit channels. We findthat in bare channels and in dilute solutions there is only minor difference in the migration behavior,
which is not sufficient for separation. In contrast, decorating the channel walls with attractive spots
arranged on a track parallel to the flow results in a reliable and novel strategy for separation of linear andring polymers that takes full advantage of the different topologies [3]. Those spots effectively capture andimmobilize linear chains, while forcing ring polymers to change their preferred orientation close to thewalls. In doing so, ring polymers are enabled to roll along the spots with a finite velocity. Furthermore,investigation of the migration in semi-dilute linear and ring polymer mixtures exhibits an astonishingdifference. Finally, we extend our studies to knotted polymers to construct knot-sensitive filters.
References[1] Kapnistos, M.; Lang, M.; Vlassopoulos, D.; Pyckhout-Hintzen, W.; Richter, D.; Cho, D.; Chang, T.;
Rubinstein, M. Nat. Mater. 2008, 7, 997–1002.
[2] Hsiao, K.-W. W.; Schroeder, C. M.; Sing, C. E. Macromolecules 2016, 49, 1961–1971.
[3] Weiss, L. B.; Nikoubashman, A.; Likos, C. N. ACS Macro Letters 2017, 6, 1426–1431.

458

MMM 2018

(H8 invited)
Modeling and Simulation of DNA Foldback Intercoil Structure*Moon Ki Kim1, Byung Ho Lee1, Soojin Jo1
1. School of Mechanical Engineering, Sungkyunkwan University

The stem-loop foldback intercoil (FBI) DNA is one of unique four-stranded DNA structures. Its intercoil
stem region is constructed by interwinding DNA duplexes in their major groove. Although the FBI DNA isclosely related with DNA homologous recombination, inversion, and deletion process, its structural anddynamic characteristics have not been yet revealed. In this study, we create a 3D FBI computer model
based on homologous and palindromic DNA base sequences and its structural validity is confirmed by avariety of topological comparison with B-DNA including its diameter and helicity such as 22 angstrom and10.5 base pairs per turn, respectively. Then, its dynamic features are investigated by normal mode
analysis (NMA). NMA results show that the stem part has more rigidity compared to the other parts during
the major bending and twisting motions. It is also cross-validated by the AFM experiment in which theheight different of stem part is apparently measured. In addition, the DNA homologous recombination
phenomenon is observed by using molecular dynamics simulation (MD) in atomic details. MD results
imply that homologous recombination of the DNA FBI structure is triggered by base flipping of the stem
region resulting in B-Z DNA junctions at the interface between stem and loop. Consequently, thiscomputational modelling and simulation study enables us to better understand both structural andbiological features of the FBI DNA.

MMM 2018

459

(H8 oral)
DNA-particle vitrimer systems*francesco sciortino1
1. Sapienza Universita' di Roma

Vitrimers are a new class of polymeric materials in which the network nodes, despite the covalentbonding, can change their bonded partners via reversible exchange reactions with unreacted sites. The
ability to dynamically rearrange the network structure and the external control (for example, via
temperature) of the rate of exchange opens up the possibility to spontaneously heal internal fractures,
recycle the material shape, and release applied stresses.
Along the lines of using DNA as a material, we have proposed an innovative design of a vitrimer system
entirely made of DNA sequences [1]. In this DNA gel, bonds can switch without breaking, providing amechanism for changing the network structure under an external driving force, retaining at all times thesame number of bonds. To implement the bond switching mechanism, we use toehold-mediateddisplacement, one of the basic processes underlying dynamic DNA nanotechnology.
Beside discussing how a DNA vitrimer can be designed in silico and in the lab, I will discuss some otherexamples of particle vitrimer systems[2] in which the bond-swithching dynamics is modeled numericallywith a three-body potential[3]. Preliminay application of this method show how the self and collective
dynamics of these particles sensitively depend on microscopic swaps. Finally, I will discuss the origin of
the observed non-dispersive (in wavevector) collective relaxation mode[4].
[1]Flavio Romano and Francesco Sciortino

Switching Bonds in a DNA Gel: An All-DNA Vitrimer
Phys. Rev. Letts. 114, 078104 (2015)[2] Lorenzo Rovigatti, Giovanni Nava, Tommaso Bellini and Francesco Sciortino

Self-Dynamics and Collective Swap-Driven Dynamics in a Particle Model for Vitrimers
Macromolecules, 51 ,1232-1241 (2018).
[3] Francesco Sciortino, Three-body potential for simulating bond swaps in molecular dynamics
Eur. Phys. J. E (2017) 40: 3 DOI 10.1140/epje/i2017-11496-5[4] Giovanni Nava, Marina Rossi, Silvia Biffi, Francesco Sciortino, and Tommaso Bellini

Fluctuating Elasticity Mode in Transient Molecular Networks
Phys. Rev. Lett. 119, 078002 (2017).

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(H8 oral)
Relation between Macroscopic Flows in a Contraction-Expansion
Channel and Dynamics of Well-Entangled Polymer Chains*Takeshi Sato1, Takashi Taniguchi1
1. Dept. of Chemical Engineering, Kyoto Univ.

We have successfully applied a multiscale simulation (MSS) method [T. Murashima and T. Taniguchi,

Europhys. Lett., 96, 18002 (2011)] to flows of a monodispersed linear entangled polymer melt in acontraction-expansion channel. In our MSS method, a macroscopic model based on the Lagrangian
picture is coupled with a microscopic polymer model through the velocity gradient tensor and the stresstensor. The smoothed particle hydrodynamics (SPH) method is employed as the macroscopic model, andthe slip-link model [M. Doi and J. Takimoto, Phil. Trans. R. Soc. Lond A, 361, 641 (2003)] is employed as
the microscopic model. Two-dimensional flows of a well-entangled polymer melt in a 4:1:4
contraction-expansion channel are examined using our MSS method. From our MSSs, we haveinvestigated both macroscopic and microscopic information. As for the macroscopic information, we havefocused on the spatial-dependent Weissenberg number (Wi). As for the microscopic information, we haveevaluated the local orientation of polymer chains, the spatial distribution of the average number ofentanglements and the number density of entanglements along a polymer chain. The states of thepolymer chains are altered mainly in the region corresponding to Wi > 1. In this region, we haveconfirmed that the polymer chains are strongly oriented, the average number of entanglements decreases
and the number density of entanglements decreases in the center part on the polymer chain and has
peaks near the tails. Furthermore, we have examined the number density of entanglements along a
polymer chain in detail. From an analysis of the number density of entanglements, we have found that aneffective advection and a hooking event play an important role in forming the number density ofentanglements. Microscopic information obtained by our MSSs will provide us with new insights for themolecular design of a polymer chain.

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461

(H8 oral)
Origin of large scale spatial organization of DNA-polymer in bacterial
chromosomes.
*Apratim Chatterji1, Tejal Agarwal2, Manjunath G.P.3, Farhat Habib4
1. IISER-Pune, India 411008, 2. IISER-Pune, India, 411008, 3. NYU, USA, 4. Inmobi, Bangalore, India 560103.

Using data from contact maps of the DNA-polymer of bacteria E. Coli (at kilo base pair resolution) as an
input to our model, we introduce cross-links between monomers in a bead-spring model of a flexible ring
polymer at very specific points along the chain. By suitable Monte Carlo Simulations we show that thepresence of these cross-links lead to a specific architecture and organization of the chain at large (micron)length scales of the DNA. We also investigate the structure of a ring polymer with an equal number ofcross-links at random positions along the chain. We find that though the polymer does get organized at
the large length scales, the nature of organization is quite different from the organization observed withcross links at specific biologically determined positions. We used the contact map of E. Coli bacteria
which has around 4642 kilo base pairs in a single chromosome. In our coarse grained flexible ring
polymer model we used 4642 monomer beads and observe that around 82 cross links are enough to
induce large scale organization of the molecule accounting for statistical fluctuations induced by thermalenergy. The length of a DNA chain of a even simple bacterial cell such as E. Coli is much longer thantypical proteins, hence we avoided methods used to tackle protein folding problems. We define newsuitable quantities to identify large scale structure of a polymer chain with a few cross-links. We havecarried out similar studies with the DNA-bacteria of C.Crecentus with cross-links at specific points
relevant to the DNA of C.Crecentus, and obtain identical conclusions. This assures us about therobustness of our results.
Published in :1. Tejal Agarwal et al J. Phys.: Condens. Matter 30 034003 (2018).
2. Tejal Agarwal, Tejal Agarwal et al EPL 121 18004 (2018).

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MMM 2018

(H9 invited)
Micromechanics and instabilities in soft composite materials*Stephan Rudykh1
1. University of Wisconsin - Madison

Soft microstructured materials enable modifications of material properties and functionalities throughapplied deformations, or other external stimuli, for example, electric, or magnetic fields [1]. The
deformation induced tunability is mainly due to controllable changes of the microstructural arrangements,
on par with material nonlinearities [2]. Moreover, the microstructures can be designed to be prone toelastic instabilities giving rise to dramatic microstructure transformations, and switchable functionalities
such as cancelling certain frequency ranges of elastic waves (through induced band gaps). In thepresentation, we will show our numerical and theoretical results for multiphase deformable composite
materials including deformable layered materials, bio-inspired nacre-like structures [3], periodic 3D fibre
composites, and periodically structured particulate materials. These numerical and theoretical results will
be illustrated by the experimental observations on 3D printed multiphase composites subjected to finite
deformations. Finally, the ways of material properties modifications via application of external magnetic
and electric fields will be explored. In particular, magnetorheological elastomers (MRE) [1] and soft
dielectric elastomers (DE) with periodic microstructures will be examined, and the coupled magneto- andelectro-mechanical stability of these active architectured materials will be analysed.

References[1] Goshkoderia, A., Rudykh, S. 2017. Stability of magnetoactive composites with periodic microstructures
undergoing finite strains in the presence of a magnetic field. Composites Part B 128: 19-29.
[2] Galich, P.I., Fang, N.X., Boyce, M.C., Rudykh, S. 2017. Elastic wave propagation in finitely deformed
layered materials. Journal of the Mechanics and Physics of Solids 98, 390-410.
[3] Slesarenko, V., Kazarinov, N., Rudykh, S. 2017. Distinct failure modes in bio-inspired 3D-printed
staggered composites under non-aligned loading. Smart Materials and Structures 26, 035053.

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(H9 oral)
Deformation analysis of UV curing 3d printed materials by combined
molecular dynamics-finite element method
Jiwon Jung1, Chanwook Park1, *Gunjin Yun1
1. Seoul National University

3d printing technology with UV cured polymer has been applied to diverse industrial areas since it has
advantages of reducing time and costs for producing complex structures. Especially in aerospaceindustries where parts with complex geometries are needed, 3d printing is said to be a suitablemanufacturing technique for such elaborate components. However, there has been little research on thepolymerization-induced deformation such as microscopic porosity and cure shrinkage strain in the 3d
printing technology to our knowledge.
In this work, molecular dynamics (MD) simulation is adopted for modeling UV induced polymerization
process to derive cure shrinkage strain. Based on the finite element method, the strain of the polymer
from MD simulation is then applied to the structure modeled by Abaqus software, to predict macroscopic
deformation. This prediction methodology is verified with experiment results observed from deformationof 3d printed structures in reality.

464

MMM 2018

(H9 oral)
Modelling and 3D Printing Kelvin Cell Acoustic Metamaterial*Huina MAO1, Peter Göransson1, John Kennedy2, Henry Rice2, Umberto Iemma3
1. Dept. of Aeronautical and Vehicle Engineering, KTH Royal Institute of Technology, 2. Dept. of Mechanical and
Manufacturing Engineering, Trinity College Dublin, 3. Engineering Department, Roma Tre University

Anisotropic metamaterial properties could be reached from design geometric structures. It can consist offunctionality unit cells including microscale complex geometries to control the wave propagation.
Structures based on regular Kelvin cells were selected and aims for design innovative devices to mitigate
the civil aviation noise.
Inspired by the ultrahigh resolution 3D printer technology, manually controlled cell geometry would bepossible. Polymer material was used in 3D printer. Samples with 0.1 mm diameter beam element kelvincell model was printed in our lab and shows certain acoustic properties to reduce noise.
To understand the fully anisotropic metamaterial behavior, inverse estimation method was applied to get
the 21 stiffness parameters in Hooke’s matrix. Optimization method was developed to design Kelvin cellstructure to target noise reduction functions. It consists in numerically fitting a solid anisotropic model on
a set of transfer functions extracted from Kelvin model. The estimated stiffness matrix is updated after
each iteration until converge to an optimal solution. Feedbacks from modelling were used to control thegeometry and material parameters in design. It would also be used to investigate modified noise
scattering patterns in acoustic metamaterial. 3D printed samples would also be tested in wind tunnel toverify the numerical analysis.
Additional sub-elements, e.g., polymer microparticles could also be imbedded in the cells to increase
viscous/damping loss and widen frequency bands. The frequency band could be adjusted according to
the application by changing the geometry. Most of the sound could be decreased in the selected
frequencies. If titanium was used in 3D printer, the cell size of the metamaterial would be much smallerthat would increase acoustic properties too. The sound absorbing capacity of the titanium metamaterials,
together with their strength and weather resistance, would also make them attractive candidates in noiseand vibration control of aircrafts.
This work is a part of European project AERIALIST (AdvancEd aiRcraft-noIse-AILeviation devIceS using
meTamaterials), which aims at the disclosure of the potential of metamaterials to envisage innovative
devices for the mitigation of the civil aviation noise (https://www.aerialist-project.eu).

MMM 2018

465

(H9 oral)
Theoretical approach for EUV resist fabrication: DFT-MD-FDM study*Muyoung Kim1, Junghwan Moon1, Joonmyung Choi3, Sungwoo Park1, Byunghoon Lee3,
Changyoung Jeong3, Maenghyo Cho2,1
1. Divison of Multiscale Mechanical Design, School of Mechanical and Aerospace Engineering, Seoul National
University, 2. Institute of Advanced Machines and Design, Seoul National University, 3. SAMSUNG ELECTRONICS CO.,LTD

Size of wafer has been reduced for improving performance and productivity in semiconductor
manufacturing industry, and Extreme Ultraviolet (EUV) light source is state of the art strategy to achieve
the goal. Fabrication of photoresist pattern is one of the main procedures in EUV lithography, and criticalproblems (pattern bridging or pinching) deteriorate as downsizing its size. Even though experimental- andtheoretical approaches have been reported to provide physical mechanism and solution of performancedegradation, technical huddles originating from complex photochemistry (chemical reaction by electron
attachment or diffusion of reactant) hinder the rigorous investigation. From this point of view, weconstructed multiscale model having sequential theoretical framework of density functional theory(DFT)-molecular dynamics (MD)- finite difference method (FDM). Our newly-developed model provides
full description of photo-triggered chemical reaction (acid activation by electron attachment and aciddiffusion-deprotection evolution) and also quantification of sub-10 nm photoresist morphology inatomistic level. This achievement will be the cornerstone of theoretical research which facilitates
fundamental understanding on important factors for EUV performance and rational design of thenext-generation PR.

466

MMM 2018

(H10 invited)
In silico design of self-assembly nanostructured polymer systems by
multiscale molecular modelling*Maurizio Fermeglia1
1. University of Trieste

One of the major goals of computational material science is the rapid and accurate prediction ofproperties of new materials. In order to develop new materials and compositions with designed newproperties, it is essential that these properties can be predicted before preparation, processing, andcharacterization. Despite the tremendous advances made in the modeling of structural, thermal,
mechanical and transport properties of materials at the macroscopic level (finite element (FE) analysis ofcomplicated structures), there remains a tremendous uncertainty about how to predict many criticalproperties related to performance, which strongly depends on nanostructure. It is then essential analyze
the structure at molecular level, with all the chemical and physical implication. Currently, atomistic level
simulations such as molecular dynamics (MD) or Monte Carlo (MC) techniques allows to predict thestructure and properties for systems of considerably large number of atoms and time scales of the order ofmicroseconds. Although this can lead to many relevant results in material design, many critical issues inmaterials design still require time and length scales far too large for practical MD/MC simulations. Thisrequires developing techniques useful to design engineers, by incorporating the methods and results of
the lower scales (e.g., MD) to mesoscale simulations. In this work, we present applications of multiscalemodeling procedures for predicting macroscopic properties strongly depending of interactions at
nanoscale scale. Several different polymer systems of interest in the area of materials sciences and life
sciences will be considered. including (i) functionalized nanoparticles in nanostructured polymer matrices(localization of gold nanoparticles in di-block copolymers and selective placement of magnetic
nanoparticles in diblock copolymer films, (ii) functionalized nanoparticles in un structured polymer
matrices (effect of chain length and grafting density) and (iii) self-assembly organization for biomedical
applications (on nanoparticles and multivalent self-assembly building blocks in nanoparticles.

MMM 2018

467

(H10 oral)
Dynamical properties of suspensions of star block-copolymers in shear
flow.
*Diego Felipe Jaramillo - Cano1, Manuel Camargo2, Christos N. Likos1
1. Faculty of Physics, University of Vienna, 2. CICBA, Universidad Antonio Nariño –Campus Farallones

Star block-copolymers (SBCs) are versatile building blocks with specific softness, functionalization, shape,and flexibility. They self-organize into a large variety of soft patchy nanoparticles, whose conformations
and patchiness can be tuned and modified by different chemical-physical parameters. The static
equilibrium properties of concentrated suspensions of SBCs have been an object of several studies
because of the rich variety of assembly scenarios that can be achieved. In this work, we took a step
forward in the study of non-equilibrium properties of suspensions of SBCs. By means of a particle-based,
multiscale simulation approach, which combines standard molecular dynamics (MD) for the star
monomers and multiparticle collision dynamics (MPCD) for the solvent particles, we investigated the
behavior of dilute suspensions of SBCs under lineal shear flow for a wide range of parameters of thesystem; the latter include the functionality, the polymerization degree and the amphiphilicity degree ofthe star, the monomer packing fraction, the solvent quality, and the shear rate. Our analysis focus on the
dynamical behavior of the at-equilibrium-formed network structures as a consequence of the patches
reorganization induced by the shear flow. The obtained results have interesting implications on thesystem’s rheological properties and viscoelastic responses in dilute bulk phases because the SBCs areable to form a variety of different intermolecular transient bonds involving rather weak ones between
individual arms and much stronger ones between multiarm patches.

468

MMM 2018

(H10 oral)
Heterogeneous flow and internal friction in amorphous carbon*Richard Jana1, Lars Pastewka1, Daniele Savio2, Volker L Dehringer3, Gábor Csányi3, Julian vonLautz2, Michael Moseler2, Peter Gumbsch2,4
1. Albert Ludwigs Universitaet Freiburg, 2. Fraunhofer-Institut für Werkstoffmechanik IWM, 3. University of Cambridge,
4. Institue for Applied Materials, Karlsruhe Institute of Technology

We use molecular dynamics simulations to probe the plastic response of representative bulk volumes of-3-3
amorphous carbon at densities from 2.0 g cm to 3.3 g cm in simple and biaxial shear. We compare

multiple interatomic potential expressions, in particular classical empirical bond-order potentials(screened Tersoff and REBO2), the modified embedded atom method (MEAM) and machine learning
approaches, in particular the Gaussian approximation potential (GAP). After an initial elastic response, thesamples yield without any strain hardening or softening. Individual plastic events are strikingly similar to
those observed for bulk metallic glasses: Like in other amorphous materials, we find that plasticity is
carried by fundamental rearrangements of regions of ~100 atoms, the shear transformation zone. We findthat STZs coalesce to forms a shear band and that the relationship between stress and pressure during
flow is well described by a Drucker–Prager law. Amorphous carbon is a prototypical single-component
network material and its pair distribution function vanishes between first and second neighbor. This
allows definition of an unambiguous nearest neighbor relationship and a mean coordination number.
Stress correlates well with mean coordination, suggesting a simple constitutive model for this material.
This relationship breaks down at low coordination numbers. We explain this with Thorpe’s constraint
counting theory, which predicts that networks become floppy below a certain value of mean coordination.

MMM 2018

469

(H10 oral)
Increasing the thermal conductivity of polymer nanocomposites filledwith carbon nanotubes via molecular dynamics simulation*Yangyang Gao1, Liqun Zhang1, Florian Müller-Plathe2
1. Beijing University of Chemical Technology, 2. Technische Universität Darmstadt

It is very important to improve the thermal conductivity of polymer nanocomposites (PNCs) to widen theirapplication. In this work, by employing reverse nonequilibrium molecular dynamics simulations in a fullatomistic resolution, we systematically investigated the effect of the chemical grafting of carbon nanotube(CNT) on the thermal conductivity of PNCs. First, the interfacial thermal conductivity is proportional to thegrafting density, while it first increases and then saturates with the grafting length. Meanwhile, the intrinsic
in-plane thermal conductivity of CNT drops sharply as the grafting density increases. Combined witheffective medium approximation, the maximum overall thermal conductivity of PNCs appears at anintermediate grafting density because of these two competing effects. In addition, two empirical formulas
are suggested, which quantitatively account for the effects of grafting length and density on the interfacialand parallel thermal conductivity. Secondly, the thermal resistance between the CNTs gradually
decreases with the increase of the grafting density and grafting length, which can be well described by anempirical equation. The heat transfer process from one CNT to another can be well described by athermal circuit model between the CNTs. Thirdly, a stronger enhancement of the thermal conductivity isrealized when chains are grafted at the end atoms of CNTs. Under deformation, the orientation of boththe chains and the CNTs improves the thermal conductivity parallel to the tensile direction, but reduces
the thermal conductivity perpendicular to it. We have quantified the contribution of the polymeralignment and the CNT alignment to the anisotropy of thermal conductivity. In general, computer
simulation is shown to have the capability to obtain some fundamental understanding of PNCs, in hopesof providing some design principles for fabricating high performance PNCs.

470

MMM 2018

(H10 oral)
Thermal transport in polymer-based nanocomposite materials acrossmultiple scales
Rajat Srivastava1, *Matteo Fasano1, Shahin Mohammadnejad1, Hernan Chavez Thielemann1,
Eliodoro Chiavazzo1, Pietro Asinari1
1. Energy Department, Politecnico di Torino

The improved physical properties of composite materials are revolutionizing many fields, from automotive
to biomedical industries, from electronics to space applications. Composites are typically made ofpolymeric matrices reinforced with macroscopic fillers, e.g. carbon fibres. Nowadays, scientists areinvestigating the possibility of introducing nanostructured fillers as wells, to further enhance the effective
properties of composites. Carbon nanostructures such as carbon nanotubes or graphene are particularly
suitable for that, because of their outstanding mechanical, electrical and thermal properties. However, theresulting nanocomposites are characterised by properties at scales spanning from nano to macro and,
therefore, should be simulated by sophisticated multi-scale approaches.
In this work, an original multi-scale approach to the thermal transport in polymer-based materials
reinforced with carbon nanotubes or graphene sheets is presented and experimentally validated. First,
molecular dynamics simulations are adopted to compute the thermal boundary resistance at thepolymer-nanofiller and nanofiller-nanofiller interfaces, according to the different structural and chemical
characteristics of the nanocomposite. The thermal conductivities of nanofillers and polymer matrix arecomputed as well, with molecular precision. Second, these nanoscale transport properties are used toestimate the effective thermal conductivity of nanocomposite, thanks to a mesoscopic simulation
approach based on Dijkstra algorithm. The accuracy of the code is verified against a finite element model.
After that, sensitivity analyses are carried out to investigate the impact of different fillers on the effective
thermal conductivity of nanocomposite. Modelling results are then validated by experiments, and themost severe bottlenecks in the thermal conductivity enhancement of nanocomposites are finallyidentified.
Our work provides guidelines towards the technical-economical optimization of novel nanostructured
materials with tunable thermal properties, with the aim to transfer them from lab experiments to industrial
exploitation.
This work has received funding from the European Union’s Horizon 2020 research and innovation
program MODCOMP under grant agreement N. 685844.

MMM 2018

471

(H11 oral)
A Hierarchical Multiscale Simulations Approach for Modeling Failure in
Polymer Matrix Composites
Xiawa Wu1, Amin Aramoon1, Christopher Woodward2, *Jaafar A. El-Awady1
1. Johns Hopkins University, 2. Air Force Research Laboratory

Polymer matrix composites (PMCs) are widely used in many automotive, marine, and aerospace
applications. Predicting the damage and failure of such systems is of crucial interest for their reliable
performance. In this work, a coarse-grained MD model has been developed to characterize the evolutionof free volume density (voids) in DGEBA polymers under loading and its subsequently plastic deformation(e.g. hardening and failure). A detailed atomic monomer is coarsened to create the tailored plastic
behavior. Cross-linked polymer networks are created under different curing conditions, including
temperature, and cross-linker functionality using a dynamic cross-linking algorithm. The effect of chainsize, and degree of cross-linking are also investigated. The free volumes are measured as a function ofstrain by fitting the largest ellipsoids between neighboring chains in the network. From these simulations
we develop a direct correlation between the evolution of plastic deformation and the free volume density.
The results of these simulations are then upscaled into a finite element simulations to model the damage
and failure of PMCs.

472

MMM 2018

(H11 oral)
Some Positive Aspect of Structural Defects in Graphene/Polymer
Nanocomposites Studied by Ab-initio, Molecular Dynamics, andContinuum Approaches*Seunghwa Yang1, Janghyuk Moon1, Maenghyo Cho2
1. School of Energy Systems Engineering, Chung-Ang University, 2. School of Mechanical and Aerospace Engineering,Seoul National University

In this study, we reveal some positive aspect of the intrinsic defect commonly observed in two
dimensional nanocarbon structure of graphene as a multifunctional reinforcement. Among the intrinsic
defects commonly observed in graphene, we focus on the crystallographic defect of theThrower-Tone-Wales (TSW) defect. Since the formation of the TSW defects in graphene involves 90
degree of rotation of the two covalently bonded neighboring carbon atoms and the resultant change inelectron density, we firstly adopt the density functional theory to investigate the adhesion characteristicsof the carbon atoms in TSW defect to the to the polypropylene (PP) monomers. In order to observe theeffect of the arbitrarily generated TSW defect on representative volume element level mechanicalproperties, we modeled transversely isotropic molecular unit cell model of single layer graphene
reinforced PP nanocomposites. The stress-strain curves in tension and shear are predicted from a
constant strain rate ensemble simulation at below the glass transition temperature of the PP matrix and at
the atmospheric pressure. At the same time, the mechanical properties of single layer graphene
embedding the TSW defect is studied using the same ensemble simulations. To correlate the locallyrippled configuration of the TSW defected sites to the macroscopic stress-strain relation, we analyzed thesurface roughness of the graphene according to the density of TSW defects. Finally, a multiphasemicromechanics model incorporating the weakened interface between graphene and PP matrix is applied
to study the stress-strain relation of poly-dispersed nanocomposites with random graphene orientation.
The two rivalling effects of the degradation of the graphene and improvement of the interfacial shear loadtransfer are evaluated from parametric studies on the effect of the aspect ratio and the volume fraction of
the defected graphene on overall stress-strain relation of poly-dispersed nanocomposites.
This research is supported by a Basic Science Research Program grant (15-113-701-010) through the
Agency for Defense Development of Korea

MMM 2018

473

(H11 oral)
Linear and non-linear viscoelastic properties of model fractal-likeaggregates polymer nanocomposites*Samy MERABIA1, Yang WANG1, François DETCHEVERRY1, Marc COUTY2, Gaëtan MAUREL2
1. CNRS and Universite Lyon 1, 2. MFP Michelin, Clermont-Ferrand

Dispersing solid fillers into a polymer matrix is a common strategy to enhance and tailor its properties.
Polymer nanocomposites (PNCs) so obtained with fractal-like aggregates have exceptional rheologicalbehavior that have long been exploited in the tire industry. However, due to disparity of time and lengthscales, our understanding of the relation between nanocomposites structure and rheology remains far
from complete.
In this contribution, we propose a mesoscopic model to describe the dynamics and the rheology ofaggregate PNCs. While aggregates are described explicitly as groups of interacting particles, we use forthe polymer matrix an implicit description based on generalized Langevin equation, that captures theaverage effect of a viscoelastic medium. These two-level description allows us to simulate large PNCs
systems containing dozens of aggregates.
We focus on the linear and non-linear viscoelastic properties of PNCs. We characterize the influence of
aggregate size, rigidity and volume fraction. We show that compared to nanoparticles, aggregates may
display levels of reinforcement considerably larger. We also demonsrate that the stress relaxation of
aggregates display long relaxation times, which originates in the slow rotation of the aggregates. As
concerns non linear properties, we concentrate on the Payne effect characterized by a drop of the storage
modulus for small deformation amplitudes. We relate the Payne effect to the deformation and alignmentof the aggregates under the imposed deformation direction. We also discuss memory effects, and inparticular the slow recover kinetics subsequent to a first deformation. All these considerations may help inbuilding connections between the macroscopic mechanical response of the PNCs and the mesoscopic
morphology of the nanofillers.

474

MMM 2018

(H11 oral)
Topological defect structure in the self-assembly of semiflexible
polymers under spherical confinement*Mihir Khadilkar1, Arash Nikoubashman1
1. Johannes GutenBerg University Mainz, Germany

Static and dynamic properties of macromolecules are often strongly affected in confinement, due to theenforced constraints on polymer motion and conformation. Moreover, for semiflexible polymers like DNA
and actin, the confinement effects also compete with the enthalpic costs of bending. Usingcoarse-grained molecular dynamics simulations, we explore the phase behavior of semiflexible polymers
confined to spheres and thin spherical shells. We observe a disordered-to-nematic ordering transition as afunction of chain stiffness accompanied by the emergence of topological defects on the surface. Each of
the configuration variables including chain length, packing density, chain stiffness and shell thickness
uniquely affects the phase behavior, including the nature and relative orientation of the defects. Systemic
trends observed could pave the way for a better understanding of the links between topological defects
and elastic properties of the macromolecules. Further, controlling the nature and locations of these
defects could also be crucial in understanding other multiscale biological processes involving these
biomolecules like chromosomal packing of DNA and gene regulation.

MMM 2018

475

(H11 oral)
Molecular dynamics simulation of the detachment force between
graphene and epoxy resin*Kazuki Mori1, Makoto Yabe2, Yuji Kohno2, Jun Koyanagi3
1. ITOCHU Techno-Solutions Corporation, 2. Yokohama National University, 3. Tokyo University of Science

We introduced a computational method to investigate the shear detachment strength at a carbon
fiber/epoxy resin interface using molecular dynamics simulations. In the computational model, thegraphene molecular structure was assumed to be that of a simple carbon fiber. The interaction energies
calculated for the epoxy bulk phase were in good agreement with the experimental data in our previous
work [1], however, the estimates of surface energy between carbon fiber and epoxy resin were missing. In
this work, we calculated the surface energy and estimated the correlation between surface energy andshear detachment strength of the carbon fiber/epoxy resin interface. All calculations were performed
using Exabyte.io platform [2]. The calculated surface energy was confirmed to be related to theexperimental shear strength: the surface energy increased with increase in the resin density neargraphene. In addition, molecular structures of bulk resin and the resin near graphene surface regions
were found to be different. Our results suggest that the carbon fiber and epoxy resin have strong
interaction at the interface. In the vicinity of graphene, the resin molecular structure deformed into aflattened state. We propose this to happen due to the influence of CH/π orbitals. The CH/π interaction
is a weak interaction (0.1 kcal/mol), however, the compound effect of the numerous weak interaction
forces leads to the rise of a strong interaction at the surface between graphene and resin. Because of this,the carbon composite becomes a high-strength material. We further simulated the stress-strain curve of
the graphene/epoxy composite model because the above interaction might be related to tensile stress.
We found that the crack formation started from the epoxy interface layer and progressed to the bulk
epoxy layer.
[1] K. Mori, N. Matsumoto, S. Nomoto, K. Tsuruta, OJCM (2017) 7, 179-184[2] https://exabyte.io/476

MMM 2018

(H11 oral)
Relation between deformation and electrical conductivity forelectroconductive polymer nanocomposites with highly segregatedstructure*Oleg V. Lebedev1, Sergey Abaimov1, Alexander Ozerin2, Anton Trofimov1, Iskander Akhatov1
1. Center for Design, Manufacturing and Materials, Skolkovo Institute of Science and Technology, 2. Laboratory of theStructure of Polymer Materials, N.S. Enikolopov Institute of Synthetic Polymer Materials of RAS

Polymer composites with highly non-uniform distribution of nanoscale filler in a matrix (filler forming aso-called segregated structure) possess many interesting properties, such as very low percolationthreshold and distinct non-linear piezoresistance effect. To study the dependence of the loss in electricalconductivity on the uniaxial deformation and 3D expansion degree, multiscale finite-element analysis was
conducted for polymer matrix containing carbon nanotubes. At the first stage of this work, a uniform
distribution of dense-packed and entangled nanotubes was created within a thin planar RVE with
polyethylene properties. The response of RVE’s electrical conductivity to a variety of loading conditions
was studied to simulate the response of different parts of a segregated structure on the nano-level to theexternal loading applied at macro-level. In particular, the planar RVE was oriented at different angles
relatively to the axis of macroscopic uniaxial loading and progressively deformed to observe non-linear
effects. For each pair angle/deformation, a value of effective electrical conductivity was calculated by an
external procedure (Python script), accounting for the Kirchhoff's circuit laws. In the result, an analytical
functional dependence of the electrical conductivity on RVE’s angle and degree of deformation wasdeveloped and compared with the experimental results obtained for uniaxially-deformed
electroconductive nanocomposite. At the second stage, a finite-element micro-level model of thesegregated structure, recreated from the experimentally observed filler distribution in a composite, wascreated. The elements of the volume corresponding to the segregated highly filled areas were modeled ashaving effective electroconductive properties that obey the previously developed conductivity andangle/deformation analytic relation. The micro-level model was uniaxially deformed and thermally treated
to introduce expansion of the matrix, and for each degree of deformation or expansion the effective
electrical conductivity of the material was calculated. For the obtained dependencies, an analyticalexpression was developed and was compared with the experimental results for a composite with highly
segregated structure.

MMM 2018

477

(P1-51)
Multi-scale modeling of DNA-dendrimers in electrolyte solutions*Natasa Adzic1, Clemens Jochum2, Gerhard Kahl2, Christos Likos1
1. Faculty of Physics, University of Vienna, 2. Institute for Theoretical Physics, Vienna University of Technology

We have studied a novel class of macromolecules, the so-called DNA-based dendrimers. They haverecently been synthesized from the enzymatic ligation of Y-shaped DNA building blocks. In order todescribe such dendrimers of various generations we have performed MD simulations employing two
independent models: a bead-spring model and the oxDNA model. The former one models each base-pairof double-stranded DNA as a single charged monomer and the interactions and interaction parameters in
the model have been carefully chosen to mimic the structural properties of a single DNA chain. The
system was immersed in water, which was modeled as a uniform dielectric and counterions wereintroduced in the system to preserve electroneutrality. Furthermore, we added salt, treating it explicitly, inorder to investigate its influence on conformational characteristics of a single dendrimer molecule. On theother hand, the oxDNA model allowed us to take a closer look into the DNA structure, treating DNA as astring of rigid nucleotides which interact through potentials that depend on the position and orientationof the nucleotides. Equilibrium properties of a single dendrimer-like DNA molecule from the first to thesixth generations obtained from these two models have been investigated and the obtained simulation
results have also been compared to the experiments. We have found an excellent agreement between thetheoretical and experimental results, which has encouraged us to use the introduced models fortheoretical analysis of novel self-assembled structures, such as cluster crystals in the bulk. The study ofthese charged dendrimer-systems is an important field of research in the area of soft matter due to theirpotential role in various interdisciplinary applications, ranging from molecular cages and carriers for drug
and gene delivery in a living organism to the development of dendrimer-based ultra-thin films in the areaof nanotechnology.

478

MMM 2018

(P1-52)
Structural and dynamical properties of star block-copolymers in shear
flow.
*Diego Felipe Jaramillo - Cano1, Manuel Camargo2, Christos N. Likos1
1. Faculty of Physics, University of Vienna, 2. CICBA, Universidad Antonio Nariño –Campus Farallones

Star block-copolymers (SBCs) have been demonstrated to constitute self-assembling building blocks withspecific softness, functionalization, shape, and flexibility. In this work, we studied the structural anddynamical behavior of an isolated SBC under a shear flow by means of particle-based multiscale
simulations (MD+MPCD) covering a wide range of system parameters, which include the functionality(number of arms of the star), the amphiphilicity degree, and the solvent quality. We systematicallyanalyzed the conformational properties of low-functionality SBC, as well as the formation of attractive
patches on their corona as a function of the shear rate. Three mechanisms of patch reorganization undershear were identified, which determine the dependence of the patch numbers and orientations on theshear rate, namely, free arms joining existing patches, a fusion of medium-sized patches into bigger ones,
and fission of large patches into two smaller ones at high shear rates [1]. As well as, the dynamics of the
SBC was investigated by means of the so-called Eckart’s frame, which allows separating pure rotationaland vibrational motions [2,3]. It is shown that SBCs display a richer structural and dynamical behavior
than athermal star polymers in a shear flow [4] and therefore they are also interesting candidates to tune
the viscoelastic properties of complex fluids. Because the conformation and dynamics of single SBCs are
expected to be preserved in low-density bulk phases, the presented results are the first step inunderstanding and predicting the rheological properties of semidilute suspensions of this kind ofpolymers.
[1] Jaramillo-Cano et al. J. Phys. Chem. B (2018) In press[2] Sablic et al. Soft Matter 13: 6988 (2017)[3] Jaramillo-Cano et al. (2018) In preparation[4] Ripoll et al. Phys. Rev. Lett. 96: 188302 (2006)

MMM 2018

479

(P1-53)
Multiscale simulation of polymeric solids for fracture processes*Takahiro Murashima1, Shingo Urata2
1. Dept. of Physics, Tohoku Univ., 2. AGC

We have developed a new multiscale simulation technique in order to investigate polymeric solids.
Macroscopic features of polymeric solids are described by finite element method and microscopic
features are described by molecular dynamics simulation. Each of finite elements has its microscopic
simulator instead of using a constitutive equation. Polymers are described by the Kremer-Grest model,
namely the bead-spring model. This model is simple but it can consider entanglements of polymers, whichcause long time hysteresis. We solve macroscopic continuum mechanics and microscopic molecular
dynamics concurrently. We have applied our multiscale simulation technique to fracture processes ofpolymeric solids under uniaxial tension.

480

MMM 2018

(P1-54)
Quantification and validation of the mechanical properties of DNA
nicks*Jae Young Lee1, Jae Gyung Lee1, Do-Nyun Kim1
1. Dept. of Mechanical and Aerospace Engineering, Seoul National Univ.

Understanding the mechanical properties at a nick site of double-stranded DNA is important to precisely
design self-assembling DNA nanostructures because they inevitably contain many nicks that are structural
discontinuities of DNA backbones. It is, however, difficult to characterize the mechanical properties of
DNA nicks experimentally due to its resolution limit while those for normal DNA double helix without nicks
have been relatively well measured. Here, we quantitatively studied the sequence-dependent
characteristics of DNA nicks at the base-pair scale using the molecular dynamics (MD) simulation. We
found that a primary structural role of DNA nicks is the relaxation of torsional constraint bysugar-phosphate backbones and that neighboring base-pair sequences affect the degree of mechanicalrigidity changes by a nick. To validate these findings, we designed DNA nanostructures sensitive to torsion
where the torsional rigidity at nick sites was regulated by using various sequences of DNA nicks andmeasured their overall twist angle using the atomic force microscope (AFM). We expect our study for nicks
offers a versatile way of fine tuning the shape and properties of DNA nanostructures.
This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korea
government (Ministry of Science and ICT) (NRF-2016R1C1B2011098, NRF-2017M3D1A1039422, andNRF-2014M3A6B3063711).

MMM 2018

481

(P1-55)
Investigating the mechanical properties of azobenzene-tethered DNA
for controlling self-assembling DNA nanostructures*Jae Gyung Lee1, Chanseok Lee1, Do-Nyun Kim1
1. Dept. of Mechanical and Aerospace Engineering, Seoul National Univ.

Advances in structural DNA nanotechnology have enabled the construction of a nanostructure withvarious shapes using self-assembling characteristics of DNA strands. Recently, there is an increasingly
higher demand for building a dynamic, morphing structure in response to environmental signals so thatDNA nanostructures can be used as a functional structure as well.
Azobenzene is one of the representative molecules that change its conformation under the change of lightsource. It switches from the trans isomer to the cis isomer when absorbing ultraviolet lights, while it isreversibly isomerized under visible lights. Multiple azobenzene molecules can be combined with DNA byintercalated between base-pair steps through threonine linkers and non-planar cis-azobenzenes
destabilize the stacking interaction of DNA bases unlike planar trans-azobenzenes. Hence,azobenzene-tethered DNA (AzoDNA) can serve as a structural motif that triggers the conformationalchange of DNA nanostructures with lights. Nevertheless, its mechanical properties and transition
dynamics are rarely known. Here, we investigate the mechanical properties of AzoDNA at its trans and cis
states by performing molecular dynamics simulation. To validate, we construct a DNA nanostructure
whose bending angles are controlled by the rigidity of AzoDNA at the hinge part.
This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korea
government (Ministry of Science and ICT) (NRF-2016R1C1B2011098, NRF-2017M3D1A1039422, andNRF-2014M3A6B3063711).

482

MMM 2018

(P1-56)
FTMP-based Modeling and Simulations of Glassy Polymers.
*Soushi Miyamoto1, Tadashi Hasebe1
1. Dept. of Mechanical Engineering, Kobe Univ.

Multiscale modeling of glassy solids, such as polymers and metallic glasses, will require mathematicallyrational as well as effective treatments of the glassy states (free volume) responsible for carrying theviscoplastic flow. For amorphous polymers, it is rephrased as “modeling network degrees of freedom(NW-DOFs)” in connection with the orientating polymer chains. The “glassy states” can then be
expressed in this context as those with curled and/or entangled chains with no net orientation, whereas
the “strength” is attributed to their fully-extended counterparts with the alignments in the load-bearing
direction, referred to as orientation hardening. To express the NW-DOFs, we introduce Finsleriangeometry for an extended description of FTMP, together with the polymer-slip system-based kinematics
proposed by Shizawa, et al. as the framework that allows direct treatments of the orientation change ofthe consisting polymer chains in a similar manner to the conventional crystal plasticity. The current
extension enables one to deal explicitly with the associated microscopic degrees of freedom of underlying
kinds, e.g., cross-linking and unzipping. Targeted material here is PMMA for modeling the early-stage
viscoplastic and the attendant softening responses as typical mechanical properties of engineering
polymers. By allocating the vecotrial field in the extended Finslerian formalism to the representative
direction of the polymer chains, together with the associated fabric tensor field, the study attempts toexpress the NW-DOFs. Assuming that the NW-DOFs are given as a function of the incompatibility, on thebasis of the “flow-evolutionary” perspectives in FTMP, the evolving “free volume” can be effectively
expressed, with which both the targeted properties are successfully reproduced. The free volume change,on the other hand, is shown to qualitatively agree with that reported in the literature.

MMM 2018

483

(P1-57)
Shock Wave Induced Damage in Tumor Cells: Experiments andSimulations*Martin Steinhauser1
1. Fraunhofer Ernst-Mach-Institute, EMI

ABSTRACT
Despite the therapeutic success of extracorporeal shock wave therapy in medicine, e.g. for disintegratingconcrements, the effects of shock waves on the damage of biological cells and their membranes remain
widely unknown. We present experimental results on the destruction of tumor cells using laser-induced
shock waves. For the simulation of biological systems such as membranes under highly transientconditions such as external shock wave load, we propose a thermodynamically consistent and energy
conserving coupling scheme between the atomistic and the continuum domain. The coupling scheme
links the two domains using the DPDE (Dissipative Particle Dynamics at constant Energy) thermostat andis designed to handle strong temperature gradients across the atomistic/continuum domain interface.
Using DPDE we investigate the effects of shock-wave impact on the damage of lipid bilayer membranes. A
coarse-grained model for the phospholipid bilayer in aqueous environment is employed, which models
single lipids as short chains consisting of a hydrophilic head and two hydrophobic tail beads. Water ismodeled by mapping four water molecules to one water bead. Using the DPDE method enables us tofaithfully simulate the non-equilibrium shock-wave process with a coarse-grained model as the correctheat capacity can be recovered. At equilibrium, we obtain self-stabilizing bilayer structures that exhibit
bending stiffness and compression modulus comparable to experimental measurements underphysiological conditions. We study in detail the damage behavior of the coarse-grained lipid bilayer uponhigh-speed shock-wave impact as a function of shock impact velocity and bilayer stability. A single
damage parameter based on an orientation dependent correlation function is introduced.
REFERENCES[1] Martin O. Steinhauser, Computational Multiscale Modeling of Fluids and Solids - Theory andApplications, Springer 2nd ed., Berlin, Heidelberg, New York 2017.
[2] Tanja Schindler, Dietmar Kröner, Martin O. Steinhauser, On the dynamics of molecular self-assembly
and the structural analysis of bilayer membranes using coarse-grained molecular dynamics, Biochimica at
Biophysica Acta, 1858, 1955-1963, 2016

484

MMM 2018

(P2-58)
Studying the Kinetics of a Self-propelled Cruiser in 2D Granular Media
under Gravity*Guo-jie Jason Gao1
1. Dept. of Mathematical and Systems Engineering, Shizuoka Univ.

We propose an extremely simple but efficient self-propelled cruiser, able to travel freely in 2D granularmaterials made of bidisperse dissipative particles under gravity. The cruiser has a round shape and asquare indentation on its edge. To move into a given direction, perpendicular or parallel to gravity, weorient the indentation in line with the desired direction, and the cruiser shifts the granular particles
entering its indent-region to its rear-half by a prescribed distance and then ejects them backward to gain
thrust for moving forward. Using molecular dynamics (MD) method, we identify three universal phases of
the cruiser during one particle-ejection process: 1) acceleration due to the ejection-thrust, 2)
deceleration by the compressed particles ahead and 3) relaxation with the decompressed particles. We
also confirm that the cruiser can travel continuously within the granular medium by successive
particle-ejection, and the cruising performance increases with the ejection-strength and decreases by theinterference from gravity. We believe our study demonstrates a novel approach to design and engineer
self-propelled machines in granular materials.

MMM 2018

485

(P2-59)
A Discrete Tetris model showing two flow regimes for hard particles
exiting a hopper with an adjustable obstacle*Tomoya Yasuno1,2, Guo-jie Jason Gao1
1. Dept. of Mathematical and Systems Engineering, Shizuoka Univ., 2. Dept. of Complex Systems Science, Nagoya
Univ.

Placing an obstacle near the orifice of a hopper has been shown experimentally and numerically to locallyenhance the flow rate for hard particles leaving the hopper under gravity. A flow rate difference between
its value near the obstacle and its maximal value without an obstacle can explain this phenomenon withminimal dynamics involved. When the obstacle sits close to the hopper orifice, the flow rate near the
obstacle is smaller than the maximal value, which corresponds to a fluidized flow regime. On the other
hand, when the obstacle is placed further from the orifice, the flow rate near the obstacle becomes larger
than the maximal value and a clogging flow regime appears. In this study, we employ a Tetris model in 2D
discretized space and successfully demonstrate the two flow regimes. Without creating overlap between
any objects in the system, our model sequentially relocates one particle at a time into its von Neumann or
Moore neighborhood closer to the hopper orifice. Our results show that in the fluidized regime, where
flow rate is low, the Moore protocol, which allows higher freedom to move particles, gives higher flow rate
than the von Neumann protocol. The trend reverses in the clogging regime, where higher freedom tomove particles renders lower flow rate.

486

MMM 2018

(P2-60)
Effect of water molecules on polymer chain motion in dense chain
ensembles of nafion membrane*Isamu Riku1, Ryoma Oka1, Koji Mimura1
1. Osaka Prefecture Univ.

Because of the high power density, high efficiency, fast start-up, and zero emission at the point of use,
proton exchange membrane fuel cells (PEMFCs) are the most promising candidates for replacing internal
combustion engines in automobiles, and are also being developed for portable and distributed stationary
power generation applications.
However, the life of PEMFCs is currently limited by the mechanical endurance of polymer electrolyte
membranes (PEMs). The failure of PEM is believed to be the result of a combined chemical andmechanical effect acting together. Recently, it is found that cyclic hydration of the membrane during theoperation cycles (start/shut down) of the fuel cell may cause mechanical degradation of the membrane.
Therefore, in this paper, to investigate such mechanical degradation of the membrane subjected to fuelcell cycles, we perform a series of molecular dynamic simulations for the membrane made from thesulfonated tetrafluoroethylene copolymer with the trade name Nafion. The effect of the water moleculeson the polymer chain motion in dense chain ensembles of nafion membrane is to be clarified.

MMM 2018

487

(P2-61)
A Coarse-Grained Model for Competitive Adsorption on the Surface of
Inorganic Nanomaterials*Stefano Poggio1, Hender Lopez2, David Power1, Vladimir Lobaskin1
1. School of Physics, University College Dublin, 2. Institute Laue-Langevin, Grenoble

The increased use of nanoparticles (NP) and nanomaterials is pushing scientific research into trying tounderstand the mechanisms governing interactions between biomolecules and inorganic materials. It is
known that, once a NP is in contact with a biological medium, a protein corona forms on its surface, andthat the nature of the corona is what regulates the interaction between the NP and the otherbiomolecules.
In this work we propose a method to coarse grain the interactions of inorganic nanomaterials in contact
with biological fluids of arbitrary composition. Biomolecules (lipids, proteins and carbohydrates) arecoarse grained by mapping their main chemical fragments onto single beads, and their interaction with
the NP surface is described a potential of mean force from atomistic simulations [2]. The NP isrepresented by a two-layer model where the surface interacts with the molecule beads by using the beads
PMF with a slab of the material, corrected by a geometric factor, while the core interacts with via van der
Waals forces calculated using Lifshitz theory. This model can describe the kinetics of competitive
adsorption of biomolecules on the surface of a NP.
We have studied the kinetics of adsorption and the corona composition of Au NPs in a biologicalenvironment with the typical composition of lung lining fluid and blood plasma. This methodology canthen be combined with adverse outcome pathway analysis to build mechanism-based predictive schemes
for toxicity assessments.
Funding: H2020 grant SmartNanotox, contract No. 686098[1] M. Rahman, S. Laurent, N. Tawil, L. Yahia, and M. Mahmoudi, Protein-Nanoparticle Interactions,volume 15 of Springer Series in Biophysics. Springer-Verlag, 2013[2] E. Brandt and A. P. Lyubartsev. Molecular dynamics simulations of adsorption of amino acid side chain
analogues and a titanium binding peptide on the TiO2 (100) surface. J. Phys. Chem. C, 119:18126, 2015.

488

MMM 2018

(P2-62)
Molecular Dynamics of Inorganic and Organic Interfaces with
Force-Field Parameters Based on DFT Simulations*Kosuke Ohata1, Hiroya Nitta1, Kenta Chaki1, Taku Ozawa1
1. JSOL Corporation

Nanocomposites are important in the engineering field. However, the controlling of the properties ofinterfaces between inorganic solid fillers and organic molecules is one of the key issues. We haveinvestigated the interaction between solids and polymers with a combination of Density Functional
Theory (DFT) and Molecular Dynamics (MD) simulations. DFT is a reliable method for calculating potential
energy surface (PES). The force field parameters for MD simulations were determined by using ourscheme for interfacial systems based on the DFT simulation. Then the scheme was applied to asolid-polymer interface. Utilizing the determined coarse-grained and full-atomistic force field parameters,
the MD simulations were conducted. In this study, we utilized SIESTA for the QM simulation and J-OCTA
for the system modeling and the MD simulation.

MMM 2018

489

(P2-63)
Molecular dynamics screening for developing compounds ofelectrolytes with high performance in lithium ion batteries*Yoshihiro Takai1, Ken-ichi Saitoh2, Masanori Takuma2, Yoshimasa Takahashi2, Tomohiro Sato2
1. Graduate school of Science and Engineering, Kansai Univ., 2. Faculty of Engineering Science, Kansai Univ.

Recently, there is great interest in development of electric vehicles, so it demands improvement in
performance of Lithium-ion Battery (LIB). For higher performance, it is important to develop new
substances used for electrolyte or electrode. In particular, electrolyte is an important chemical factor formoving lithium ions between positive and negative electrodes in the battery. When the amount of ionsmoving is enhanced, the performance of the battery will be effectively improved. But there are hundredsof thousands of compounds as candidates for electrolytes, so we need to screen and choose ones fromthese many compounds. In this research, we perform atomistic evaluation about various characteristics ofpossible compounds of electrolyte (such as viscosity, ionic conductivity, degree of dissociation anddiffusion coefficient) by mainly using molecular dynamics (MD) simulations. In evaluating at a molecularlevel, we can understand how the molecular level structure and properties affect the behavior ofelectrolyte. Molecule models we are using are ethylene carbonate (EC), fluoro ethylene carbonate (FEC),
propylene carbonate (PC), butylene carbonate (BC), γ-butyrolactone (GBL), γ-valerolactone (GVL),
dimethyl carbonate (DMC), ethyl-methyl carbonate (EMC), diethyl carbonate (DEC), and lithium
hexafluorophosphate (LiPF6). An electrolytes system in which 1 mol of LiPF6 is mixed per 1 L of single
solvent (solvent + 1M-LiPF6) is simulated. The results suggest that we can determine a criterion for thescreening of superior compounds based on information about molecular structures and properties ofelectrolyte. It is found that the smaller solvent molecules that easily diffuse contribute to the higher ionic
conductivity of electrolytes. This is because diffusion coefficient of Li cation is greatly affected by that ofsolvent molecules. It is also found that solvation structure and size around Li cation take large effect on its
diffusivity.

490

MMM 2018

(P2-64)
Molecular dynamics study on transmission mechanism of torsional
deformation in cellulose nanofibers with hierarchical structure*Kentaro Takada1, Ken-ichi Saitoh2, Masanori Takuma2, Yoshimasa Takahashi2, Tomohiro Sato2
1. Graduate School of Science and Engineering, Kansai Univ., 2. Faculty of Engineering Science, Kansai Univ.

Cellulose nanofiber (CNF) has high strength comparable to steel, and it shows low environmental load
during a cycle of production and disposal. Beside it has many excellent properties and functions such as
high rigidity, light-weight, flexibility and shape memory effect, so it is expected as a next-generation newmaterial. CNF is a fibrous and nano-sized substance produced by decomposition of bulk-type cellulose
which is a main component of plants. Usually it is constituted by many cellulose micro fibrils (CMFs) inwhich molecular chains of cellulose are aggregated in a crystal structure. It is also possible to makecomposite material of CNF together with other components, and then a new material with lightweight aswell as high strength and high toughness will be realized. In such case, knowledge of mechanicalproperties for each CMF units is important. Since actual fibrils are complicatedly intertwined, it is also
crucial to elucidate the transmission mechanism of force and deformation not only in one fibril but also in
between fibrils. Indeed, how the dynamic and hierarchical structure composed of CMFs responds tobending or torsion, which includes gradient of stress and strain, is an interesting issue. However, little is
known on torsional characteristics (shear modulus, torsional rigidity, etc.) concerning CMF. In general, ina wire-like structure, it is difficult to enhance torsional rigidity and strength, compared with tensile ones.
Therefore, in this study, we try to build a hierarchical model of CNF by multiplying CMF fibers and to
conduct molecular dynamics simulation for torsional deformation, by using a hybrid modeling between
all-atoms and united-atoms models. First, shear modulus was estimated for one CMF fibril and it showed avalue close to the experimental values. In addition, it is revealed that intermolecular hydrogen bonds(HBs) are dynamically changed and the HB mechanism is likely to work as strong resistance in torsional
deformation.

MMM 2018

491

(P2-65)
Bubble dynamics of foam flow around an obstacle*Antti Puisto1, Juha Koivisto1, Leevi Viitanen1, Mikko J Alava1
1. Aalto University, Department of Applied Physics

Foams are one of the rare examples of simple yield stress fluids. Formed of bubbles embedded in liquidthey are often studied in the context of jamming [1]. Foam flow is interesting not only because the start-up
requires a dynamical unjamming transition (yielding), but also due to novel technological applications
related to forming technologies [2].
Here, we study a foam flow through a 2D Hele-Shaw shell with an obstacle, forming a constriction. For thispurpose, we use bubble scale dynamics model (the Durian bubble model), which we extend with theappropriate descriptions for the boundary effects coming from the walls and the top and bottom plates.
We observe a negative wake behind the obstacle, analogous to the one observed in gas bubble motion ina viscoelastic medium. There, the medium is succesfully described by an Oldroyd-B model. This suggeststhat in the present conditions, the foam acts as a typical viscoelastic fluid, rather than the expected
elastoviscoplasticity [3]. We compare the simulations data against experiments, foam intruder
experiments, where we find a similar flow pattern. We find a reasonable agreement in the flow dynamics
and the overshoots between the experimental data and the bubble model. Finally, we identify aviscoelastic timescale, which determines the magnitude of the velocity overshoot.
[1] G. Katgert, B. P. Tighe, M. van Hecke, Soft Matter 9, 9739 (2013).
[2] A. M. Al-Qararah, T. Hjelt, A. Koponen, A. Harlin, J. A. Ketoja, Colloids Surf. A 467, 97 (2015).
[3] S. Bénito, C. -H. Bruneau, T. Colin, C. Gay, F. Molino, Eur. Phys. J. E 25, 225 (2008)

492

MMM 2018

(P2-66)
Shape Deformation and Mechanical Relationship of Ionic Droplet
Under An Electric Field
Bo-Yu Shih1, *Wei-Chun Lin1, Alice Hu2, Hsuan-Teh Hu1, Yu-Chieh Lo3
1. Department of Civil Engineering,National Cheng Kung University, 2. Department of Mechanical and Biomedical
Engineering, City University of Hong Kong, 3. Department of Materials Science and Engineering, National Chiao Tung
University

Nano-sized symmetric double Taylor cone forms a capillary liquid bridge while electric field applied on
Ionic liquids (2-dimethyl-3-propylimidazolium- bis(trifluoromethylsulphonyl)imide). This ionic liquid
bridge size is around 20 nm. We attempt to understand the critical dimensions and stability criteria fornano-bridge forming mechanism. Therefore, we conduct molecular dynamics simulation under electricfield to investigate ionic droplets electrohydrodynamic behavior with different structures and potentialfunctions. Factors that affect liquid bridge size through extensive parameter are studied thoroughly in thiswork. We also investigate the influence of changing velocity field and shape deformation under different
electric field conditions. The mechanical relationship between electric stress, Coulomb electrostatic force
and the intermolecular interactions are analyzed. Through this complete studies, surface tension
coefficient and ionic liquid viscosity are obtained. Results show that shape deformation and size of liquid
bridge are mainly controlled by surface tension coefficient and viscosity.

MMM 2018

493

(P2-67)
Multiscale modeling of surface functionalized graphene/polymer
nanocomposites: insight into the effect of covalent grafting*Jeong-ha Lee1, Seunghwa Yang2
1. Dept. of Energy Engineering, Chung-Ang Univ., 2. Dept. of Energy Systems Engineering, Chung-Ang Univ.

Polymer nanocomposites is applied in various industries to improve their mechanical, electrical, thermal
properties instead of using pure polymer. In view of improving mechanical properties of polymer
nanocomposites, graphene nanosheets is typical reinforcement due to its excellent properties. However,
agglomeration of graphene nanosheets in polymer reduces their mechanical and thermal properties.
Besides, weak cohesive energy between graphene and polymer by poor van der waals interaction causes
slip condition in their interfacial region. To improve their intrinsic weak interfacial strength and dispersion
properties between graphene and polymer nanocomposites, surface treatment of graphene such ascovalent grafting or functionalization on graphene nanosheets have been generally used ingraphene/polymer nanocomposites.
In this study, multiscale modeling approach for pristine and covalently functionalized graphene included
polypropylene nanocomposites is implemented. Representative unit cell consists single-layered graphene
and polypropylene matrix is modeled with three-dimensional periodic boundary conditions. Different
number of covalent grafting on graphene is considered to investigate effect of grafting density. In
molecular dynamics simulations, reactive forcefield for hydrocarbon structure is used to describe
carbon-carbon bond breakage in graphene. Through statistical ensemble simulations, thermoelastic
behavior of graphene/polypropylene nanocomposites are determined with grafting density differences atsingle-layered graphene interface. For equivalent continuum modeling to account for covalent grafting,the mean field micromechanics model is supplemented to characterize interfacial and interphase
properties of nanocomposites in accordance with number of covalent grafting. In heat of vaporization
perspective, correlation of covalent grafting and dispersion inside the polymer matrix is examined.

494

MMM 2018

(P2-68)
Durability Analysis on the Environmental Aging of epoxy-based
Nanocomposite*Sunyong Kwon1, Janghyuk Moon3, Man Young Lee2, Seunghwa Yang3
1. Dept. of Energy Engineering, Chung-Ang Univ., 2. Agency for Defense Development, 3. Dept. of Energy Systems
Engineering, Chung-Ang Univ.

Aging is a slow and steady process which occurs by various environmental factors including moisture, uv
light, changes in temperature and pressure. Hygroscopic aging of polymer nanocomposite occurs byconsistent exposure to moisture in service condition. As bounded water in the material causes
microscopic changes in chemical and physical structure of the composite material, it eventually leads toswelling, plasticization, degradation of mechanical and interfacial properties. Thus, to properly examine
the long-time process of aging, correlation between aging time-structure-corresponding properties should
be developed. Therefore, in this study, multiscale bridging method incorporating atomistic approach ofmolecular dynamics (MD) simulation and continuum modeling is presented.
To define the relationship between aged structure and corresponding properties, MD simulation is firstly
adopted. Different crosslinking ratio of 30% to 70% is established by crosslinking reaction between
bisphenol F type epoxy (EPON862®) resin and triethylenetetramine (TETA) curing agent. A single layered
defect-free graphene is added as fiber reinforcement in the nanocomposite structure. Also, to observe thehygroelastic behavior of nanocomposite, weight fraction of 0, 2, 4wt% water is included in the
nanocomposite unit cell. After isobaric-isothermal (NPT) ensemble simulation, diffusion coefficient ofwater, coefficient of moisture expansion (CME), elastic modulus and cohesive zone law of
epoxy/graphene nanocomposite models are predicted. Based on the results of MD simulation,equilibrium hygroelastic constitutive models incorporating interfacial properties between epoxy andgraphene are used to accurately measure the nanoscale effect observed in MD simulations with moisture.

MMM 2018

495

Symposium I(I1 invited)
Grain Growth at the Nanoscale: The Coupling of Stress and Grain
Boundary Motion*Peter Voorhees1, Kevin McReynolds1,3, Akinori Yamanaka2
1. Northwestern University, 2. Tokyo University of Agriculture and Technology, 3. National Institute for Standards andTechnology

Phase field crystal (PFC) models have been used to describe a wide range of phenomena from grain
growth to solidification and dislocation motion in crystals. The method allows the atomic scale motion
and defect formation to be determined on diffusive timescales. Following a brief introduction to the PFC
method, the structure and dynamics of grain boundaries will be discussed. Using two-dimensional andthree-dimension PFC simulations, we find that the atomic-scale structure of the boundary gives rise toqualitatively new grain growth kinetics as well as to grain rotation and grain translation. These new modesof growth can give rise to periodic changes in the morphologies of the grains as they grow or shrink. We
find that grain translation is a result of the climb, glide, and elastic interactions of the dislocations thatcomprise the grain boundary, as well as dislocation interactions at trijunctions. We also find conditions
where the stress generated during grain boundary motion can lead to a cessation of grain growth.

496

MMM 2018

(I1 oral)
Verification of grain growth models by time-resolved 3D experimentsin pure iron
*Jin Zhang1,2, Yubin Zhang2, Wolfgang Ludwig3, David Rowenhorst4, Henning F Poulsen2, Peter W
Voorhees1
1. Northwestern University, 2. Technical University of Denmark, 3. European Synchrotron Radiation Facility, 4. The US
Naval Research Laboratory

Predicting the grain growth behavior of polycrystalline materials is important in the design of material
properties in many engineering applications. Various analytical models are proposed to predict thegrowth behavior, e.g., grain size distribution from the Hillert’s theory or local growth behavior, e.g., thegrowth rate from the MacPherson-Srolovitz theory. However, experimental verification of these models islacking. In this work, we present a unique comprehensive dataset with 1300 grains and 15 annealing
time-steps, which allows us to test various models for grain growth. In particular, we focus on the
MacPherson-Srolovitz theory and address to what extent various statistical ensembles can bemeaningfully described by the model and to what extent it is required to include anisotropy. A
comprehensive statistical analysis shows that geometrical properties averaged over the entire grain
ensemble are well described by the model, but the properties and evolution of the individual grains
exhibit substantial scatter.

MMM 2018497

(I1 oral)
Coarse-grained, three-dimensional modeling of defects at low-angle
grain boundaries with the amplitude expansion of the phase field
crystal model*Marco Salvalaglio1, Rainer Backofen1, Ken R. Elder2, Axel Voigt1
1. Institute of Scientific Computing, Technische Universität Dresden, 01062 Dresden , 2. Department of Physics,
Oakland University, Rochester, 48309 Michigan

The Phase-Field Crystal (PFC) approach describes the dynamics of the local atomic probability density on
atomic length scales and diffusive time scales [1]. It generally requires a fine spatial discretization, which
limits the application of the method to small systems. The so-called amplitude expansion of the PFC
model (APFC) is known to solve this issue, as it accounts for the evolution of the slowly varying amplitudesof the atomic probability density [2]. However, a limited number of parameters are present in the modeland this poses some restrictions in the quantitative description of material properties. Moreover, theapplication of this model to three-dimensional systems has not been extensively explored.
We illustrate the modeling of defects at low-angle grain boundaries (GBs) forming between tilted crystals
by means of the APFC model in two and three dimensions. This is achieved through a Finite Element
Method with advanced numerical features such as adaptive mesh refinement exploiting the features of
the APFC approach. Moreover, an extension of the model to control the energy of defects and GBs is
proposed [3]. The possibility to describe dislocation networks at planar and spherical grain boundaries is
illustrated for different lattice symmetries, namely triangular/honeycomb in 2D as well as body-centeredcubic and face-centered cubic in 3D. The dynamics of spherical grain boundaries is also addressed indetail. In particular, the anisotropic shrinkage of spherical grain is addressed, revealing general qualitative
features independent of the specific rotational axis and crystal symmetry [4] in agreement with recent
atomistic calculations.
[1] K. R. Elder, M. Katakowski, M. Haataja, and M. Grant, Phys. Rev. Lett. 88, 245701 (2002)[2] N. Goldenfeld, B. P. Athreya, and J. A. Dantzip, Phys. Rev. E 72, 020601 (2005)[3] M. Salvalaglio, R. Backofen, A. Voigt, K. R. Elder, Phys. Rev. E 78, 184104 (2017)[4] M. Salvalaglio, R. Backofen, K. R. Elder, A. Voigt, arXiv:1803.03233

498

MMM 2018

(I1 oral)
Estimation of Grain Boundary Anisotropy using Multi-phase-field
Model based on the Ensemble Kalman Filter*Akinori Yamanaka1, Yuri Maeda1, Kengo Sasaki1
1. Tokyo University of Agriculture and Technology

Grain growth is one of the most fundamental microstructure evolutions in metallic materials. Because thesize of crystal grains strongly affects mechanical properties of materials, it is essential to predict the grain
growth behavior during polycrystalline grain growth and recrystallization. Recently, the multi-phase-fieldmodel has been widely used for simulating the grain growth behavior and predicting average grain sizeand grain size distribution. In order to simulate a realistic grain growth behavior by the multi-phase-field
simulation, we need to use accurate data of grain boundary properties, for example, anisotropic grainboundary mobility and grain boundary energy. However, it is difficult to identify the anisotropic grainboundary properties from experimental results by trial-and-error. This study proposes a data assimilation
methodology for estimating grain boundary properties by incorporating experimental results into themulti-phase-field simulation of polycrystalline grain growth. We construct the data assimilation
methodology using the Ensemble Kalman filter. We demonstrate that the anisotropic grain boundary
energy can be successfully estimated from three-dimensional distributions of polycrystalline
microstructure by the data assimilation method proposed in this study.

MMM 2018

499

(I1 invited)
Multiscale Modelling of Graphene from Nano to Micron Scales*Tapio Ala-Nissila1
1. Aalto and Loughborough University

Over the last few years novel 2D materials and thin heteroepitaxial overlayers have attracted intense
attention due to their unusual properties and important technological applications. Many properties suchas thermal conductivity and electrical transport are intimately coupled to the large scale mechanical andstructural properties of the materials whose modeling is a formidable challenge due to a wide span ofscales involved. In this talk, I will review progress in structural multi-scale modeling of 2D materials andthin overlayers [1], and graphene in particular [2], based on the Phase Field Crystal (PFC) model
combined with standard microscopic modeling methods (classical MD and Quantum DFT). The PFC
framework allows one to reach diffusive time scales for structural relaxation of the materials at the atomic
scale, which facilitates quantitative characterisation of domain walls, dislocations, grain boundaries, andstrain-driven self-organisation up to almost micron length scales. This allows one to study e.g. thermal
conduction and electrical transport in multi-grain systems [3]. References: 1. Elder et al., PRL vol. 108,
226102 (2012); PRB vol. 88, 075423 (2013); JCP 144, 174703 (2016). 2. Hirvonen et al., PRB 94,
035414 (2016). 3. Fan et al., PRB vol. 95, 144309 (2017); Hirvonen et al., Sci. Reps. 7, 4754 (2017); Azizi
et al., Carbon 125, 384 (2017); Fan et al., Nano Lett. 7b1072 (2017).

500

MMM 2018

(I2 invited)
Energy and dynamics of grain boundaries based on underlying
mircrostructure*Yang Xiang1
1. Hong Kong University of Science and Technology

Grain boundaries are the interfaces of grains with different orientations in polycrystalline materials.
Energetic and dynamic properties of grain boundaries play essential roles in the mechanical and plastic
behaviors of the materials. These properties of grain boundaries strongly depend on their microscopic
structures. We present continuum models for the energy and dynamics of grain boundaries based on thecontinuum distribution of the line defects (dislocations or disconnections) on them. The long-range
elastic interaction between the line defects is included in the continuum models to maintain stablemicrostructure on grain boundaries during the evolution. The continuum models is able to describe bothnormal motion and tangential translation of the grain boundaries due to both coupling and sliding effects
that were observed in atomistic simulations and experiments.

MMM 2018

501

(I2 oral)
Grain growth in ultrafine-grained thin films: A 3D problem*Ahu Oencue1, Thorsten Halle2, Dana Zoellner3
1. Institute of Experimental Physics, Otto-von-Guericke University, Magdeburg, 2. Institute of Materials and Joining
Technology, Otto-von-Guericke University, Magdeburg, 3. B CUBE Center for Molecular Bioengineering, TU Dresden,
DresdenGrain microstructures of polycrystalline solids have an massive impact on materials properties. For
investigations of bulk materials it is generally assumed that surfaces effects are insignificant. In contrast, inthin films surface effects become non-negligible. If during grain growth in such films the average grain size
reaches the order of the layer thickness, grain growth slows down or even comes to a halt.
While analytic theories of nano- and microcrystalline grain growth are often in good agreement withnumerical results using computer simulations with respect to, e.g., average growth kinetics, particularly
analytic grain size distributions or topological correlations between grains rarely capture the experimental
features. One reason of this disagreement can be found in the simple fact that the experimental samples
are of 3D nature, but are commonly measured in 2D and compared with 2D simulations and analytic
theories.
In the present work, we model grain growth in ultra-fine grained thin films using a three-dimensional
Monte Carlo Potts model. In particular, we take into account that a reduction in grain size frommicrometer to nanometer increases the importance of higher order grain boundary junctions, namely
triple lines. The results are compared to experimental measurements of ultrafine-grained thin metallic
films in three dimensions undergoing grain growth.

502

MMM 2018

(I2 oral)
The Kinetics of Ideal Grain Growth: A Large Scale Monte Carlo
Simulation*Xiangge Qin1
1. School of Materials Science and Engineering, Jiamusi University

Most of the metallic materials are polycrystalline aggregates, understanding the grain growth kinetics andcontrolling the grain size are important for predicting the microstructure of polycrystalline materials andpreparing bulk nanomaterials. Although some theoretical equations for the ideal grain growth kinetics
have been proposed, it is difficult to achieve an ideal grain growth process experimentally.
A considerable number of Monte Carlo simulations of the grain growth process have been reported, but
none of the simulation results accurately match the ideal three-dimensional grain growth kinetics. In thispaper, Monte Carlo method was used to simulate the three-dimensional grain growth process of metal.
The effect of model temperature and model size on the dynamic simulation results of grain growth wasstudied. The grain size distribution during grain growth was analyzed. Large-scale simulation results showthat the ideal grain growth kinetics accords with the theoretically predicted parabolic law, and the grain
size distribution at the steady state stage is consistent with the Weibull distribution function.

MMM 2018

503

(I2 oral)
Understanding the energetics of grain boundary motion in terms ofcompatible transformations and optimal transport theory*Ian W Chesser1, Brandon Runnels2, Elizabeth Holm1
1. Carnegie Mellon University, 2. University of Colorado Colorado Springs

In this study, we investigate two approaches to understanding the complex dependence of grainboundary mobility on temperature, crystallography, and driving force observed in experiments andmolecular dynamics simulations. We first calculate energy barriers for unit mechanisms associated with
faceted grain boundary migration, and show that although these barriers are useful in understanding
trends in grain boundary mobility such as mobility type classification, they are difficult to enumerate for alarge number of boundaries. As a second approach to explore the kinetics of grain boundary motion, we
model GB mobility using Bain strains and dissipation energy, in the sense of martensitic phasetransformations and twinning. This framework provides new insight into faceted grain boundary motion byinterpreting it as a competition between compatible transformations at the boundary informed byapproximate energy barriers to motion computed in an optimal transport framework. We establish a yield
criterion for grain boundary migration via the interplay between optimal transportation energies,
temperature, and applied load, and compare our results to existing shear coupling data in the literature.

504

MMM 2018

(I2 invited)
Microstructure Stabilization and the Herring Condition*Jeremy K Mason1, Erdem Eren1
1. University of California, Davis

Consistency of properties is critical for materials performance, and fundamentally depends on thestabilization of the microstructure. Existing stabilization techniques often rely on chemical modifications,with the solute preferentially segregating to the interfaces or precipitating as small second phaseparticles. The idea to instead stabilize a microstructure by modifying the crystallographic degrees of
freedom is part of the broader subject known as grain boundary engineering. This has been mostsuccessful with materials that form low-energy annealing twins, though this severely restricts the materials
to which the concept can be applied.
We propose that, rather than increasing the fraction of low-energy boundaries, grain boundary
engineering can most effectively stabilize a microstructure by increasing the fraction of boundaries withboundary plane orientations at cusps in the energy landscape. The resistance of such boundaries toreorientation can pin boundary junction lines in two-dimensional materials and boundary junction points
in three-dimensional materials. This is discussed in the context of the two-dimensional Herring condition
and the three-dimensional analogue, which is apparently not widely known in the literature.
This leads to the idea that the microstructure of materials with arbitrary chemical compositions could bestabilized by introducing a misorientation distribution that makes many singular boundary planeorientations available; the material would then be allowed to evolve to a local minimum of the overallgrain boundary energy, with the junction points pinned in metastable configurations. Simulation of such a
process requires the implementation of accurate and general equations of motion for the junction points.
We finally describe our ongoing development of a finite element-based microstructure evolution code forthis purpose.

MMM 2018505

(I3 invited)
Interaction of moving grain boundaries with solutes: bridging timescales between atomistics and continuum*Yuri Mishin1
1. George Mason University

Solutes can strongly interact with grain boundaries (GBs) and impact their thermodynamic and kinetic
properties. Some of the most interesting effects of solute segregation include the segregation-induced GB
phase transformations and the solute drag/pinning effect. An even more interesting is the effect of GB
motion on GB phase transformations. Studying these effects by molecular dynamics is highly problematicdue to the limited time scale of the method. The talk will present two alternative methods for studying
phase transformations in moving GBs. One method is based on a semi-analytical discrete GB model in abinary solid solution. The regular solution model predicts first-order phase transformations in GBs, which
can be shown by a GB phase coexistence line and GB spinodals of the bulk phase diagram. The model
overcomes the time-scale limitation and treats both GB thermodynamics and GB dynamics within aunified framework. It gives direct access to the solute drag force and the GB free energy, which are
difficult to compute by atomistic simulations. GB migration can be modeled on different timescales andover a wide range of velocities in both the transient and steady-state regimes. The simulations reveal
interesting effects, such as kinetic stabilization of metastable or even unstable GB phases, dynamic GB
phase transformations, and the dynamic GB hysteresis. Other interesting effects include the break-way
events in which the GB leaves the segregation atmosphere behind and forms a new segregation. This new
segregation atmosphere can represent the same of a different GB phase. It is shown how the discretemodel reduces to phase-field model in the limit of wide segregation region. The phase-field simulations
within this model yield similar results as the discrete model but have certain computational advantages.
The results of this study can be broadly interpreted in terms of extremum principles of non-equilibrium
thermodynamics, which predict dynamic stabilization of thermodynamically unstable but kineticallyfavored phases. Extension of the models to 2D and 3D systems and more accurate thermodynamic
treatments will be discussed.

506

MMM 2018

(I3 oral)
Phenomenological model for prediction of interaction parameters ingrain boundary segregation*Pavel Lejcek1, Siegfried Hofmann2
1. Institute of Physics, AS CR, Praha, 2. Max-Planck-Institute for Intelligent Systems, Stuttgart

Grain boundary segregation is a phenomenon studied both experimentally and theoretically for decades.
The interest in this phenomenon is evoked not only by its close relationship to temper embrittlement but
also by its ability to stabilize nanocrystalline structures by solute segregation in the concept of Grain
Boundary Engineering.
A full description of the exact temperature and solute concentration dependence of grain boundary
segregation requires reliable values of segregation enthalpy and entropy as well as solute interaction
coefficients. However, experimental studies of all these values are rather limited, and theoretical studies
are usually restricted to segregation energy at zero Kelvin. It is obvious that such a database does notallow a full description of the grain boundary segregation in most systems of interest.
In this contribution we present a semi-empiric method enabling prediction of all required thermodynamic
parameters –ideal enthalpy, ideal entropy and real binary interaction (Fowler) coefficient –which, in
combination, describe fully the segregation of any segregant at any temperature and at individual grainboundaries. This method is based on the relationship between the segregation enthalpy and the solid
solubility limit, and on the enthalpy-entropy compensation effect. The data for numerous solutes in alphairon are predicted and they are compared to available data in the literature.

MMM 2018

507

(I3 invited)
Influence of Solutes at Grain Boundaries on Phase Transformations
and Mechanical Response*Stephen M Foiles1, Nathan Heckman1, Christopher Barr1, Fadi Abdeljawad1, Khalid Hattar1,
Brad Boyce1
1. Sandia National Laboratories

Segregation of solute atoms to grain boundaries is a well-known phenomenon which can modify grainboundary structure, energy and mobility among other properties. In this talk, we present computations of
the segregation of Au at grain boundaries in Pt and corresponding experimental validation. The existenceof a grain boundary phase transformation between high- and low-segregation states will be demonstrated.
The impact of this segregation on the mechanical response of both individual grain boundaries and grainboundary networks will be examined. This work shows that for nanocrystalline samples, the stability of thegrain structure against thermal or deformation induced grain growth is enhanced, but that ductility is
reduced.

508

MMM 2018

(I4 invited)
Atomistic modeling of helium segregation to grain boundaries intungsten and its effect on de-cohesion*Enrique Martinez Saez1, Blas Pedro Uberuaga1, Brian D Wirth2,3
1. Material Science and Technology Division, MST-8,Los Alamos National Laboratory, Los Alamos, 87545 NM, USA, 2.
Department of Nuclear Engineering, University of Tennessee, Knoxville, TN 37996, United States of America, 3. Oak
Ridge National Laboratory, PO Box 2008, MS-6003, Oak Ridge, TN 37831, United States of America

Due to their low sputtering yield, low intrinsic tritium retention, high melting point, and high thermal
conductivity, W and W alloys are promising candidates for the divertor region in a magnetic fusion device.
Transmutation reactions under neutron irradiation lead to the formation of He and H particles that
potentially degrade material performance and might lead to failure. High He fluxes ultimately lead to theformation and bursting of bubbles that induce swelling, a strong decrease in toughness, and a nanoscale
microstructure that potentially degrades the plasma. Understanding the behavior of He in polycrystalline
W is thus of significant importance as one avenue for controlling the material properties under operating
conditions. In this work we study the interaction of both substitutional and interstitial He atoms withvarious grain boundaries in pure W and the effect of the He presence on the system response to externalloading. We observe that He segregates to all the interfaces tested and decreases the cohesion of thesystem at the grain boundary. Upon tension normal to the interface, the presence of He significantly
decreases the yield stress, which depends considerably on the bubble pressure. Increasing pressure
reduces cohesion, as expected. More complex stress states result in more convoluted behavior, with He
hindering grain boundary sliding upon simple shear.

MMM 2018

509

(I4 oral)
Thermodynamic properties of bcc Fe grain boundaries withsegregation of 3d-transition-metal solutes*Zhuo Xu1, Shingo Tanaka1, Masanori Kohyama1
1. AIST

Grain boundaries (GBs) are one of the key factors to manipulate the material properties. The solutesegregation and solute drag effects are remarkable for the thermodynamic properties of GBs, where themicroscopic atomic and electronic structures affect the macroscopic behaviors of GB evolution. We havedeveloped the local analysis method to get the local-energy and local-stress of each atom [1], which is of
assistance to unveil the microscopic mechanism. In the present study, we firstly focused on the Σ11(332)and Σ3(111) <110> symmetrical tilt GBs in bcc Fe with 3d-transition-metal (TM) solutes. We performed
ab-initio calculations together with the local analysis, and observed that the segregation behaviors of
3d-TM solutes can be classified into early TMs (Sc, Ti and V), middle TMs (Cr and Mn), and late TMs (Co,Ni and Cu). The early and late TMs prefer to segregate at the looser and tighter sites of GBs, respectively.
The local-energy decomposition indicates that the segregation of early TMs is dominated by thestabilization of surrounding Fe atoms at a GB, while that of late TMs is dominated by the repulsion fromthe bulk as well as the stabilization of a TM-atom itself. Furthermore, corresponding to the segregationenergy of each 3d-TM segregated to the looser or tighter site of GBs, by the multiscale modeling, weestimated the solute drag effects on the mobility and interface energy of GBs, and the grain growth rate.
Finally, we discuss the cases of general GBs, and compare the results with experiment.
[1] Y. Shiihara et al., Phys. Rev. B 81, 075441 (2010). S. Bhattacharya et al., J. Mater. Sci. 49, 3980 (2014).

510

MMM 2018

(I4 oral)
A new thermodynamic model for the austenite-ferrite massive
transformation in Fe-C-X alloys.
*Alexandre MATHEVON1, Michel PEREZ1, Damien FABREGUE1, Veronique MASSARDIER1,
Philippe ROCABOIS2, Patrice CHANTRENNE1
1. INSA Lyon, 2. FIVES KEODS

Predictions of the ferrite to austenite or austenite to ferrite transformation kinetics during heating and/or
cooling of ternary Fe-C-X alloys is challenging because diffusion coefficients of interstitial andsubstitutional elements are very different in both phases. The model described in this paper, is based on
the prediction of (i) concentration profiles for all elements and (ii) interface velocity, that corresponds tothe minimization of the Gibbs energy (i.e. phase field approach). The total energy of the system is given via
a dynamic coupling with Thermocalc database.
The ferrite/austenite interface is assumed to have a finite width. This allows to deal with a unique
diffusion profile for all species. Element fluxes are derived from their chemical potential gradient. Anotheradvantage of such an approach is the possibility of introducing a potential depth within the interface,leading to solute atoms segregation, that is believed to control transformation kinetics in ternary systems.
During the transformation, it is observed that the model automatically switches between different
equilibrium conditions (from out-of-equilibrium transformation to para-equilibrium, local equilibrium withand without partitioning until ortho-equilibrium). To our knowledge, it constitutes a real improvement inphase transformation modeling. This model has been calibrated on decarburization experimental kinetics
reported in the literature in various Fe-C-X systems. This model shows an interface motion slowing downwith the addition of a potential depth at the interface in agreement with experimental observations. By
extension, the model has been used on isothermal and non-isothermal heating for complex Dual Phasesteels and compared with experiments.

MMM 2018

511

(I4 oral)
Multiscale simulation of solid phase sintering of nano copper powder*Xinyu Zhu1, Xiangge Qin1
1. School of Materials Science and Engineering, Jiamusi Univ.

Powder metallurgy is a processing technology that uses metal powder compacts to produce near net
shape parts through sintering and is suitable for mass production. Quantitative prediction of
microstructure evolution and sintering kinetics during powder sintering is of great significance forunderstanding the sintering mechanism and optimizing the sintering process.
In this paper, the packing structure model of nano copper particles was established using the discreteelement method as a powder compact. The solid phase sintering of the powder compact was studied bymolecular dynamics simulation using embedded atom method. The sintering mechanism anddensification kinetics of the nano powders were analyzed and compared with the experimental results.
The effects of diffusion mechanism and grain growth on the sintering densification process are discussed
in detail.
Keywords: powder sintering, diffusion, grain growth, discrete element, molecular dynamics

512

MMM 2018

(I5 invited)
Heterogeneous disconnections nucleation mechanisms during grainboundary migration*Nicolas Combe1,2, Frederic Mompiou1, Marc Legros1
1. CEMES Toulouse, France, 2. Univ. Toulouse, UPS, France

In specific conditions, grain boundary (GB) migration occurs in polycrystalline materials as an alternative
vector of plasticity compared to the usual dislocation activity. The shear-coupled GB migration, theexpected most efficient GB based mechanism, couples the GB motion to an applied shear stress. The
migration of the GB occurs through the nucleation and motion of disconnections.
We report a detailed theoretical study of the elementary mechanisms occurring during heterogeneous
disconnections nucleation. Using molecular simulations, the absorption of a $1/2[110]$ edge bulk
dislocation in a symmetric $\Sigma{17}(410)$ $[001]$ tilt GB generates an immobile disconnection in the
GB. We show that the shearing of this GB induces its migration and reveals a new GB migration
mechanism through the nucleation of a mobile disconnection from the immobile one. Energy barriers andyield stress for the GB migrations are evaluated and compared to the migration of a perfect GB. As
expected, the migration of imperfect GB is easier than the one of perfect GB. An immobile disconnection
in a BG can thus operate as a source of disconnections driving the GB migration.

MMM 2018

513

(I5 oral)
Motion of Grain Boundaries Based on Disconnections*Chaozhen Wei1,2, Luchan Zhang5, Yang Xiang2, Jian Han4, Spencer Thomas4, kongtao Chen4,David J. Srolovitz3,1
1. HKUST Jockey Club Institute for Advanced Study, Hong Kong University of Science and Technology, 2. Departmentof Mathematics, Hong Kong University of Science and Technology, 3. Materials Science and Engineering, MechanicalEngineering and Applied Mechanics, Computer and Information Science, University of Pennsylvania, 4. Materials
Science and Engineering, University of Pennsylvania, 5. Department of Mathematics, National University of Singapore

We propose a novel approach to simulate the evolution of polycrystalline microstructures based upon adisconnection model for grain boundary (GB) kinetics. The model incorporates surface tension, appliedstress, and jumps in chemical potential across GBs. The model also includes disconnection nucleationand mobility. Disconnections are line defects that lie solely with GB and are characterized by both a
Burgers vector and a step height, as set by the GB bicrystallography. We first derive a continuum equationof motion for individual GBs and then for GB triple junctions (TJ) within a polycrystalline microstructurethat rigorously accounts for conservation of disconnection Burgers vectors and step heights and couples
the GBs meeting at the TJ. We then implement this model in a continuum simulation of GB dynamics
without TJs, with TJs and in a polycrystalline microstructure. The resultant simulations provide clear
demonstrations of the importance of including a crystallography-respecting microscopic model ofmicrostructure evolution and the intrinsic coupling between stress, capillarity, and microstructure
connectivity in microstructure evolution.

514

MMM 2018

(I5 invited)
Dislocation-mediated boundary motion, dislocation-boundaryinteraction, and their effect on the mechanical behavior in fcc
materials*Bob Svendsen1,2, Jaber Rezaei Mianroodi2, Juan Li3, Christoph Kirchlechner3, Gerhard Dehm3
1. Material Mechanics, RWTH Aachen University, 2. Microstructure Physics and Alloy Design, Max-Planck-Institut fürEisenforschung GmbH, 3. Structure and Nano-/Micromechanics of Materials, Max-Planck-Institut für EisenforschungGmbH

In the current work, mechanisms for dislocation-mediated motion of and deformation at coherent
boundaries in fcc materials is investigated along with the resulting mechanical behavior.
On the modeling side, molecular dynamics (MD) simulations are carried out on bulk single and bi-crystal
Cu, the latter containing two Σ3(111) boundaries. These are subject to loading conditions varying
between shear loading parallel to the boundaries and perpendicular uniaxial loading. In the case of pure
shear loading, MD results demonstrate that <112> shearing on Σ3(111) results in monotonic flow
depending on the shear sign, whereas <110> shearing leads to oscillatory flow. As it turns out, thisdifference in behavior can be related to corresponding changes in the coincidence site lattice (CSL). In
particular, in the case of <112> shearing, new potential boundary positions in the CSL occur every three
atomic layers. Depending on the shear sign, however, one of these positions is closer to the current
boundary position than the other. Since less energy is required for the boundary to shift to the closer
position, the boundary moves to this position. In contrast, <110> shearing results in new potential
boundary positions in the CSL having the same distance to the current boundary. Consequently, none ofthese is favored energetically, resulting in oscillatory motion of the boundary between these.
On the experimental side, nano-indentation experiments with 2μm sized spherical indenter tips are used
to study the impact of coherent Σ3(111) boundaries on the yield strength distribution in copper. The
indents are performed either inside a grain or close to a coherent Σ3(111) boundary aligned normal to
the sample surface. The force-displacement curves show elastic loading following Hertz´s predictions
until a sudden displacement burst –a “pop-in” –is observed. The maximum shear stress beneath theindenter tip at the pop-in force is interpreted as yield stress. The statistical behavior of the yield stress isanalyzed via cumulative probability (CP) plots. The CP plots show a significantly lower average yield stressat the boundary with respect to the single grains and an extremely narrow distribution at the boundary.
The later finding suggests that the mechanism responsible for dislocation source activation is
omnipresent at the boundaries.

MMM 2018

515

(I6 invited)
Migration mechanisms of faceted vicinal grain boundaries*Sherri Hadian Hadian1, Blazej Grabowski1, Mike W Finnis2, Jörg Neugebauer1
1. Max-Planck-Institut fuer Eisenforschung, 2. Imperial College London

We report molecular dynamics simulations and their analysis for grain boundaries vicinal to the Sigma 7
symmetric tilt boundary of the type {1 2 3} in aluminium. When minimized in energy at 0K a grainboundary of this type exhibits nano-facets that contain kinks. We observe that at higher temperatures of
migration simulations, given extended annealing times, it is energetically favorable for these nano-facets
to coalesce into a large terrace-facet structure. Therefore we initiate the simulations from such a structureand study as a function of applied driving force and temperature how the boundary migrates. We find themigration of a faceted boundary can be described in terms of the flow of steps. The migration isdominated at lower driving force by the collective motion of the steps incorporated in the facet, and at
higher driving forces by the step detachment from the terrace-facet junction and propagation of steps
across the terraces. The velocity of steps on terraces is faster than their velocity when incorporated in thefacet, and very much faster than the velocity of the facet profile itself, which is almost stationary. A simple
kinetic Monte Carlo model matches the broad kinematic features revealed by the molecular dynamics.
Since the mechanisms seem likely to be very general on kinked grain boundary planes, the step flow
description is a promising approach to more quantitative modeling of general grain boundaries.

516

MMM 2018

(I6 oral)
Multiscale model for the structure and energy of low-angle general
grain boundaries in Al, Cu and Ni*Shuyang Dai1, Yang Xiang2, David Joseph Srolovitz3
1. Wuhan University, 2. Hong Kong University of Science and Technology, 3. University of Pennsylvania

We present a multiscale model to describe the structure and the energetics for various grain boundaires.
The model incorporates both the anisotropy elasticity in each grain and the first-principle calculation
informed interaction between two grains across the grain boundary, i.e., the generalized stacking-fault
energy based upon the disregistry. The force balance between these two contributions determines the
structure. We apply this approach to determine the structure and energetics of several twist grain
boudnaries and tilt grain boundaries in FCC metals such as Cu, Al and Ni.

MMM 2018

517

(I6 invited)
The influence of normal stress on the structural transformation andmigration of grain boundaries*Chuang Deng1, Mohammad Aramfard1
1. University of Manitoba

It is now widely accepted that stress plays important roles on the kinetics of grain boundaries. Past studies
on grain boundary kinetics by atomistic simulation mainly focused on those under shear deformation byrecognizing that shear coupling is one of the most fundamental modes of grain boundary motion.
However, grain boundaries in polycrystalline materials under realistic loading conditions rarely experience
pure shear deformation and stresses with both shear and normal components are more common. In thiswork, we used molecular dynamics simulations to investigate (1) how normal stress would influence theground state structures of a few types of special CSL grain boundaries and (2) how such structural change
would further influence the modes and energy barrier for migration in those grain boundaries.
We found that while both constant and cyclic normal stresses can facilitate the transition in grainboundary structure among its metastable states, the influences of tension and compression are not thesame and vary among different types of grain boundaries. It is also found that the macroscopic straincaused in the materials due to the structural transformation in grain boundaries under cyclic stress can bewell described by Coble creep, which implies a possible new mechanism of Coble creep that has been
overlooked before. Furthermore, the grain boundary structural transformation can either lower or raise theenergy barrier for grain boundary migration depending on the grain boundary type, which can be
explained by the relative easiness of atomic shuffling in each grain boundary based on their DSC lattice.

518

MMM 2018

(I7 invited)
Insights into recrystallisation: atomistic simulations of the properties ofgrain boundaries in heavily deformed material*Chris P Race1
1. University of Manchester

A critical process in the development of a desirable microstructure in polycrystalline metals is that ofrecrystallisation, in which new grains of defect-free material nucleate and grow within the defect-laden
microstructure of deformed material.
Thus, recrystallisation involves the creation and migration of grain boundaries between pristine and highly
defective crystal. The structure and properties of these grain boundaries will be strongly affected by theplastic deformation in the defective grain. Furthermore, the driving force for their migration will be theelimination of the deformation defects. This force will be larger in magnitude and different in character tothe driving forces present in other cases of grain boundary migration, such as grain growth. Grainboundary properties under these conditions have, however, received comparatively little attention.
Here we present insights from atomistic models of grain boundaries between cold-worked and defect-freegrains which begin to uncover trends in their static and dynamic properties under conditions ofrecrystallisation.

MMM 2018

519

(I7 oral)
Continuum Dislocation Dynamic Based Grain Fragmentation Modelingfor Severe Plastic Deformation in FCC Metals*Ali Al-Hadi I. Kobaissy1, Georges Ayoub2, Mu'Tasem Shehadeh1
1. Department of Mechanical Engineering, American University of Beirut, 2. Department of Industrial and
Manufacturing Systems Engineering, University of Michigan-Dearborn

In this work, the mechanical response as well as the microstructural features of face-centered cubic (fcc)
metals subjected to severe plastic deformation (SPD) are investigated. A multi-scale framework that
couples crystal plasticity (CP) scheme with continuum dislocation dynamics (CDD) model is proposed to
mimic the loading conditions during Equal Channel Angular Pressing (ECAP) processes. Several aspects ofthe deformation process have been considered including texture evolution, the evolution of statisticallystored dislocations (SSDs) and geometrically necessary dislocations (GNDs), and the fragmentation of thegrains and its effect on the overall mechanical response. The framework is applied to a reference volume
element (RVE) in which the grains are distributed and assigned a position. Within the model, each grain isallowed to split into 1024 new smaller grains which subsequently lead to strain hardening and grain
refinement. The latter is modeled by accounting for the grain-grain interaction, for which the concept of
the GNDs is incorporated into the mean free path of the dislocations. GNDs are assumed to be induced
by grain boundaries that restrict the free deformation of a grain and result in an increase of stresses
leading to the grain size reduction. The grain refinement procedure is triggered when the misorientationthreshold between subgrains is exceeded. The calibration of the model parameters is performed using
torsion test of pure copper material. The simulations results of generated texture and grain size reduction
are in very good agreement with experimental data available in the literature.

520

MMM 2018

(I7 oral)
Atomistic Investigation on Interaction of Stress-Assisted Grain
Boundary Motion with Crack*Mohammad Aramfard1, Chuang Deng1
1. Univ. Manitoba

Stress-assisted grain boundary motion is a prominent phenomenon in crystalline materials. On the other
hand, crack/voids are common features in physical materials, thus the interaction of a moving grainboundary mediated by stress with such features are important to study the microstructural evolution of
crystalline materials under severe plastic deformation. In this work three major behaviors are studied
using atomistic simulations when a stress-mediated moving grain boundary interacts with crack, i.e., crack
healing, crack propagation and sub-grain formation. The underlying macromechanism is described using
atomistic configurations for each case and the effect of loading type, i.e., monotonic or cyclic is
described. It is shown that metastable phases are the main reason for crack heal-ability of grainboundaries. The effect of material type which is in terms of stacking fault energy is also explored.

MMM 2018

521

(I7 invited)
Effect of grain boundary structure on its Dynamic Response using
Molecular Dynamics*Saryu Fensin1, Timothy Frolov2
1. Los Alamos National Lab, 2. Lawrence Livermore National Laboratory

Grain boundaries (GBs) can play an important role in governing the mechanical behavior and damage
evolution of a material during both quasistatic and dynamic loading. However, the general consensus of
the shock physics community has been that minute details about the GB structure should not affect the
response of a material to dynamic loading. In this paper, we present results of molecular-dynamics
simulations investigating whether or not small changes in boundary structure are 'recognized' by theshock wave and can in turn affect the spall strength of a material. As a test case, we will study grainboundaries in both Copper and Tungsten with similar orientation relationships but varying local
structures. Specifically, we propose to use grand-canonically optimized structures for grain boundaries
that retain their ordered structure even at elevated temperatures.

522

MMM 2018

(I8 invited)
New Approaches for Understanding Nanocrystalline Mechanics:
Physical Microstructures, Grain Boundary Descriptors andDeformation Mechanisms*Garritt Tucker1, Jacob Tavenner1, Ankit Gupta1
1. Colorado School of Mines

Engineering interest in nanocrystalline (NC) materials has been founded on the potential to improvemechanical properties such as increased strength/hardness, while scientific interest stems from thealternative fundamental mechanisms that are operative. Compared to their coarser-grained counterparts,
the influence of interfaces (i.e., grain boundaries (GB)) becomes more significant in NC materials. Current
simulation techniques for understanding NC mechanics rely on non-physical microstructures, first-order
grain boundary descriptors that poorly capture the complexity of interfacial structure-property
relationships, and a lack of quantitative approaches that can accurately capture the specific contributionof different deformation mechanisms. In this study, we propose utilizing higher-order GB descriptors to
improve interfacial understanding, while determining polycrystalline behavior through networks
connecting bulk and boundary behaviors. We perform atomistic modeling (e.g., Molecular Dynamics)
studies of both favored CSL and general high-angle GBs for analyzing empirical and structural descriptors
from atomic-scale behavior at the interfaces. These descriptors then aid in our GB network modeling tounderstand larger-scale polycrystalline behavior by unraveling the complexity surrounding thecompetition/cooperation between different deformation mechanisms. The importance of choosing
physically-based atomistic microstructures and proper equilibration techniques in such simulations arealso discussed. Finally, by utilizing continuum-based kinematic metrics, which can resolve the individual
contribution of various deformation mechanisms such as GB and dislocation-mediated deformation to thetotal strain in the material, we help to further unravel the complex microstructural deformation behavior in
NC metals.

MMM 2018

523

(I8 oral)
Grain Boundary Sliding: the best supporting role in ductile localization*Alexandre Dimanov1, Jean Raphanel1, Michel Bornert2, Eva Héripré3, Mathieu Bourcier1,
Ababacar Gaye2, Alexandre El Sabbagh1, Wolfgang Ludwig4, Andrew King5
1. LMS, CNRS-UMR7649, Ecole Polytechnique, 2. Laboratoire Navier, CNRS-UMR8205, Ecole des Ponts, 3. MSSMAT,
CNRS-UMR8579, Centrale-Supélec, 4. MATEIS, UMR5510, INSA-Lyon, 5. SOLEIL Synchrotron, Beam line Psiché

Viscoplastic deformation of polycrystalline materials conditions many aspects of our everyday life fromindustrial hot forming of metallic items and durability of engineered structures, to glacier flow or plate
tectonics powered by convection of Earth’s mantle rocks. Usually, polycrystalline viscoplastic
deformation is largely based on crystal slip plasticity (CSP). However, grain boundary sliding (GBS) may
become a dominant mechanism at high temperatures, small grain sizes and low strain rates. Bothmechanisms are often considered to act in parallel and to contribute independently to the global
behaviour. We considered different classes of polycrystalline materials, such as Silicates, NaCl andAluminium, combining high temperature rheological investigations and micromechanical testing with insitu SEM multi-scale observations. In the latter case, we performed full field strain measurements, basedon digital image correlation (DIC). For NaCl we could perform micromechanical tests in situ synchrotron
X-ray microtomography and obtain both 2D and 3D strain fields. We show that for all of the consideredmaterials CSP and GBS are not independent, but co-operative. Depending on microstructure and loading
conditions, each one may be either the dominant strain cumulative mechanism, or a secondary
mechanism, allowing for accommodation of the local strain incompatibilities related to the previous one.
Both are necessary to ensure macroscopically homogeneous flow. We show examples of minor but crucial
contribution of GBS to ductile localization phenomena and strain propagation throughout the
microstructure.

524

MMM 2018

(I8 oral)
Polycrystal plasticity with anisotropic grain boundary energy describedon the five-dimensional grain boundary space*Nikhil Chandra Admal1, Javier Segurado2,1, Jaime Marian1
1. University of California Los Angeles, 2. IMDEAAt high temperatures, grain boundaries are not static entities but can migrate in response tothermo-mechanical forces brought about by temperature, external stresses, and internal microstructure.
This gives rise to a wide array of dynamic behavior, including recovery, recrystallization, grain growth, etc.
In this talk, we present a recently developed three-dimensional polycrystal plasticity model driven by asingle energy functional that captures both grain boundary (GB) energy and bulk elastic energy inresponse to plastic deformation. The model represents any arbitrary grain boundary using geometricallynecessary dislocations (GNDs), and the GB energy is constructed as a function of the GNDs. Moreover, incontrast to previous phase-field approaches for grain boundary evolution, our model’s energy functionalis described on the entire five-dimensional grain boundary space allowing the consideration of bothmisorientation and inclination in the grain boundary energy.
The model is parameterized using grain boundary energy data from atomistic simulations, after which it isused to simulate the microstructure evolution of polycrystalline samples. The framework is used first to
simulate the recrystallization process in a three dimensional polycrystal without external loads. Finally, thedeformation of polycrystals under external loads is simulated for a few selected cases.

MMM 2018525

(I9 invited)
Grain Boundary Microstates Under Irradiation: A Moment in Time?
*Mitra L Taheri1
1. Drexel University, Department of Materials Science & Engineering

Understanding the sink efficiency of interfaces under irradiation is of paramount importance to tailoringmaterials for radiation tolerance. Using in situ irradiation coupled with precession electron diffraction
analysis, defect absorption was tracked for increasing dose. Denuded zones were found to collapse, but inthe absence of any detected changed in macroscopic degrees of freedom of the grain boundaries. Eachgrain boundary denuded zone experienced changes different doses, indicating a direct observation of thedifference in grain boundary “immunity” to irradiation depending on character. Since a change in sink
efficiency is likely due to a change in point defect absorption at the boundary, this indicates thatsomething about the structure of these boundaries has changed. The grain boundary macroscopic
character remains the same, however, leading to the conclusion that a change in microscopic characterhas occurred, possibly due to the formation of extrinsic defects on the boundary. To analyze this further,simulations were used to explore grain boundary microstates loaded with defects (mimicking a collisioncascade) and assessed for vacancy formation energies. Overall, the results present a foundation forimproving sink efficiency descriptions under irradiation, and take a step forward in understanding
complex interfacial dependencies. Additionally, results will be shown for studies of grain boundary
structure and stability, including faceting, in the context of grain boundaries acting as “phases.” Theseadvances present the possibility that the extent to which a grain boundary is at “equilibrium” controls
many of its properties, and that its equilibrium is possible to be tuned.

526

MMM 2018

(I9 oral)
The Role of Grain Boundaries under Long-Time Radiation*Jing Luo1, Yichao Zhu1, Xu Guo1, Yang Xiang3, Stephen Jonathan Chapman2
1. Dalian University of Technology, 2. Mathematical Institute, University of Oxford, 3. Department of Mathematics, The
Hong Kong University of Science and Technology

Materials containing a high proportion of grain boundaries offer significant potential for the developmentof radiation-resistent structural materials. However, a proper understanding of the connection between
the radiation-induced microstructural grain boundary behaviour and its impact at long natural time scales
is still missing. To bridge this gap in time scales, a rigorously coarse-grained formulation describing thecoupled evolution of point defects and low-angle tilt grain boundaries is proposed. The derived
formulation captures well the radiation-induced climb behaviour of grain boundary dislocations, whichleads to asymmetry in grain shape evolution. It also reveals that the presence of point defect sources
within a grain further accelerates its shrinking process, and radiation tends to elongate the twin boundary
sections. Based on the proposed formulation, it is predicted for the first time that the minimum dimensionof a polycrystalline aggregate needed for shielding the point defect concentration to a prescribed level
scales with its grain boundary fraction at a sublinear rate.

MMM 2018527

(I9 invited)
Quantifying point defect fluxes to interfaces and the role of interface
structure*Shen J Dillon1
1. University of Illinois

Point defect fluxes to and from interfaces can dominate the response of materials exposed to irradiation,
their equilibration after thermal perturbation, and their sintering. However, little is known experimentally
about how non-equilibrium point defect concentrations equilibrate, how their concentration gradientsdevelop in the lattice, and the role of interface structure in affecting sink efficiency. This talk describes
experimental approaches to probing these phenomena using localized tracer diffusion measurements andirradiaiton induced creep in model sample geometries.

528

MMM 2018

(I10 invited)
Structure and Mobility of Dissociated Vacancies at Twist Grain
Boundaries and Screw Dislocations in Ionic Compounds*Blas Pedro Uberuaga1, Enrique Martinez1, Kedarnath Kolluri1, Xiang-Yang Liu1
1. Los Alamos National Laboratory

Interfaces, grain boundaries, and dislocations are known to have significant impact on the transport
properties within materials. Even so, it is still not clear how the structure of interfaces influence themobility and concentration of carriers that are responsible for transport. Using low angle twist grainboundaries in MgO as a model system for semi-coherent interfaces more generally, we examine thestructural and kinetic properties of vacancies. These boundaries are characterized by a network of screw
dislocations. Vacancies of both types, Mg and O, are strongly attracted to the dislocation network,
residing preferentially at the misfit dislocation intersections (MDIs). However, the vacancies can lower
their energy by splitting into two parts, which then repel each other along the dislocation line between
two MDIs because of electrostatic repulsion, further lowering their energy.

This dissociated structure has important consequences for transport, as the free energy of the dissociatedvacancies decreases with decreasing twist angle, leading to an increase in the net migration barrier for
diffusion as revealed by molecular dynamics simulations. Similar behavior is observed in BaO and NaCl,
highlighting the generality of the behavior. We analyze the structure of the dissociated vacancies as a pairof jogs on the dislocation and construct a model containing electrostatic and elastic contributions that
qualitatively describe the energetics of the dissociated vacancy. Finally, we examine the nature of thismechanism in other ionic compounds without a rocksalt structure. Our results represent the firstvalidation of a mechanism for vacancy dissociation on screw dislocations in ionic materials first discussed
by Thomson and Balluffi in 1962.

MMM 2018

529

(I10 invited)
Reverse engineering the kinetics of grain growth
Mingyan Wang1, Jules Dake1, Søren Schmidt2, *Carl Krill1
1. Institute of Micro and Nanomaterials, Ulm University, 2. Dept. of Physics, Technical University of Denmark

At first glance, the strategy for modeling grain growth seems obvious: simply combine an expression
governing the physics of grain boundary (GB) migration with a treatment of the connectivity of theboundary network, such that shrinkage of one grain corresponds to growth of its neighbor. However, 70
years of analytic models and three decades’ worth of computer simulations have left us with seemingly
intractable discrepancies between theory and experiment—concerning, for example, the shape of theself-similar grain size distribution; the occurrence of growth stagnation at long annealing times; and, moststrikingly, the emergence of a bimodal size distribution during instances of so-called “abnormal” grain
growth.
In the world of industry, when repeated attempts to solve a technological challenge end in failure,
employees sometimes try to “reverse engineer” a competitor’s product, dissecting the interplay
between the device’s internal parts to uncover its working principle. Afflicted with a similar sense ofdesperation, we applied this approach to the phenomenon of grain growth! Exploiting three-dimensional
x-ray diffraction (3DXRD) microscopy, we investigated thermally induced coarsening in two different Al
alloys. From microstructural snapshots interspersed between isothermal annealing steps, we were able to
track the morphology, misorientation and migration of thousands of GBs over time, acquiring the basis fora robust statistical analysis of local growth kinetics. The results allow extracting dependencies of reduced
mobility (the product of GB mobility and energy) on GB misorientation and inclination. In one specimen,
the measured dependency is consistent with expectations for normal grain growth, but, in the other case,
we find evidence for boundary kinetics that lie beyond the scope of standard models.

530

MMM 2018

(I11 invited)
Recent advances in the Full-Field modeling of microstructural
evolutions using a finite-element level set integrated framework.
*Daniel Pino Munoz1, Thomas Toulorge1, Charbel Moussa1, Nathalie Bozzolo1, Marc Bernacki1
1. Mines ParisTech / PSL Research University

Full field modeling of microstructure evolutions at polycrystalline scale is nowadays a very active field of
research. Within this context we have been working in tight collaboration with industrial partners in order
to be able to predict microstructural evolutions of metallic material due to recrystallization and grain
growth phenomena. This collaboration had lead us to the development of a numerical framework basedon the Level-Set method that allows to account fro complex microstructural evolution. These complex
phenomena includes grain growth, Smith-Zener pinning and, in particular, static, dynamic andpost-dynamic recrystallization. In this way the motion of the grain boundaries of microstructures issimulated accurately.
The general numerical framework will be presented along with recent developments that allowed us tosignificantly improve the performance of our approach. Our on-going developments aim at theenhancement of the existing capabilities of our numerical tools in order to account for high grainboundary energy and mobility. Different examples will the used to illustrate the robustness of thenumerical approach as well as its capabilities to accurately simulate microstructural evolutions. Finally,some current challenges for the forthcoming years will be discussed.

MMM 2018

531

(I11 invited)
The effect of strong anisotropic grain boundary energy and mobility on
microstructure formation and evolution: mesoscale modeling andsimulation*Brandon Runnels Runnels1, Josep Maria Gras Ribot1, Ian Chesser2
1. University of Colorado Colorado Springs, 2. Carnegie Mellon University

Nanocrystalline materials have exceptional mechanical properties. Because of their small microstructurallength scale, their macroscopic properties are dramatically influenced by grain boundaries (GBs). In thiswork we develop an algorithm for constructing the Allen-Cahn equation for grain boundary migration
including an orientation-dependent, nonconvex, anisotropic boundary energy. The energy minimizing
morphology for boundaries with nonconvex energy is faceted, but lacks a lengthscale, resulting inunstable solutions in phase field gradient flow. It is therefore necessary to include a second-orderregularization to penalize corners To compute the variational derivative of this complex free energy, we
simplify by transforming into the eigenbasis of the Hessian. This reduces the computation of principal
curvatures to second derivatives with respect to the second and third principal axes, which preservescomputational efficiency.To incorporate realistic boundary behavior, the lattice-matching model is used
to calculate boundary energy for arbitrary orientations, on-the-fly. Simulations are conducted for
microstructure evolution in a real-space implementation using adaptive mesh refinement. Finally, we
propose a continuum understanding of GB motion as a shear transformation goverened by compatibility.
Optimal boundary transformations are determined using a systematic process for shear identification, andby computing the minimum energy barrier for each. Such transformations typically require an atomic“shuffle” meaning that they atoms do not transform in the Cauchy-Born sense. The resulting optimaltransformation can then be incorporated at the mesoscale by modeling the elastic energy as a multiwellpotential. This provides a continuum context for understanding disconnections and faceted boundary
migration.

532

MMM 2018

(I11 invited)
A Machine Learning Exploration of Grain Boundary Mobility
Mechanisms
Leila Khalili1, Eric R Homer2, *Srikanth Patala1
1. North Carolina State University, 2. Brigham Young University

The mobility of grain boundaries plays an important role in governing the kinetics of microstructural
evolution in every class of polycrystalline materials. Of particular interest is the role of bicrystallography,
characterized by the macroscopic crystallographic degrees of freedom, on the underlying atomistic
mechanisms governing grain boundary mobility. In this talk, I will present an algorithm to automatically
identify such mechanisms that give rise to mobility of an interface. We use machine-learning methods,
inspired by recent work in disordered solids, to correlate local structure with the susceptibility forrearrangement of grain boundary atoms. We show that it is possible to automatically identify mobility
mechanisms of grain boundaries with a diverse range of crystallographic character.

MMM 2018

533

(I12 invited)
Formation and shrinkage of grain boundary loops in two-dimensional
colloidal crystals
Francois Lavergne1, Arran Curran1, Dirk Aarts1, *Roel Dullens1
1. University of Oxford

Understanding the dynamics of grain boundaries in polycrystalline metals and alloys is crucial to enabletuning their mechanical properties. From an experimental point of view, grain boundaries in colloidal
crystals are convenient model systems since imaging their dynamics requires only simple optical
microscopy and they can be manipulated using optical tweezers. The formation and kinetics of grainboundaries are closely related to the topological constraints imposed on their complex dislocation
structure. As such, loop-shaped grain boundaries are unique structures to establish such a link because
their overall topological “charge” is zero due to their null net Burgers vector.
Here, we study the formation and shrinkage of such grain boundary loops by creating them on demand via
a local rotational deformation of a two-dimensional colloidal crystal using an optical vortex. In particular,
we observe that a grain boundary loop only forms if the product of its radius and misorientation exceeds a
critical value. In this case, the deformation is plastic and the grain boundary loop spontaneously shrinks at
a rate that solely depends on this product while otherwise, the deformation is elastically restored. We
show that this elastic-to-plastic crossover is a direct consequence of the unique dislocation structure ofgrain boundary loops. Our results thus reveal a new general limit on the formation of grain boundary loops
in two-dimensional crystals and elucidate the central role of defects in both the onset of plasticity and the
kinetics of grain boundaries.

534

MMM 2018

(I12 oral)
Growth and characterization of two-dimensional poly(quasi)crystals*Petri Hirvonen1, Gabriel Martine La Boissonière2, Zheyong Fan1, Cristian Achim3, Nikolas
Provatas4, Ken R. Elder5, Tapio Ala-Nissila1,6
1. Dept. of Applied Physics, Aalto Univ., 2. Dept. of Mathematics and Statistics, McGill Univ., 3. Dept. of Chemical
Engineering, Univ. of Concepción, 4. Dept. of Physics, McGill Univ., 5. Dept. of Physics, Oakland Univ., 6. Dept. ofMathematical Sciences, Loughborough Univ.

We use a simple two-mode phase field crystal (PFC) model to simulate grain growth in realistic
two-dimensional model systems of square and hexagonal, as well as of 10- and 12-fold symmetricquasicrystal lattice symmetries. Modeling the evolution of poly(quasi)crystals had remained a challenge
until the arrival of PFC, giving access to long diffusive time scales. We characterize the model systems
using a powerful new method for segmenting and analyzing grain structures. This method generalizes
effortlessly to all lattice types of even-fold rotational symmetry. To our knowledge, our characterization
method is the first of its kind so far for quasicrystals.
The grain structure segmentations produced by the characterization method are found to agree very wellwith the corresponding human-made segmentations. We observe power-law scaling of the average grain
size as a function of the simulation time, and log-normal grain size distributions for all lattice types
considered. Similarly, irrespective of the lattice type, grain misorientation distributions appear remarkably
flat, indicative of little correlation in the lattice orientations between neighboring grains. Average numberof neighboring grains is between 5 and 6. We are currently working on improving our statistics to cut
down the error margins and to verify or to refute the universality of these, and of further related,distributions.

MMM 2018535

(I12 oral)
A Parallel Algorithm for High Resolution 3D Phase Field Simulations of
Polycrystalline Solidification*Pavel Strachota1, Aleš Wodecki1
1. Czech Technical University in Prague

We deal with numerical solution of a three-dimensional phase field model of solidification in single
component anisotropic materials. In this contribution, we extend the model by crystal orientationtransformation. A robust algorithm is then developed to simulate the growth of multiple grains with anarbitrary number of random crystallographic orientations and a fully resolved 3D dendritic geometry. In
the first part, the model and the hybrid parallel implementation of the algorithms are explained. The
second part is devoted to demonstrating the effect of mesh-related numerical anisotropy, investigation of
the parallel efficiency, and the simulations of complex polycrystalline solidification on very fine meshes.

536

MMM 2018

(I12 oral)
Properties of β/ω phase interfaces in Ti and their implications onmechanical properties and ωmorphology*Shuo Cao1, Qing-Miao Hu1, Rui Yang1, Yong Jiang2
1. Institute of Metal Research, Chinese Academy of Science , 2. Central South University

The ωphase strengthens β-Ti alloys effectively as a precipitate phase but decreases the plasticity withincreasing size dramatically. To understand the strengthening and embrittlement mechanism of ωphasein the β-Ti alloys, the property of the β/ω interface is demanded. In this regard, we calculated theinterface energies (γint ), cleavage energies (γcl ), and generalized stacking faults energy surfaces of the β/ω interfaces with different orientations using a first-principles method based on density functional
theory. The results indicate that the strong anisotropy of the interface energy explains the ellipsoidmorphology of the ωphase with its major axis parallel to the <111>β direction. The calculated cleavageenergies and unstable stacking fault energies γus are determined. By comparing the γcl/γus ratio of bothinterfaces and those of the bulk ωand βphases, we conclude that the interfaces are prone to crack
initiation than the bulk and contribute to the ω-embrittlement of the β-Ti alloy.

MMM 2018

537

(P1-58)
Phase-field modeling of anisotropic grain growth with incorporation of
Sigma 3 CSL grain boundaries.
*Kunok Chang1
1. Kyung Hee Univ.

Understanding of grain growth is one of the major on-going challenges of materials science. Since grain
growth is complex multi-physics phenomena, it is extremely difficult to describe grain growthphenomenon analytically with the completed theory. Therefore, a number of researchers have tried toinvestigate grain growth using computational techniques. Among them, the phase-field method has beenused one of the efficient and strong tools. So far, 3D grain growth with isotropic grain boundary energy
has been intensively studied and their microstructural characteristics have been quantitatively analyzed.
On the other hand, since anisotropic 3D grain growth is way more complicated than isotropic
phenomena, only a few attemps have been made with simplified assumptions. In this study, we performed
anisotropic 3D grain growth with consideration of Sigma 3 CSL grain boundary. To perform the modelingwithin realistic time window, we implemented OpenMP parallelized technique. We analyzed how degreeof anisotropy in grain boundary energy and fraction of CSL boundaries affects growth kinetics andevolution of microstructural characteristics.

538

MMM 2018

(P1-59)
Direct mapping from molecular dynamics to phase-field simulations
for accurate prediction of grain growth*Eisuke Miyoshi1, Tomohiro Takaki1, Yasushi Shibuta2, Munekazu Ohno3
1. Kyoto Institute of Technology, 2. The University of Tokyo, 3. Hokkaido University

For numerically predicting grain growth, two different approaches have been used: atomistic simulations
typified by molecular dynamics (MD); and continuum-based treatments including the Monte-Carlo,
phase-field, and vertex methods. The former can model the spontaneous nucleation process thatprecedes grain growth. However, due to the large computational cost, it is difficult to simulate grain
growth until the late stage using only atomistic methods. On the other hand, continuum-based models
allow for relatively efficient computations. In particular, the multi-phase-field (MPF) model [I. Steinbach
and F. Pezzolla, Physica D, 134 (1999) 385], which is an extension of the phase-field model topolycrystalline systems, is widely employed in recent years as a prominent tool for simulating grain growthwith accuracy and efficiency. Nevertheless, the MPF model cannot directly reproduce the nucleation
phenomenon. Considering the strong dependence of grain growth behaviors on the initial structure, there
is a pressing need for a means of providing realistic initial structures for MPF simulation.
In this study, we aim to achieve more accurate and efficient prediction of grain growth by exploiting themerits of atomistic and continuum simulations. To this end, we propose a method to convert
MD-generated atomic configurations into the MPF interfacial profiles; this enables us to perform MPF
grain growth simulations in succession to MD nucleation simulation. Furthermore, using the proposed
method, MPF and MD grain growth simulations from the same initial structure are directly compared, via
which the difference between each simulation result is quantified. Through the detailed investigation of
the causes of the difference, a way to improve the accuracy of the MPF model is discussed.

MMM 2018

539

(P1-60)
Density functional theory plus Hubbard U study of the segregation of
Pt to the CeO2-x grain boundary
Guoli Zhou1, Pan Li1, Qingmin Ma1, *Zhixue Tian1, Ying Liu1
1. Hebei Normal Univ.

Grain boundaries (GBs) can be used as traps for solute atoms and defects, and the interaction between
segregants and GBs is crucial for understanding the properties of nanocrystalline materials. In this study,
we have systematically investigated the Pt segregation and Pt-oxygen vacancies interaction at the sigma 3(111) GB in ceria (CeO2). The Pt atom has a stronger tendency to segregate to the sigma3 (111) GB
thanto the (111) and (110) free surfaces, but the tendency is weaker than to (112) and (100). Lattice
distortion plays a dominant role in Pt segregation. At the Pt-segregated-GB (Pt@GB), oxygen vacancies
prefer to form spontaneously near Pt in the GB region. However, at the pristine GB, oxygen vacancies canonly form under O-poor conditions. Thus, Pt segregation to the GB promotes the formation of oxygen
vacancies, and their strong interactions enhance the interfacial cohesion. We propose that GBs fabricatedclose to the surfaces of nanocrystalline ceria can trap Pt from inside the grains or other types of surface,resulting in the suppression of the accumulation of Pt on the surface under redox reactions, especially
under O-poor conditions

540

MMM 2018

(P1-61)Interfacial charge transfer and enhanced photocatalytic mechanism
for Bi2WO6/BiOCl heterostructure: A first-principles theoretical study*Pan Li1
1. Hebei Normal Univ.

First-principles calculations based on density functional theory are used to explore the interfacial
structure and properties of the Bi2WO6/BiOCl heterojunction aiming at gaining insights into thephotocatalytic mechanism of the Bi2WO6/BiOCl heterojunction. Bi2WO6/BiOCl interface has a goodlattice match, with the interface formation energy is -4.67eV. The calculated band alignment between the
Bi2WO6 and BiOCl reveals that the valence band offset and conduction band offset between BiOCl and Bi2
WO6 are 0.37 eV and 1.02 eV, respectively. The calculated Mulliken charge population and electron
difference density maps reveal that there is a self-induced internal electric field along the perpendicular
direction to the layers in the BiOCl and Bi2WO6. Based on the obtained work function and band edge
positions of BiOCl and Bi2WO6, the formation mechanism of the internal electric field at the interface of Bi
2WO6/BiOCl heterostructure is studied. The existence of band offsets and the internal electric field canfacilitate the separation of the photo-generated electron-hole pairs, resulting in the enhanced
photocatalytic activities of the heterostructures.

MMM 2018

541

(P1-62)
Investigation of abnormal grain growth conditions by phase-field
method*Nobuko Mori1, Eisuke Miyoshi1, Tomohiro Takaki1
1. Kyoto Institute of Technology

To improve the properties of materials by controlling their textures and grain sizes, it is essential toaccurately predict abnormal grain growth, a phenomenon through which a few grains undergo
preferential growth. In addition, the abnormal grain growth is considered to be one of the origins for the
nucleation of recrystallized grains and, thus, is of great importance for modelling recrystallization
processes.
As a prominent theory of abnormal grain growth, that proposed by Humphreys (here referred to as thecellular microstructural stability (CMS) theory) is well known. In this theory, a complicated polycrystalline
microstructure, where grain size, boundary energy, and boundary mobility are not uniform, is simplified asa cellular microstructure model. The model consists of two ingredients: a specific grain and its
surrounding matrix with uniform grain size and boundary properties. This modelling makes it possible todescribe the abnormal grain growth behavior of the specific grain using only three parameters, i.e., its
relative size, boundary energy, and boundary mobility normalized by those of the matrix. However, thevalidity of this theory has not been confirmed yet neither in experiment and simulation.
In this study, we aim to evaluate the applicable range of the CMS theory via systematic two-dimensional
numerical simulations. As the numerical model, the multi-phase-field model [2] is employed, enabling
accurate treatment of curvature-driven grain boundary migration in polycrystalline systems. Grain growthsimulations are performed while varying the size, boundary energy, and boundary mobility of a specificgrain in the matrix. By comparing the simulated results with the prediction of the CMS theory, we reveal
the validity and applicable range of the theory.
[1] F. J. Humphreys, Acta Mater., 45, (1997), 4231.
[2] I. Steinbach, F. Pezzolla, Physica D, 134, (1999), 385.

542

MMM 2018

(P1-63)
Phase Field Crystal Modeling of Mechanism of Strain-Driven for
Nucleation and Grain of Deformed-Grain*Ying-Jun Gao1
1. Guangxi University

The phase-field-crystal (PFC) method is used to investigate dynamic strain-induced a splitting of grainboundary (GB) to generate a deformed grain with high strain energy and to drive it to grow in two
dimensions. The simulated results show that the essence of the splitting process of the original GB is thatnew deformed grain is nucleated, and results in formation of high-density ensembles of mobile lattice
dislocation that is capable of plastic flow localization (deformed banding). The GB migration is theprocess of the new deformed grain with higher strain energy consuming the original grain to extend. The
deformed grain stores the strain energy through climbing of the dislocation, as well as changing theorientation of the original grain. The deformed grain growth (DGG) is the acceleration process of the
speed, and its area extension is proportional to the time square. The rule of the time square of the DGG
can also be deduced by establishing the dynamic equation of the strain-driven for the dislocation and thedeformed grain extension.

MMM 2018

543

(P1-64)
Diffusion and trapping of hydrogen at grain boundaries scale in fcc
polycrystalline nickel: some implications of the atomic volume and theinterstitial self-stress*xavier Feaugas1, jiaqi Li1, abdel malek Hallil1, abdelali oudriss1, arnaud metsue1, jamaa
bouhattate1
1. university of La Rochelle

The mobility of hydrogen in metals is a key parameter for understanding the basic mechanisms ofhydrogen embrittlement (HE). This problem is directly related to the mechanisms of diffusion and trappingof hydrogen within a specific metallurgical state. These mechanisms depend on the various
microstructural heterogeneities and in particular the grain boundaries (GBs), triple junction (TJs) andseveral defects (dislocations, vacancies …). Although a number of theories have been proposed todescribe the role of GBs for hydrogen diffusion and segregation, none of them is able to give an exact
answer. In present work we report our recent works, which support the investigation of diffusion andtrapping of hydrogen in two elementary systems: nickel single crystals and bi-crystals to highlight the
impact of GB on hydrogen mobility in polycrystalline fcc metals. We developed a methodology combining
experimental tools (electrochemical permeation / TDS, HRTEM, EBSD) and numerical methods(FEM-COMSOL / EAM-LAMMPS). We propose to screen several bi-crystals of pure nickel with different
grain boundaries (Σ11-50°30<110>{311}, Σ11-129°30<110>{332}, Σ3-70°30<110>{111} and Σ5-37°<100>{310}).
The results allow us to associate the short-circuit diffusion and trapping phenomena to the grainboundaries and defect characters (excess volume, defects density and distribution …). In each situation,
we highlight the importance of the self-stress on the processes of diffusion and segregation. The
segregation energy of hydrogen depends on the nature of the site (the local free volume and the elastic
energy associated with the incorporation of solute). The diffusion of hydrogen is directly influenced by thenature of the grain boundary (the free volume and the distribution of the segregation sites). Our results, atthe atomic scale, show a correlation between the solubility and the free volume of the grain boundary.
The grain boundaries with a higher free volume have more favorable diffusion paths for hydrogen than in
the crystal lattice and at the same time more segregation sites.

544

MMM 2018

(P2-69)
Topological evolution of the microstructures of thin films during grain
growth*Ahu Oencue1, Thorsten Halle2, Dana Zoellner3
1. Institute of Experimental Physics, Otto-von-Guericke University Magdeburg, 2. Institute of Materials and Joining
Technology, Otto-von-Guericke University Magdeburg, 3. B CUBE Center for Molecular Bioengineering, TU Dresden

Historically, metallography has been the two-dimensional characterization of materials microstructures by
optical microscopy. Consequential problems have long been known: A two-dimensional section through athree-dimensional object gives us only a very poor idea about size and form of the object. The same holds
for the complex grain boundary networks of various kinds of polycrystalline materials. Therefore, many
attempts have been made to gain three-dimensional information experimentally. Nevertheless, in
simulations and analytical theories thin films are commonly still treated as two-dimensional objects
making comparisons with three-dimensional experimental data rather hard.
In the present work, based on experimental measurements, grain growth in metallic thin films isinvestigated in detail by three-dimensional Monte Carlo Potts model simulations focusing particularly on
the transition from bulk-like growth to columnar microstructures. Changes not only in average growth
behaviour from a linear increase of the average grain area with annealing time to near-stagnation, butparticularly temporal changes in local topology and individual growth kinetics, e.g., in terms of the
Lewis-law as well as of the von Neumann-Mullins-law are discussed.

MMM 2018

545

(P2-70)
Molecular dynamics simulations of phase transformations in
nanocrystalline NiTi shape-memory alloys*Won-Seok Ko1
1. University of Ulsan, 93 Daehak-ro, Nam-gu, Ulsan, 44610

Molecular dynamics simulations are performed to investigate temperature- and stress-induced phasetransformations in nanocrystalline nickel-titanium shape-memory alloys. Our results provide detailed
insights into the origins of the experimentally reported characteristics of phase transformations at thenanoscale, such as the decrease of the transformation temperature with grain size and the disappearanceof the plateau in the stress-strain response. The relevant atomic scale processes, such as nucleation,
growth, and twinning are analyzed and explained. We suggest that a single, unified
mechanism--dominated by the contribution of a local transformation strain--explains the characteristics ofboth temperature- and stress-induced phase transformations in nanocrystalline nickel-titanium.

546

MMM 2018

(P2-71)
An attempt to connect migration of grain boundaries to their atomic
structures with help of Molecular Dynamics
Zakaria El Omari1, *Sylvain Queyreau1, Charlie Kahloun1, Brigitte Bacroix1
1. Universite Paris XIII, Sorbonne Paris Cite, Laboratoire des Sciences des Procedes et des Materiaux, LSPM UPR
3407

Grain Boundary (GB) migration is at the origin of microstructure evolutions in a large variety of crystalline
materials. One of the many challenges to fully understand GB motion has to do with the GB vast panel ofatomic configurations and existence of several motive forces or impacting parameters. In this context,
atomistic simulations have proven to be particularly useful since GB migration can be investigated underwell defined conditions, and large scale systematic investigations are now possible (i.g. [1]).
In this work we report a Molecular Dynamics investigation of the migration of a large panel of CSL GB infcc Ni. In order to construct lesser known GB with mixed tilt+twist or asymmetric character, we orient andconstrain the simulation domain to the CSL lattice defined by the two crystal orientations, in a fashion
very similar to the approach proposed in [2]. GB motion may be initiated by a synthetic driving force as
defined in [3]. As a result, very different temperature behaviour are observed ranging from athermal, tothermally activated and non monotonous thermal behaviour. Different behaviours are sometimes
observed for very similar GBs, confirming the important of the GB atomic structure over the macroscopic
geometrical parameters describing GB.
In an attempt to rationalise these results we developed an automated post-processing of the atomic
configurations into a discrete modelling of GB in terms of intrinsic dislocations and disconnections whenpresent. This analysing tool is applied to a dozen of simple GB and a correlation is made when possible
between elementary migration mechanisms -atomic shuffling or disconnection motion- and discrete
structure of GB.
[1] DL Olmsted, EA Holm, SM Foiles, Acta Mater. 57, 3704 (2009).
[2] AD Banadaki, S Patala. J. Appl. Cryst. 48, 585 (2015).
[3] K.G.F. Janssens, D. Olmsted, E.A. Holm, S.M. Foiles, S.J. Plimpton, P.M. Derlet, Nat. Mater. 5, 124(2006).

MMM 2018

547

(P2-72)
Disconnection interaction in Cu grain boundaries*Christian Brandl1
1. Karlsruhe Institute of Technology

Recent molecular dynamics (MD) simulations and transmission electron microscopy indicate that thegrain boundary (GB) migration in asymmetric GB plane orientation is mediated by the nucleation andmigration of disconnections in the GB plane. The collective motion and reaction of disconnections also
initiates the formation of facets as the agglomeration of disconnections into a disconnection arrays.
In MD simulations we address the interaction of disconnection and disconnection dipoles at zero stress in
S3 and S7 GBs. The diffusive rearrangement at finite temperate is analyzed in terms of one-dimensionalrandom walks and the drift signatures are used to deduce the interaction strength and the disconnection
core interaction. The implications of the disconnection-interaction on the collective migration ofasymmetrical GBs is discussed in context of grain coarsening in fcc metals at elevated temperatures andthe transition to stress-driven grain coarsening in nanocrystalline metals.

548

MMM 2018

(P2-73)
Phase-field Approach to Thermo-mechanical Behavior ofThrough-silicon Vias
Wooju Lee1, *jaekeun yoon1, Dongchoul Kim1
1. Sogang Univ.

Three-dimensional stacking of silicon chips via Through-silicon vias (TSVs) is an innovative technique forelectronic devices due to the drastically shortened electrical path which leads to the faster operation.
When TSVs are exposed to high temperature, the higher coefficient of thermal expansion (CTE) of Cu
generate the out-of-plane extrusion of Cu from TSV, so-called Cu pumping, which may damage the above
lying silicon chip. Furthermore, the thermal expansion is irreversible because the Cu grains are coarsened
during the annealing. The comprehensive understanding of Cu pumping mechanism according to thegeometry of TSV and annealing conditions is indispensably required to ensure the reliability of electronic
devices. A finite element analysis has been used to predict the Cu pumping. However, the finite element
method does not incorporate the grain coarsening mechanisms that reduce the elastic energy generated
by the thermal expansion. Here, we propose a phase field model that is modified to consider the thermal
expansion of Cu polycrystalline during annealing process. The phase field model is the most suitable
method to model the evolution of microstructures, since it has benefits for incorporating multiple
mechanisms simultaneously. In this study, the significant mechanisms of Cu pumping, including grainboundary migration, thermal expansion, and interfacial characteristics, are considered into the freeenergy functional of the phase field model

MMM 2018

549

(P2-74)
Grain-Growth in Nanocrystalline Metals under Ion Irradiation: A
Thermal Spike Model*Djamel Kaoumi1, Robert Birtcher2, Arthur Motta3
1. North Carolina State University, 2. Argonne National Laboratory, 3. Penn State University

Grain growth was observed in nanocrystalline metallic foils in situ in a transmission electron microscopein a wide range of irradiation doses, temperature (from 20K to 773K) for four different pure metals (Zr, Pt,
Cu and Au). The average grain size increased monotonically with ion fluence and similarly to thermal grain
growth, the ion-irradiation induced grain growth curves could be best fitted with curves of the type:. With
respect to temperature, the experimental results showed the existence of a low-temperature regime(below about 0.15–0.22Tm), where grain growth is independent of the irradiation temperature, and athermally assisted regime where grain growth is enhanced with increasing irradiation temperature. A
model is proposed to describe grain growth under irradiation in the temperature-independent regime,
based on the direct impact of the thermal spikes on grain boundaries. In the model, grain-boundary
migration occurs by atomic jumps, within the thermal spikes, biased by the local grain-boundary curvaturedriving. The experimental results will be presented as well as the model proposed to describe
grain-growth kinetics in the low-temperature regime (cryogenic temperatures).

550

MMM 2018

Symposium J(J1 invited)
A multiscale failure analysis for layered composites with statisticalaccount of manufacturing defects*Ramesh Talreja1
1. Texas A&M University

As composite materials have broadened in their applications from aerospace to automotive and energy
industries, their cost-effectiveness has become critical. Since in most cases, a major cost lies in the
manufacturing process, the effects of defects on performance has gained renewed attention. Traditional
failure analysis has either not considered effects explicitly or has accounted by these in artificial ways. The
work to be presented develops a statistical simulation of manufacturing defects such as nonuniform fiber
distribution within the matrix by quantifying deviations from uniformity based on fiber mobility during the
manufacturing process. Algorithms are implemented to generate representation of the “real”
microstructure with quantified anomalies. The representative volume element realizations are subjected
to boundary conditions under which the local stress fields are computed. A hierarchical energy based set
of failure criteria are used to determine occurrence of the sequence of failure events and theirprogression. By parametric studies, the effects of defects as well as constituent properties on failure
behavior under different loading conditions is clarified.

MMM 2018

551

(J1 invited)
Multiscale Modeling of Fiber Reinforced Materials for Future
Aerospace Structures*Anthony M Waas1
1. U. Washington, Seattle, WA 98195; also Univ. of Michigan, Aerospace Engineering, Ann Arbor, MI 48109

The current practice of designing composite aerospace structures relies on extensive testing, coupled to abottom-up, pyramidal building block approach, to ensure structural integrity and damage tolerance.
Reducing the number of tests can lead to a substantial decrease in total design cost of many vehicles.
Cost reduction is enabled by developing high fidelity computational models which can provide valuable
information regarding the performance of a structure up to and including failure. Composites, because of
their heterogeneity, display a rich variety of damage (dissipation) and failure (two piece) mechanisms
starting at the atomistic scale and progressing up in length scale. An acute understanding of the physics
and mechanics of these mechanisms, at different scales, is essential in order to develop physically soundtheories and attendant computational methods for predicting the structural performance of a composite
structure. With a view towards addressing future, robotically manufactured polymer matrix composite
structures for lightweight vehicle applications, this talk will address a multiscale computational modelingframework that the author has developed over the past decade to model and predict the structural
performance of polymer matrix composites. Nonlinear material response, including damage and failure, isincorporated in conjunction with geometric nonlinearity to predict damage evolution and failure that isobserved in the laboratory for a variety of examples. Issues related to mesh objectivity will be addressed
and the importance of this aspect in numerical predictions will be highlighted.

552

MMM 2018

(J1 invited)
A-DiSC (Adaptive Discrete-Smeared Crack) Model for Multi-Scale
Progressive Damage Analysis of Composite Structures*Tong-Earn Tay1, Xin Lu1, Vincent Beng-Chye Tan1
1. National University of Singapore

Laminated fibre-reinforced composite structures often fail in a complex progressive damage process
involving interaction of mulitple failure mechanisms, including matrix cracking, interfacial delamination,
local micro-buckling and fibre rupture. Modeling these multi-scale mechanisms in a single simulation with
sufficient fidelity to the physics while retaining computational efficiency still presents a formidable
challenge. Discrete crack models (DCMs)1-3 offer excellent fidelity, especially for modeling multiple crack
interactions, but are impractical and inefficient for structures larger than coupon sizes. Recently, an
adaptive DCM has been proposed to improve computational efficiency4. On the other hand, smeared
crack models (SCMs)5,6 are suitable for describing diffuse damage such as micro-cracks but unable toproperly account for local coupling effects and final fracture. In this presentation, an adaptive
discrete-smeared crack model (A-DiSC) is proposed, whereby the DCM is initially used to model matrix
cracks and their interaction with delaminations via cohesive interface elements, but non-critical cracks arelater converted to diffused damage using SCM. The transitional region from DCM to SCM is effected
through satisfaction of certain criterion based on the principle of energy conservation3. The proposed
A-DiSC model is applied to the tensile failure analysis of open-hole composite laminates.

MMM 2018

553

(J1 oral)
Analysis for the Plane Problem of Layered Magnetoelectric Composite
with Collinear Interfacial Cracks*Wenxiang Tian1, Zheng Zhong1,2
1. School of Aerospace Engineering and Applied Mechanics,Tongji Univ, 2. School of Science, Harbin Institute ofTechnology

A fracture problem of collinear interfacial cracks in layered magnetoelectric composites is studied. The
magnetic permittivity of the piezoelectric material and the dielectric constant of the piezomagnetic
material are considered. The poling axis of the piezoelectric and piezomagnetic material are all assumed
to be perpendicular to the interfacial cracks and an in-plane magnetic potential difference or voltage isapplied along the poling axis, which results in in-plane deformation of the composite. Thus, a planefracture problem is studied in the present paper. A system of singular integral equation of the second kindwith Cauchy kernel is obtained by means of Fourier transform and further solved by using Jacobi
polynomials. The primary interfacial fracture mechanics parameters, such as the stress intensity factors,
the electric displacement intensity factors, the magnetic inductions intensity factors and the energy
release rates are then obtained. Analysis reveals that the material combination and the thickness ratio of
piezomagnetic/ piezoelectric layer has significant influences on the stress, the electric displacement andthe magnetic inductions on the interface versus the thickness of the active response layer. In terms of thedefined dimensionless parameters Zsigma, ZB, ZD, which represent the interface material mismatch underin-plane deformation, three different cases can be categorized for each parameter, i.e. Zsigma>1,Zsigma<1
and Zsigma=1; ZB>1, ZB<1 and ZB=1; ZD>1, ZD<1 and ZD=1; where the stress curves, magnetic inducions
curves and electric displacement curves display different monotony, respectively. Based on the three
dimensionless parameters, we can define another four dimensionless parameters CKI,CKII,CKD,CKB, which
represent the cracked interface material mismatch under in-plane deformation, different cases can be
categorized for different parameters, where the corresponding intensity factor curves display different
monotony.

554

MMM 2018

(J2 invited)
Simulation-based Design of Bioinspired Impact-resistantNanocellulose Films with Bouligand Microstructure*Sinan Keten1
1. Northwestern University

The Bouligand structure, which features a helicoidal layup of in-plane uniaxial arranged fibers, has been
widely observed in biomaterials with superior impact-resistant properties. However, the underlying
mechanisms for the damage tolerance remain poorly understood, posing great challenges for the designand development of bio-inspired materials with optimized microstructures. Interestingly, manybio-sourced nanomaterials such as cellulose nanocrystals (CNCs) achieve helicoidal ordering throughself-assembly. As a step towards mimicking impact tolerant biomaterials, here we present systematicallycoarse-grained molecular dynamics simulations of nanoscale projectiles impacting CNC films with
Bouligand structure as a model system. We report the specific ballistic limit velocity and energy
absorption as metrics to quantify impact performance of CNC films. We discover that Bouligand structureswith low pitch angles (10-30 degrees) show optimal ballistic resistant performance, and significantly
outperform quasi-isotropic baseline structures. Intriguingly, decreasing interfacial interactions helps
enhance the performance under impact through allowing dissipative inter-fibril and interlayer sliding
events to occur more readily without severe fibril fragments. We show that improved interfacial sliding,enhanced wave propagation, larger in-plane crack opening and through-thickness crack twisting
contribute to the improved energy dissipation during projectile penetration for CNC Bouligand films with
optimal pitch angles. This study reveals structural and chemical factors that govern the optimal design ofBouligand structures made from 1D nanomaterials for protective applications. Concluding remarks will
include comparative analyses on other thin film materials, including layered graphitic and metallic
nanostructures through scaling and theoretical arguments, as well as potential strategies for mechanicalproperty improvement through better nanocellulose interface design.

MMM 2018

555

(J2 invited)
Amelogenesis: Nature’s 3D printing system for multi-scale laminates*brian n cox1
1. arachne consulting

Ameloblasts are pencil-shaped cells with writing (secretory) tips at one end, which mass together in 2D
sheets, standing on their writing ends. As they write (secrete protein scaffolds), they slide past one
another and/or swap places, so that the secretions of the whole cell population form complex patterns in
3D space as the secreted material accumulates (amelogenesis is additive manufacturing). The secreted
protein is the spatial template for the chemical reactions that result in mineralized dental enamel. By
forming patterned microstructures, the cells impart exceptional fracture properties to the enamel.
How does each ameloblast know where and how fast to move at any instant during the writing process, sothat it will contribute correctly to the desired pattern? There is no analogue of an external computerissuing instructions to all cells (the human engineering approach to 3D printing). Instead, we postulatethat cells acquire the timing and positional information they need by sensing their evolving local strain
environment: as the population moves, global shape changes map onto local strains around individual
cells, and vice versa. Analyzing the mapping from local to global is a complex 3D problem in nonlinear
wavelike cell motions. Its solution leads to deduction of cell “response functions” (rules that state cellactions for a given strain state) that will lead, when assigned to all cells, to correct patterns.
Having discovered the right response functions, we can account for much of the movement ofpattern-forming ameloblasts in, e.g., the mouse incisor, including: the complex 3D trajectory of each cell
during amelogenesis as layered structures are formed; switching on and off of secretion; the matching of
the rate of enamel manufacture to the rate of eruption of the mouse incisor; and the appearance andspatial wavelength of “cohorts” of ameloblasts, which are an instance of spontaneous segmentation of ahomogeneous population into a periodic structure.

556

MMM 2018

(J2 oral)
Role of geometrical features on mechanical properties in bio-inspired
staggered composites
Aman Mahar1, *Siladitya Pal1
1. Indian Institute of Technology Roorkee

Nacre (mother of pearl), bone, and teeth boast a prominent amalgamation of mechanical facets such as
strength, toughness and stiffness. Merger of these properties in engineering materials is still a challenge.
Nacre, which is an inner shell layer of many sea-shells, primarily made of 95% of brittle mineral (calcium
carbonate), however; it exhibits superior mechanical properties as compared to their major constituents.
Micro-structure of nacreous layer reveals the staggered 'brick-bridge-mortar' architecture in which
polygonal shaped mineral tablets are stacked and organic layer and mineral bridges act as a glue andinterconnection between platelets, respectively. Mimicking of the architecture of these biologicalmaterials has captured the attention of research community for development of high performancecomposites. However, an in-depth knowledge of structure-properties-function relationship is still lacking.
Therefore, the goal of the present study is to provide a detailed understanding on the role of geometricparameters towards the origin of extraordinary mechanical properties.

In the present study, we will develop a predictive modeling framework to understand the influence ofstaggered architectured composite on emergent mechanical behaviors under quasi-static loading
condition. Therefore, a finite element framework will be adopted and implemented in parallel computing
enviroment. The state-of-art cohesive zone models will be incorporated to account the failure ofinter-layers and mineral bridges, separately. Using the model, we will provide systematic parametric studyof geometrical features such as overlap ratio, aspect ratio, bridge density and bridge distribution on
emergent strength, stiffness and toughness. Finally, detail design map will be constructed that can assist
to develop novel architectured composites with tailored mechanical properties.

MMM 2018

557

(J2 oral)
Dynamic homogenization for acoustic metamaterials*Celia Reina1, Chenchen Liu1
1. University of Pennsylvania

A variational coarse-graining framework for heterogeneous media in the spirit of FE^2 methods is
proposed, which allows for a seamless transition from the traditional static scenario to dynamic loading
conditions, while being applicable to general material behavior. The method automatically delivers thecoarse-grained macroscopic equations of motion together with a computational multiscale solver.
Newmark method is employed for the time discretization, and excellent accuracy, stability, andcomputational time saving have been certified through the comparison with the standard single scale
finite element solver. In addition, the capability of the multiscale solver to capture wave scattering has
been demonstrated by the simulations in locally resonant sonic materials.

558

MMM 2018

(J2 oral)
Cracking behavior of ferrite-pearlite pipeline steel with hierarchicaland concurrent multiscale modeling schemes*Taolong Xu1,2, Wei Wang1, Xiaojiao Deng1, Anlin Yao1, Jinghong Fan3,2
1. Southwest Petroleum University of China, 2. International Institute of Material Multiscale Modeling, 3. Alfred
University

Attempts of two schemes with atomistic-based multiscale simulations for lamellar ferrite-pearlite pipeline
steel are evoked in this work. They are hierarchical multiscale method by atomistic-based Cohesive Zone
Model (CZM) and concurrent multiscale method by extended Generalized Particle Dynamics Method(XGP), where XGP proposed recently by author and coauthors (Fan, Eng. Fract. Mech., 2017) is anextension of the GP method (Fan, Multiscale Model. Simul., 2009) in which finite element (FE) nodes areconnected with the outermost particles, thus encouraging accurate crack-tip parameters are obtained
due to reduce artificial effects on the atom-node boundary. The former one bridging crack propagation at
the atomistic and mesoscopic scale by local energy release rate Gc extracted from ferrite-cementite grainboundary, where Gc characterized by exponential and trilinear traction-separation law (TSL) complied into
VUMAT in finite element software, using the unique cohesive element length (Xu, Eng. Fract. Mech.,2016), tension specimen FE models are designed to reveal the accuracy of the intrinsic correlation
between the atomistic and mesoscopic scale under a stress intensity factor, and will be verified by thefracture test result, which proves the practicability of concurrent multiscale method to solve the cracking
behavior of engineering material. The latter one investigates the lamellar ferrite-cementite micro-structurewith extra size compared to conventional multiscale method such as the quasicontinuum (QC) method,for crack-tip behavior, the GP method provides the possible way to qualify the layer thickness characterizeof ferrite, as well as the model size effect via XGP multiscale analysis under one-order larger than GP
model, XGP method develops a sufficiently large model bases on proven accuracy may open a new
avenue to study the cracking behavior in lamellar structure, also explore a new types of multiscale
methods which can improve the accuracy in bridging atomistic and continuum scales. However, eachsimulation scheme, Bagaryatskii orientation relationship between ferrite and cementite within pearlite isdeveloped by Voronoi geometric method, and then the crystal boundary of lowest energy state is
obtained by using conjugate gradient method and annealing under NPT ensemble.

MMM 2018

559

(J3 invited)
Fundamentals of Generalized Particle (GP) Multiscale Methods with
Applications to Analyses of Alternatively-Arranged Soft and Hard
Layers*Jinghong Fan1
1. Kazuo Inamori School of Engineering, Alfred University

Alternatively-placed soft and hard layers are investigated using multiscale generalized particle methods,
namely the GP and/or XGP methods (Fan 2009, Xu, Fan et al. 2016, Fan et al. 2017). In the methods,
material models are divided into several domains with generalized particles of different scales. The
distinguished characteristics of the GP methods are that each higher scale particle is formed by lumping
the lower scale particle or atoms but their primary cell structures are all the same as the atomistic one,such as BCC, FCC and HCP. The fundamentals of this type of multiscale analysis are based on the local
bottom-up multiscale analysis. Specifically, the position, mass and physical property of the upper-levelparticle is determined by all lower-level particles (e.g. atoms) within the individual particle’s neighborlink cell (NCL), thus the material behavior naturally follows the displacement and motion of these atoms
consisted of the material. In addition, the numerical algorithm follows typical molecular dynamics.
Significant results obtained for pearlite steels with alternatively-placed ferrite (soft) and cementite (hard)layers include: Observed progressive failure process via formation and evolution of multiple slip bands
which produce void nucleation, growth, coalescence and failure; found strong dependence of material
behavior on loading direction with respect to the layer plane and strong dependence of microstructure
size effects of mechanical behavior on the extrinsic or model size effects. The latter confirms the necessityand effectiveness to use the XGP, short for extended GP method, in developing large simulation models
and using the asymptotical method to find the critical model size.

560

MMM 2018

(J3 oral)
The effect of layer thickness ratio on the plastic deformation
mechanisms of nonoindented Ti/TiN nanolayered composite: A
molecular dynamics study*Georges Y Ayoub1, Wei yang2, Iman Salehinia3, Bilal Mansoor2, Hussein Zbib4
1. Dept. of Industrial and manufacturing system Engineering, Univ. of Michigan, 2. Dept. of Mechanical Engineering,Texas A&M Univ., 3. Dept. of Mechanical Engineering, Northern Illinois Univ., 4. School of Mechanical and Materials
Engineering, Washington State Univ.

Molecular dynamics simulations were performed to identify the underlying deformation mechanisms
controlling the plastic behavior of nanoindented nanoscale multilayered Ti/TiN. MD simulations wereconducted on pure Ti and pure TiN as well as on four different layer-thickness ratios of Ti/TiN multilayers,
Ti:TiN=1, 2.5, 4, and 7.5. The Ti layer thickness varied from 2nm to 15nm. The plastic deformation of
nanoindented pure Ti was dominated by the formation of dislocation loops resulting from basal partial
dislocations, while very few perfect dislocations that tie dislocation loops together were observed. The
plastic deformation of nanoindented pure TiN was controlled by the activation of perfect dislocation
propagation along the 111 plane that dissociates into two partials. Depending on the thickness ratio,
either dislocation pile-up or single dislocation crossing through the interface was the controlling plastic
deformation mechanism of nanoindented Ti/TiN multilayers. For metal layer thicknesses above 5nm,
significant dislocation pile-ups were observed at the interface of the quad-layered samples. The Ti/TiN
multilayer with a thickness ratio of 1:1 with individual layer thickness of 2nm exhibited the highest
strain-hardening rate. At this length scale, the activation of dislocation sources requires very high stresses,
and the single dislocation crossing process is the most dominant deformation mechanism. The initiationof plasticity in the TiN layer occurs at a high level of stress since there is no dislocation pile-up at theinterface.

MMM 2018

561

(J3 oral)
The attenuation of stress wave propagation in multilayer structure*Fengyuan Yang1, Zhanli Liu1
1. Tsinghua University

In recent years, many biological structures with excellent impact resistance have been studied, such as
the stomatopod dactyl club, fish scales, etc. These biological structures have a multi-layered structure andeach layer structure has different material properties. These specific bio-layered structures not only havehigher hardness and modulus, but also have higher dissipative stress wave performance. In this paper, theinfluence of the material properties and the arrangement order of the multilayer structure on the
propagation of stress waves is studied, in order to obtain a multilayer structure with higher attenuationstress wave energy. The propagation of stress wave in the material of viscoelastic layer changes with thefrequency of stress wave. The transmission coefficient of the stress wave in the multilayer structure has
frequency effect and size effect.

562

MMM 2018

(J3 oral)
Multiscale molecular-dynamics simulations of structure andmechanics of polymer nanocomposites*Alexey Lyulin1,2
1. Group Theory of Polymers and Soft Matter, Eindhoven University of Technology, 2. Center for Computational
Energy Research, Department of Applied Physics, Eindhoven University of Technology

Using multiscale modelling approach we performed molecular-dynamics simulations of bothcoarse-grained and detailed atomistic (polyimide R-BAPB) amorphous polymer melts consisting of
non-entangled, non-crosslinked polymer chains. The inorganic filler surfaces were mimicked either bysolid walls, or by explicit insertion of the filler particles. The rise in the glass-transition temperature withincrease of the filler fraction was accompanied by a monotonic slowing-down of the relaxation of theincoherent scattering function on all simulated length scales. The filler surface roughness could also leadto slower segmental relaxation. Higher dynamic fragility was observed for smaller film thicknesses.
The cyclic shear deformation is performed to characterize macroscopic properties of the systems before
and after filler insertion. The reported coarse-grained simulations show a strong decrease of thenanocomposites storage modulus with increasing strain amplitude, which is accompanied by a maximum
in the loss modulus (the so-called Payne effect); the onset of the softening is observed in the linear regimeof deformation at strain amplitude of about 0.01. Moreover, the dependence of the storage modulus on
the instantaneous strain exhibits both softening and hardening regimes, in agreement with Large
Amplitude Oscillatory Shear (LAOS) experiments. The observed hardening is caused by the shear-induced
decrease of the non-affine diffusion of the polymer segments due to filler particles acting as effective
crosslinks between polymeric chains and, hence, hindering diffusion. Moreover, the formation of glassy
immobile layers at the nanoparticle interface strongly increases the storage modulus at low strain
amplitudes. The strain softening with increasing strain amplitude is connected to the mobilization of theseglassy layers and an increase in the dynamic heterogeneity of the polymer matrix.

MMM 2018

563

(J4 invited)
Validation of analytical models for ply cracking of general symmetriccomposite laminates*TOMONAGA OKABE1
1. Department of Aeroscpace Engineering, Tohoku University

Validation of analytical models for ply cracking of general symmetric composite laminates
Polymer matrix composites (PMCs) that have high specific strength and specific rigidity have been used inaerospace fields. Laminates made by stacking unidirectional fiber-reinforced lamina, which havepredefined mechanical properties, are commonly used. Fiber-reinforced laminates have a specific fracturemode such as transverse cracking, delamination, and breakage of the fiber. Of these fracture modes,
transverse cracking occurs in the earliest stage. In laminates with plies in different fiber orientations,transverse cracks can form from defect in a given ply, and grow traversing the thickness of the ply andrunning parallel to the fibers in that ply. The most direct effect of transverse cracking is reduction of thethermomechanical properties of the laminate, including changes in the effective values of Young's moduli,
Poisson's ratios, and thermal expansion coefficients. Substantial transverse cracking may give rise to moredeleterious forms of damage, or provide pathways for the entry of moisture and corrosive liquids. Thus,
although this damage mode is not critical from a final fracture point of view, it can result in significant
degradation in the properties of laminates. Therefore, it is necessary to understand the mechanicalbehavior of laminate including transverse cracks. The continuum damage mechanics (CDM) is considered
to be a valid approach to this issue. This approach utilizes the internal state variable and damage variable
to consider the effect of transverse cracks on the reduction of stiffness.
In this study, the three-dimensional local stress field (3-D LSF) model was formulated at first. Secondly, a
Tohoku continuum damage mechanics (TCDM) model, which is an energy-based model of ply cracking ofgeneral composite laminates, was presented using the 3-D LSF model and the CDM approach. Especially,
the damage variable d2 in the direction normal to the fiber was derived for the ply including transverse
cracking as a function of transverse crack density using the 3-D LSF model. Finally, the validity of the
TCDM model was estimated by comparing the NPL model and the experiment results.

564

MMM 2018

(J4 invited)Two-way coupled modeling of lithium diffusion and diffusion induced
finite elastoplastic bending of bilayer electrodes in lithium-ion
batteries
Jun Yin1, *Junqian Zhang1
1. Shanghai University

A fully coupling model for diffusion induced finite elastoplastic bending of bilayer electrodes inlithium-ion batteries is proposed. The effect of mechanical stress on lithium diffusion is accounted for by
the mechanical part of chemical potential derived from Gibbs free energy along with use of logarithmic
stress and strain. Eight dimensionless parameters, which govern the stress-assisted diffusion and the
diffusion induced elastoplastic bending, are identified. It is found that the finite plasticity starting from theinterface of bilayer increases the chemical potential gradient and thereby facilitates lithium diffusion. The
full plastic flow makes the abnormal lithium concentration distribution possible that the concentration at
the lithium inlet is lower than the concentration at the interface (downstream). The increase in thicknessof active layer during charging is much larger than the eigen-stretch due to lithiation, and this excess
thickening is found to be caused by the lithiation induced plastic yield.

MMM 2018

565

(J4 oral)
Micro-Scale Model of Thermomechanics in Solidifying Saturated
Porous Media*Michal Benes1, Alexandr Zak1
1. Czech Technical University in Prague

In the contribution, we discuss the model of solidification of melt occupying pores of the porous medium
with grains intact but participating in the heat transfer. The research motivation is given by developmentof advanced materials as well as by climate changes. The model is based on coupled heat conduction
equation and the phase-field. We present the model and several computational studies.
Solidiffication inside the porous medium is accompanied by complex processes affected by the material
composition, micro-scale interfaces between phases within the medium, bulk properties of the presented
phases, and ambient physical conditions. Volumetric changes of the liquid presented in pores subjectedto phase change conditions is one of crucial phenomena. Due to the generic inhomogeneity of volume
occupied by the solidifying porous medium, we focus on treating the phase transition at microscale. We
have developed a micro-scale model describing mechanical, thermal, and phase change processes withina small sample of a porous medium. The phase change is described in the Lagrangian framework bymeans of the energy, Navier, and phase-field equations. A coupling of multi-physics and multiple phases is
introduced. The model provides spatio-temporal dependencies of primary variables, the resulting forces
exerted on grain surfaces by the change in specific volume due to phase transition, and possibly, themean values of the key quantities useful for upscaling. The role of the model is demonstrated on several
computational studies which follow recently published results [1-2].
[1] Žák A., Beneš M. and Illangasekare T.H. Analysis of Model of Soil Freezing and Thawing, IAENG
International Journal of Applied Mathematics, Volume 43, Issue 3, pp. 127--134, September 2013[2] Žák A., Beneš M., Illangasekare T. H., Trautz A. Mathematical Model of Porous Medium Freezing at
Micro-Scale, to appear in Communications in Computational Physics, 2018566

MMM 2018

(P1-65)
Multiscale Model for Interlayer Defects in Heterogeneous Bilayer
Material*Shuyang Dai1, David Joseph Srolovitz2, Yang Xiang3
1. Wuhan University, 2. University of Pennsylvania, 3. Hong Kong University of Science and Technology

We present a multiscale model to describe the interlayer defects in bilayer materials. The model
incorporates both the anisotropy elasticity of each layer and the first-principle calculation informedinteraction between two layers, i.e., the 3-dimensional generalized stacking-fault energy. The force
balance between these two contributions determines the structure. We apply this approach to determine
the structure and energetics of twisted bilayer material. In twisted bilayer graphene, two distinct, modified
Moiré structures are observed. We also study the buckling twisted heterogeneous bilayer material such as
bilayer graphene/hexagonal boron nitride. We investigate the structure and corresponding dislocation
network due to various misfits and twist angles between two layers. The relaxation of the Moiré structure
reduces the symmetry and increases the period of the bilayer material. Our model agrees well with theatomistic results. An analytical description is developed based on the obtained structural features.

MMM 2018

567

Symposium K(K1 oral)Supercomputer Post-K Project “Challenge of Basic Science–Exploring Extremes through Multi-Physics and Multi-Scale
Simulations”
1*Momoji Kubo
1. Tohoku Univ.

Japanese government plans to construct a supercomputer Post-K at 2021, which is expected to have over18

10

flops. This calculation speed is 100 times faster than the present supercomputer K in Kobe, Japan.

Japanese government started 13 projects (9 priority issues and 4 exploratory challenges) to develop
original simulators for the supercomputer Post-K. Our Post-K project “Challenge of Basic Science–Exploring Extremes through Multi-Physics and Multi-Scale Simulations” (Leader: Prof. Momoji Kubo,
Institute for Materials Research, Tohoku University) was selected as one of four exploratory challenges atJune, 2016 and started at August, 2016. The summary of our Post-K project is as follows.
Supercomputer K significantly advanced individual sciences on exploring extremes such as fracture ofmaterials, earthquake, atmospheric flow, ocean flow, volcanic eruption, magma, and extreme properties.
On the basis of the above individual understanding obtained by supercomputer K, our Post-K project aims
to solve multi-physics and multi-scale problems for exploring extremes by developing new calculation
methods beyond the limits of calculation accuracy and possibility and by bridging the different physics
and scales. Our Post-K project consists of four sub-challenges.
Sub-Challenge A: Fracture and Catastrophe(Sub-Leader: Prof. Momoji Kubo, Tohoku University)
Sub-Challenge B: Phase Transitions and Flows(Sub-Leader: Prof. Toshihiro Kawakatsu, Tohoku University)
Sub-Challenge C: Structures and Properties of Materials in Deep Earth and Planets(Sub-Leader: Dr. Toshiaki Iitaka, RIKEN)
Sub-Challenge D: Fundamental Quantum Mechanics and Informatics(Sub-Leader: Prof. Naoki Kawashima, The University of Tokyo)
In the supercomputer Post-K project, our group is focusing on the fracture and catastrophe of materials.
Our recent outcomes on the following three topics will be presented in the conference.
Friction and wear processes of diamond-like carbon thin films. Stacking fault generation processes ofpolycrystalline ceramics. Sintering processes of Ni/YSZ electrode in solid oxide fuel cell.

568

MMM 2018

(K1 invited)
Universal avalanche statistics across 16 decades in length: Fromnanocrystals (and neurons) to earthquakes and stars?
*Karin Dahmen1
1. University of Illinois at Urbana Champaign

Slowly-compressed nano-crystals, bulk metallic glasses, rocks, granular materials, and the earth all deform
via intermittent slips or “quakes”. We find that although these systems span 12 decades in length scale,they all show the same scaling behavior for their slip size distributions and other statistical properties.
Remarkably, the size distributions follow the same power law multiplied with the same exponential cutoff.
The cutoff grows with applied force for materials spanning length scales from nanometers to kilometers,
indicating an underlying nonequilibrium phase transition. A simple mean field model for avalanches ofslipping weak spots explains the agreement across scales. It predicts the observed slip-size distributions
and the observed stress-dependent cutoff function. The analysis draws on tools from statistical physics
and the renormalization group. The results enable extrapolations from one scale to another, and from one
force to another, across different materials and structures, from nanocrystals to earthquakes. Connections
to neuron avalanches in the brain and recent observations on stars will also be discussed, extending therange of scales to 16 decades in length.
References:[1] J.T. Uhl, S. Pathak, D. Schorlemmer, X. Liu, R. Swindeman, B.A.W. Brinkman, M. LeBlanc, G. Tsekenis, N.
Friedman, R. Behringer, D. Denisov, P. Schall, X. Gu, W.J. Wright, T. Hufnagel, A. Jennings, J.R. Greer, P.K.
Liaw, T. Becker, G. Dresen, and K.A. Dahmen, Scientific Reports 5, 16493 (2015).
[2] N. Friedman, A. T. Jennings, G. Tsekenis, J.-Y. Kim, J. T. Uhl, J. R. Greer, and K. A. Dahmen, Phys. Rev.
Lett. 109, 095507 (2012).
[3] J. Antonaglia, W.J. Wright, X. Gu, R.R. Byer, T.C. Hufnagel, M. LeBlanc, J.T. Uhl, and K.A. Dahmen,
Physical Review Letters 112, 155501 (2014).
[4] J. Antonaglia, X.Xie, M. Wraith, J.Qiao, Y. Zhang, P.K. Liaw, J.T. Uhl, and K.A. Dahmen, Nature Scientific
Reports 4, 4382 (2014).
[5] N. Friedman, S. Ito, B.A.W. Brinkman, L. DeVille, K. Dahmen, J. Beggs, and T. Butler, Phys. Rev. Lett.
108, 208102 (2012).
[6] M.A. Sheikh, R.L. Weaver, and K. A. Dahmen, Phys. Rev. Lett. 117, 261101 (2016).

MMM 2018

569

(K1 oral)
Predicting avalanches and failure: wood and paper*Mikko Alava1, Juha Koivisto1, Tero Mäkinen1, Leevi Viitanen1
1. Aalto University

Natural and man-made structured materials like wood and paper exhibit complex response to externalloading. We have demonstrated the presence of deformation avalanches in wood compression in analogy
to other laboratory-scale experiments mimicking earthquakes. I will review this and the subsequentpredictability of large events and discuss the relation of this to the wood microstructure, based on
deformation analyses and tomographic studies [1-3]. Fiber networks - paper - exhibit a very complex
creep response in, again, analogy to most other materials. The predictability of sample failure with orwithout an initial defect/notch depends on the presence of regularities in the rheological response, suchas the localization of creep strain or the formation of a large Fracture Process Zone [4-6]. I will discuss theexperimental signatures that are of relevance for predictability, and what coarse-grained models teach us.
[1] T. Mäkinen et al., Phys. Rev. Lett. 115 (055501).
[2] L. Viitanen et al., J. Stat. Mech. 053401 (2017).
[3] T. Mäkinen et al., unpublished.
[4] J. Rosti et al., Phys. Rev. Lett. 105, 100601 (2010).
[5] J. Koivisto et al. Phys. Rev. E 94, 023002 (2016).
[6] L. Viitanen et al., submitted, Physical Review Applied.

570

MMM 2018

(K1 oral)
System-spanning shear avalanches induced by thermal structural
relaxation in metallic glasses*Tomoaki Niiyama1, Masato Wakeda2, Tomotsugu Shimokawa4, Shigenobu Ogata3
1. College of Science and Engineering, Kanazawa Univ., 2. Research Center for Structural Materials, National Institutefor Materials Science, 3. Department of Mechanical Science and Bioengineering, Osaka University, 4. Faculty of
Mechanical Engineering, Kanazawa University

Metallic glasses (MGs) are one of the most attractive materials because of their excellent properties [1].
However, brittle fractures are an obstacle for MGs to be applied as structural materials. This brittlenessoriginating from shear band nucleation is determined by structural relaxation using thermal annealing [2].
Thus, the influence of structural relaxation upon the nucleation of shear banding that can induce
catastrophic failure in MGs is a significant challenge in material science.
For this challenge, we focus on the avalanche behavior that provides sudden massive deformations andspatiotemporal correlation in solid plasticity [3]. Thus, this avalanche behavior is deeply connected to the
localization of plastic deformation and catastrophic failure in MGs.
In this study [4], we investigate the geometry of the shear transformation avalanches that exhibit thepower-law statistics using molecular dynamics simulations of shear deformation in two thermallyprocessed MG models that are based on a less-relaxed glass and a well-relaxed glass. The simulation
showed a shear-band like heterogeneous pattern in the well-relaxed glass model, whereas theless-relaxed model exhibits homogeneous deformation patterns. Considering the spatial correlation
functions of the non-affine least square displacements of atoms during each each avalanche event, wereveal that the regions an avalanche developed in well-relaxed glasses tend to be anisotropic whereas
those in less-relaxed glasses are isotropic. Moreover, a temporal clustering feature of the direction ofavalanche propagations and a considerable correlation between the anisotropy and size of an avalanche
in the well-relaxed glass model are demonstrated.
[1] A. L. Greer, Science 267, 1947 (1995).
[2] G. Kumar, et al., Acta Mater. 57, 3572 (2009).
[3] M. C. Miguel, et al., Nature 410, 667 (2001).
[4] T. Niiyama, et al., arXiv:1804.00852 (2108).

MMM 2018

571

(K2 invited)
Predictability of catastrophic failure in porous media*Ian Main1, Ferenc Kun2, Jeremie Vasseur3, Andrew Bell1
1. University of Edinburgh, 2. University of Debrecen, 3. Ludwig Maximilian University, Munich

Recent discrete element models for the processes leading up to material failure provide a very close
match to experiment in the case of high porosity, highly disordered materials. On approach tocatastrophic, system-sized failure the number of micro-cracks and their associated acoustic emissions(AE) increase at a rate marked by a smooth inverse power law, defining a failure time at the singularity in
AE rate. This behaviour is reminiscent of a second-order phase transition. At the same time thedeformation becomes progressively more localised on an incipient optimally oriented fault plane, and thescaling exponent b for the frequency-magnitude distribution of the acoustic emissions decreases to aminimum near the failure time. On the other hand, a simple elastic fracture mechanics for an ideal
ordered, uniform solid with a single pre-existing crack provides no warning of incipient failure. In between
these limits there are clear precursors, but failure occurs suddenly, and earlier than predicted by the
inverse power-law model, more reminiscent of a first-order phase transition. We develop a mean fieldmodel for a population of cracks emanating from pores in an otherwise uniform medium to explain thissystematic error in the predicted failure time. The correction depends non-linearly on the porosity,
specifically the distance between pores in the starting model, and tends to zero in the limiting case ofhigh-porosity materials. It provides a good match to aggregate data obtained from experiments on a rangeof materials, both natural and synthetic. We show the behaviour scales very well to a range of data fromearthquakes prior to volcanic eruptions, including quasi-periodic ‘drumbeat’ long-period earthquake
signals preceding a recent large vulcanian explosion at Tungurahua volcano, Ecuador. Unfortunately, suchsignals are not yet detectable prior to large earthquakes above a null hypothesis of conventional
epidemic-type earthquake triggering models.

572

MMM 2018

(K2 oral)
Deciphering the dynamics of precursors to failure in quasi-brittle
solids: an inspiration for understanding the statistics of earthquakes ?
*Laurent Ponson1, Estelle Berthier1, Vincent Démery2
1. Institut Jean le Rond d'Alembert, CNRS - Sorbonne University, Paris, 2. Gulliver, CNRS - ESPCI, Paris

Quasi-brittle failure results from the evolution of a large number of interacting microcracks growing
through the material microstructural disorder. Despite this complexity, quasi-brittle materials under slowly
increasing compressive load exhibit a remarkably robust failure behavior: During a first stage, damage
grows and accumulates through bursts of failure events that are localized both in space and time. This
earthquake-like dynamic is characterized by scale free statistics with exponents that vary weakly with thetype of materials and the loading conditions. Ultimately, the damage localizes into a macroscopic bandthat leads to the catastrophic failure of the specimen.
In this study, we investigate theoretically the physical mechanisms underlying intermittency andlocalization during quasi-brittle failure. Elasticity is shown to promote long-range interactions between thedamaging elements constituting the specimen and to drive the collective response of the array of
microcracks. To capture this cooperative dynamic, we encapsulate the interactions in an elastic kernelthat derives from the continuum mechanics of elasto-damageable solids. We then show how it can beused to (i) disentangle the statistical properties of precursors to failure and explain their scale-freestatistics and (ii) predict the onset of localization and the emerging fracture pattern. Our theoreticalpredictions are critically compared to experimental and numerical observations made during thecompressive failure of disordered quasi-brittle solids.
The relevance of this theoretical framework for unravelling the statistics of earthquakes is finallydiscussed.

MMM 2018

573

(K2 oral)
Avalanche precursors and fracture strength in the limit of highdisorder*Ferenc Kun1,2, Viktória Kádár1
1. Department of Theoretical Physics, University of Debrecen, 2. Institute for Nuclear Research (Atomki), Debrecen

Most of the materials have an inherent disorder which appears at different length scales in the form of
dislocations, flaws, microcracks, grain boundaries, or internal frictional interfaces. Under mechanical load,
this quenched disorder plays a decisive role in the emerging fracture process: Increasing the extension ofsamples a size effect emerges, i.e. the ultimate strength of disordered materials is a decreasing function of
their size. On the microscale the fracture process is composed of a large number of crack nucleation propagation - arrest steps which generate a sequence of precursory cracking avalanches. This cracklingnoise is of ultimate importance to forecast natural catastrophes such as landslides and earthquakes, andthe catastrophic failure of engineering constructions.
We investigate how the amount of microscale disorder affects the emerging fracture process focusing on
the size scaling of the ultimate fracture strength and on the time series of avalanche precursors. We
consider a fiber bundle model where the strength of fibers is described by a power law distribution.
Tuning the amount of disorder by varying the power law exponent and the upper cutoff of fibers' strength,
in the mean field limit an astonishing size effect is revealed: For small system sizes the bundle strength
increases with the number of fibers and the usual decreasing size effect is only restored beyond acharacteristic size. We show that the extreme order statistics of the micro-scale disorder is responsible forthis peculiar behavior, which can be exploited for materials' design.
In the limit of very high disorder the avalanche activity does not show any acceleration so that no
signatures of the imminent catastrophic failure can be identified. Limiting the disorder to a finite range anacceleration period emerges with precursors, however, the predictability of the catastrophic event
depends on the details of the tail of the disorder distribution.
[1] V. Kádár, Zs. Danku, and F. Kun, Physical Review E 96, 033001 (2017).
[2] Zs. Danku and F. Kun, J. Stat. Mech.: Theor. Exp. 2016, 073211 (2016).

574

MMM 2018

(K2 oral)
Jump statistics of epicenters in thermally induced cracking of fiber
bundles*Naoki Yoshioka1, Ferenc Kun2, Nobuyasu Ito1,3
1. RIKEN Center for Computational Science, 2. Department of Theoretical Physics, University of Debrecen, 3.
Department of Applied Physics, Graduate Schoold of Engineering, The University of Tokyo

Based on a fiber-bundle model we investigate epicenters of breaking events in thermally activated creep
rupture. Breaking events are initiated by thermal fluctuation of load followed by load redistribution whichleads to an avalanche of breakings. Using recently introduced kinetic Monte Carlo algorithm for athermally activated fiber-bundle model, it is shown that jump distance between epicenters of consecutive
breaking events has a power-law distribution with a non-trivial exponent. We also discuss relations
between burst size and other quantities such as jump distance of epicenters and waiting time of breaking
events.

MMM 2018

575

(K2 oral)
Time dependent fracture under unloading in a fiber bundle model*Reka Korei1, Ferenc Kun1
1. Department of Theoretical Physics, University of Debrecen

Failure of materials can also occur due to unloading from a previously applied stress level. Excavation
during underground engineering rapidly releases stress which can result in rock bursts. Similar conditions
may also occur on much larger length and time scales at the emergence of earthquakes: crustal unloading
due to near-surface mass redistribution (water, ice or quarried material) can affect the subsurface stressfield altering seismic activity and being also responsible for rupture activation and induced earthquakes.
Fracture processes under unloading present a high degree of complexity which makes it difficult toachieve a general understanding.
To consider this problem, we investigate the process of sub-critical fracture which occurs when unloadingfrom an initial load. We use a fiber bundle model of time dependent deformation and rupture which
captures the slow damaging of loaded fibers and their immediate breaking when the local load exceeds
the fibers' fracture strength. We focus on the case when a constant sub-critical load gives rise to failure ina finite time so that unloading may prevent the final breakdown. We show that the system has two phases:
at rapid unloading only partial failure occurs and the sample has an infinite lifetime, however, slow
unloading results in global failure in a finite time. We demonstrate that the transition between the phasesof finite and infinite lifetime occurs as a continuous phase transition.
The unloading process is accompanied by breaking bursts of fibers with a varying rate. We show bycomputer simulations that in the regime of finite lifetime the initial relaxation is followed by a shortacceleration period of bursting activity towards failure which is described by the Omori law. Based on thepattern of the time varying burst rate we propose a method to forecast the impending failure underunloading.

576

MMM 2018

(K3 invited)
Creep of strongly disordered materials: Plasticity, damage andapproach to failure*Michael Zaiser1, David Fernandez Castellanos1
1. Inst,. of Materials Simulation, Dept of Materials Science, FAU University of Erlangen-Nuremberg

Stochastic plasticity models have recently become popular for modelling the deformation behavior ofstructually disordered materials, from amorphous solids over crystals with disordered microstructure togeomaterials. We generalize such models to account for thermally activated creep processes in
conjunction with structural damage. We show that the thus modified models exhibit three-stage creep
curves where failure occurs as a finite-time singularity of the creep rate in conjunction with strong
localization of deformation in a catastrophic shear band. Deformation occurs as a stochastic sequence of
discrete avalanches with a rate that accelerates towards failure in the form of an inverse Omori law.
Immediately before failure, the system switches to a regime of mechanical activation where the avalanche
statistics exhibits universal features. We compare our findings with experimental data on failure of rocksamples and discuss relations with other stochastic models of plasticity and fracture.

MMM 2018

577

(K3 oral)
Creep rupture and Omori-Utsu law: Fiber bundle model approach*Takahiro Hatano1, Subhadeep Roy2
1. University of Tokyo, 2. Norwegian University of Science and Technology

We study the dynamical aspects of a statistical-mechanical model for fracture of heterogeneous media:
the fiber bundle model with various interaction range. Although the model does not include any nontrivial
elementary processes such as nonlinear rheology or stochasticity, the system exhibits creep-like behaviors
under a constant load being slightly above the critical value. These creep-like behaviors comprise threestages: in the primary and tertiary stages, the strain rate exhibits power-law behaviors with time, which are
well described by the Omori-Utsu and the inverse Omori laws, respectively, although the exponents arelarger than those typically observed in experiments. A characteristic time that defines the onset of
power-law behavior in the Omori-Utsu law is found to decrease with the strength of disorder in thesystem. The analytical solution, which agrees with the above numerical results, is obtained for themean-field limit. Beyond the mean-field limit, the exponent for the Omori-Utsu law tends to be even larger
but decreases with the disorder in the system. Increasing the spatial range of interactions, this exponent isfound to be independent of disorder and to converge to the mean-field value. In contrast, the inverse
Omori law remains independent of the spatial range of interaction and the disorder strength.

578

MMM 2018

(K3 oral)
Temperature dependent shear friction in metallic glass*Akio Ishii1, Tomoaki Niiyama2, Takahiro Hatano3, Tomotsugu Shimokawa2, Shigenobu Ogata1
1. Osaka Univ., 2. Kanazawa Univ., 3. Tokyo Univ.

Sharp local sliding such as shear band is frequently observed in bulk metallic glass subject to mechanicalloading, and it usually leads to a global mechanical instability and then a catastrophic failure of metallic
glass. Although sliding induced frictional force is a key to understand the stability of metallic glass, thedetails has not been fully clarified yet. Using a micromechanics-based kinetic Monte Carlo tensile test
simulation of metallic glass [1], we computed a temperature and velocity dependent shear frictional force
in metallic glass in experimental time scale. We found a negative velocity dependency (velocity
weakening) within certain temperature range and thus a velocity dependency transition from negative topositive (velocity hardening) at a transition temperature as the temperature increases. The similar trend
has been observed in rock as well [2].
[1] P. Zhao, et al., International Journal of Plasticity, 40 pp.1–22 (2013).
[2] C H. Scholz, nature, 391 pp.37-42 (1998).

MMM 2018

579

(K3 oral)
Mechanism of controlled crack formation induced by memory effect ofclay paste*Akio Nakahara1, Rokuya Hayashi1, Tomoki Hiraoka1, Yousuke Matsuo1, So Kitsunezaki2
1. Nihon Univ., 2. Nara Women's University

Wet clay paste remembers the direction of vibration and flow it has experienced and, when the paste isdried, the direction of desiccation cracks strongly depends on the memory of such motions. To find out
the mechanism of memory effect of vibration, we perform experiments to rewite memory in paste by
applying additional vibration to the paste along different direction. Experimental results are found to beconsistent with theoretical models which are based on residual stress theory.

580

MMM 2018

(K3 oral)
Effects of Shockwave-Induced Nanobubble Collapse on Precision
Polishing : Molecular Dynamics Study*yoshimasa aoyama1, Jingxiang Xu2, Yuske Ootani1, Nobuki Ozawa1, Momoji Kubo1
1. Dept. of Materials Science, Tohoku Univ., 2. Dept. of Mechanical Engineering, Shanghai Ocean Univ.

Chemical mechanical polishing (CMP) is used for the planarization of semiconductor materials. However,some of them are much hard. Therefore, the designing of efficient CMP process of hard materials is
required. In order to achieve an efficient CMP process, we focused on a nanobubble. When the
nanobubble collapses, the liquid around the bubble forms a jet which creates a water hammer shock.
Therefore, we suggested that applying the jet increases the efficiency of CMP process. To establish theefficient CMP, revealing the effects of the nanobubble collapse on precise polishing is required. In orderto reveal the effects of the nanobubble collapse on a solid surface, we performed nanobubble collapse
simulation on a solid surface by molecular dynamics method. First, we made the nanobubble in water
solvent by removing the solvent molecules spherically. Next, we applied shockwave and performed
nanobubble collapse simulation on the solid surface. In order to clarify the effects of one nanobubble
collapse, we compared the nanobubble collapse simulations and only shockwave simulation. In the
nanobubble collapse simulation, a jet was generated by the nanobubble collapse and the structuralchange in the solid surface was larger compared with the shockwave simulation. This suggests that ananobubble would be useful to increase the removal rate of a solid surface. Next, in order to clarify theeffects of different number of nanobubbles, we compared the nanobubble collapse simulations with one
and two nanobubbles. We found that the structural change by the two nanobubbles simulation was larger
than that by the one nanobubble simulation. When the jet passes the water solvent, the stream of the jet
decays. However, when the generated jet passes another nanobubble, the stream of the jet does notdecay, because there are no water molecules in the nanobubble. Therefore, increasing the number of
nanobubbles would be useful to improve the removal rate.

MMM 2018

581

(K4 invited)
Disclination dipole model of kink deformation in layered solid*Akihiro Nakatani1, Xiao-Wen Lei2
1. Dept. of Adaptive Machine Systems, Osaka Univ., 2. Dept. of Mechanical Engineering, Univ. of Fukui

Kink deformation is commonly observed in layered solid for a wide variety of scales from atomistic layers
to geological structures, and their structural similarity means possibility of universal application of
deformable body mechanics. In our previous works, the deformation mechanism of kink deformation in alayered solid with a single-slip system has been studied and it is shown that the fundamental process ofkink deformation is represented by disclination dipole model.
In this study, first, we review the relationship between generalized continuum theories and thedisclination dipole dynamics from a multiscale point of view.
Secondly, nucleation of kink deformation under a compressive force in the direction parallel to the layers
is studied using configulational force of disclination dipole based on instability theory, in which thedeformation mechanism is discussed from the viewpoint of the instability theory with the Maxwell's equalarea rule.
Third, we use cellular automaton as a discretized model of disclination dipole theory, in which each layer
is divided into segments and the amount of Frank vector of each segment is encoded using integers. In
this study, total strain energy is calculated as sum of the local bending strain energy of each layer andinter-layer energy, in which the local strain energy density is evaluated as a linear function of square ofcurvature and inter-layer energy is assumed to be expressed by a form of Lennard-Jones 12-6 potential.
The Metropolis Monte Carlo Method is adopted to evolve the state of the material system. The result of
this scheme is verified on the classical problem of buckling of Euler's column with comparing analyticalsolution. After that, simulation of kinking deformation is examined for various values of the inter-layer
strength parameter.

582

MMM 2018

(K4 oral)
Large-scale coarse-grained molecular dynamics simulations on
fracture processes of lamellar structure in crystalline polymers*Yuji Higuchi1
1. The University of Tokyo

Understanding the fracture processes of crystalline polymers such as polyethylene by molecular theory is
one of the big challenges, which contributes to the increase in the toughness of polymeric materials inindustry. To reveal the fracture process, coarse-grained molecular dynamics simulation is effective;
however the fracture processes of the most fundamental crystal structure, lamellar structure consisting ofamorphous and crystalline layers, in polyethylene has not been revealed. The reason is the difficulty of theconstruction of the lamellar structure due to the small simulation size in the order of 104 beads. Thus, we
propose a crystallization method for the large-scale lamellar structure in the order of 106 beads andperform the fracture simulation.
In the fracture process of the lamellar structure by coarse-grained molecular dynamics simulation,mechanical properties are consistent with the experiment, confirming the validity of the simulation results.
We also reveal that the movement of chain ends from amorphous layers to crystalline layers causes thedeformation and void generation in the amorphous region, indicating that the chain ends act as defect [1,2]. In the large scale simulations, the buckling of crystalline layer and its fragmentations are observed,
which can be compared directly with the experimental observation by electronic microscope. Ourlarge-scale coarse-grained molecular dynamics simulations are useful to reveal the fracture process ofpolymers at molecular level. We develop the above molecular technology which can reveal the fracture
processes of polymeric materials at the molecular level and successfully apply it to the fracture process of
double network gels [3].
[1] Y. Higuchi et al., Macromolecules 50, 3690-3702 (2017).
[2] Y. Higuchi, Polym. J. in press (2018).
[3] Y. Higuchi et al., Macromolecules in press (2018).

MMM 2018

583

(K4 oral)
Grain boundary sliding within the entropy production rate theory*Tetsuo Mohri1
1. IMR, Tohoku University

It is a challenging task to clarify the underlying principle of deformation behavior of an alloy. In hightemperatures, an alloy undergoes various deformation modes driven by dislocations movement, grainboundary sliding etc. Under a given strain rate (tensile test) or a given stress (creep test), how does analloy adjust internal structures to accommodate externally imposed deformation conditions such as
deformation rate and applied stress? This is the question the author addresses in the present paper. Bytaking a grain boundary sliding as an example, we formulated deformation behavior in terms of averagerotation rate of a grain and density of rotating grains based on entropy production rate theory within thelinear regime. The physical implications of grain boundary sliding at an extremum of entropy production
rate are main focus of the discussion, and thermodynamic similarity between grain boundary sliding anddislocations driven deformation is addressed. Furthermore, it will be briefly touched upon how to extend
the formulation of grain boundary sliding for an alloy to larger scale deformation phenomena.

584

MMM 2018

(K4 oral)
Molecular Dynamics Simulation on Intergranular Cracking Mechanismof Iron Material in High Temperature Pressurized Water Environment*Qian Chen1, Jingxiang Xu1, Yusuke Ootani1, Nobuki Ozawa1, Momoji Kubo1
1. Institute for Materials Research, Tohoku University

Iron-based materials are widely used in power plants, and usually serve in high temperature pressurizedwater environment. The severe water environment causes stress corrosion cracking (SCC), resulting in
potentinally catastrophic accidents. It is therefore important to understand the chemical reaction
dynamics at the iron-water interface and the SCC mechanism during failure process in order to improve
corrosion resistance. However, it is difficult to achieve that by experimental observation, particularly inhigh temperature pressurized water. Thus, the SCC mechanism for iron-based materials in such anenvironment is still unclear. In this study, we employed molecular dynamics simulation to study the failure
process in high temperature pressurized water environment. To investigate chemical reactions, a reactive
force field was used in this study. Since intergranular cracking is more common than transgranular
cracking during the fracture processes of iron materials, a Σ5(310) grain boundary was modeled. To
investigate the crack growth process under stress conditions, we applied external tension along theperpendicular direction of the pre-crack model with water arranged on the surface. The simulation results
showed structural change due to plastic deformation around the pre-crack, and the structural change wasfound to be attributable to twinning by partial dislocations. In addition, small cracks were observed in thelateral direction of the pre-crack tip. To investigate the effect of high temperature pressurized water, we
also simulated the model without water for comparison. It was revealed that crack propagation was
promoted by suppressing structural relaxation due to the chemical reactions in the high temperaturepressurized water condition.

MMM 2018585

(K5 oral)
Effects of a bulk-region size in the first-principles tensile test of a grainboundary*Masanori Kohyama1, Hao Wang2, Shingo Tanaka1
1. AIST, 2. Shanghai Univ.

The first-principles tensile test (FPTT) of a grain boundary (GB) is a powerful tool to clarify intrinsic tensile
strength and failure process of a GB, according to natural behaviors of electrons and atoms,
corresponding to a slow tensile test at zero temperature [1]. This is quite effective to clarify the effects ofsegregated impurities [2]. This can provide intrinsic energy-strain and stress-strain curves of a GB, whichshould be useful to construct separation-energy curves for “cohesive zone model” in a continuum
model as a multi-scale simulation technique. However, there are several unsolved issues in interpreting
FPTT results compared to real mechanical properties of GBs. In the present study, we investigate theeffects of the bulk-region size in the GB supercell, where usually two symmetric interfaces of acoincidence-site lattice (CSL) GB are alternately stacked between bulk slabs with some thickness. We
show that the bulk-region thickness seriously affect the features of energy-strain and stress-strain curves
around the failure point by comparing the FPTTs of the same Al GB with different bulk-region thicknesses,
which are also analyzed by local-energy and local-stress techniques [3]. The difference is dominated by
the relation between the Griffith and stress conditions for failure, depending on the bulk-region thickness.
From this viewpoint, we re-interpret our previous FPTT results of Al GBs with various segregated impurities[2].
[1] M. Kohyama, Phil. Mag. Lett. 79, 659 (1999); Phys. Rev. B, 65, 184107 (2002)[2] G. H. Lu et al., Phys. Rev. B, 73, 224115 (2006)[3] H. Wang et al., Modell. Simul. Mater. Sci. Eng. 25, 015005 (2017)586

MMM 2018

(K5 oral)
Combined analysis of first-principles calculations and fracture
mechanics experiments on intergranular embrittlement of an alloy
steel*Masatake Yamaguchi1
1. Center for Computational Science and e-Systems, Japan Atomic Energy Agency

A significant loss of fracture toughness (KIc) is induced by intergranular (grain boundary; GB) segregationof metalloid solute (Sb, Sn, and P) in a Ni-Cr steel. However, the atomistic mechanism has not beenclarified from a multiscale point of view. From first-principles calculations, it is shown that segregatedsolute with higher energetic stability on fracture surfaces causes a larger linear reduction in the ideal work
to intergranular fracture (2γint ); i.e. the energy difference between a GB and its two fracture surfaces.
The combined analysis with first-principles calculations and fracture mechanics experiments foundseveral orders of magnitude more energy loss in KIc for a specific range in the 2γint within only a few

tenths of J/m2. These results illustrate that the GB of steel has the threshold energy of atomic cohesion
under which catastrophic failure occurs [1]. [1] M. Yamaguchi and J. Kameda: Phil. Mag. 94, 2131-2149(2014).

MMM 2018

587

(K5 oral)First-principles local energy analysis of grain boundary segregation of
sp-elements on bcc Fe*Kazuma Ito1,2, Somesh Kr. Bhattacharya3, Shingo Tanaka3, Masanori Kohyama3, Hideaki Sawada
2, Shigenobu Ogata1

1. Osaka University, 2. Nippon Steel & Sumitomo Metal Corporation, 3. National Institute of Advanced Industrial
Science and Technology

In polycrystalline materials, grain boundary segregation of impurity elements and alloying elements may
have a great influence on the material properties. Particularly in steel materials, it is known that the
sp-elements such as P and S segregate at the grain boundary, thereby remarkably lowering hot ductility
and low temperature toughness. However, the microscopic mechanism of grain boundary segregation ofthese elements has not been fully clarified.
In this study, grain boundary segregation energies for sp-elements (Al,Si,P and S) in bcc Fe are calculated
by first-principles calculations. Furthermore, to investigate the microscopic mechanism, the contributionof each physical origin to the grain boundary segregation energies is evaluated numerically by using
first-principles local energy analysis. We will also discuss the relationship between the contribution ofeach physical origin and the geometric structure or the electronic state of the grain boundary.

588

MMM 2018

(K5 oral)
Fast and scalable prediction of local energy at grain boundaries:
Machine-learning based modeling of first-principles calculations*Tomoyuki Tamura1,2, Masayuki Karasuyama1,2,3, Ryo Kobayashi1,2, Ryuichi Arakawa1, Yoshinori
Shiihara4, Ichiro Takeuchi1,2,5
1. Nagoya Institute of Technology, 2. Center for Materials Research by Information Integration (CMI2), Research andServices Division of Materials Data and Integrated System (MaDIS), National Institute for Materials Science (NIMS), 3.
PRESTO, JST, 4. Toyota Technological Institute, 5. RIKEN Center for Advanced Intelligence Project

A GB is the interface between two grains or crystallites in a polycrystalline material, and the atomic
configurations and chemical bonds near GB are distinct from those of the bulk crystal. Thus, theproperties of materials with GBs can greatly differ from those of a single crystal. Since it is possible toimprove various properties of materials significantly by GB engineering, it is of great importance toinvestigate physical and chemical properties of each atoms or local regions near GBs. By virtue of thedevelopment of efficient computational techniques of large-scale density functional theory (DFT)
calculations and the rapid progress of parallel computers including supercomputers, we can deal withrelatively large supercells for GB models. But, it needs much computational costs to perform DFT
calculations with larger supercells, and it is impossible to cover all GB models.
We proposed a new scheme based on machine learning for the efficient screening in GB engineering. A
set of results obtained from DFT calculations for a small number of GB systems is used as a training data
set. In our scheme, by partitioning the total energy into atomic energies using a local-energy analysis
scheme, we can increase the training data set significantly. We use atomic radial distribution functions
and additional structural features as atom descriptors to predict atomic energies and GB energies
simultaneously using the least absolute shrinkage and selection operator (LASSO). In the test study with
fcc-Al [110] symmetric tilt GBs, we could achieve enough predictive accuracy to understand energychanges at and near GBs at a glance, even if we collected training data from only ten GB systems. The
present scheme can emulate time-consuming DFT calculations for large GB systems with negligible
computational costs, and thus enable the fast screening of possible alternative GB systems.

MMM 2018

589

Symposium L(L1 invited)
Flow and failure of (amorphous) materials - a nonequilibrium phasetransition?
*Peter Schall1
1. University of Amsterdam

Amorphous materials show intriguing mechanical properties that are of fundamental interest and of great
importance for applications. While the aging, deformation and failure have been longstanding researchquestions for conventional glasses, recent soft glasses including colloidal suspensions, foams andemulsions have spawned new interest and new perspectives on glassy flow. In these systems, flow is
ubiquitous: it is easily induced by small applied stresses, and the underlying flow and failure mechanisms
can be conveniently studied at the particle scale. Simulations and experiments on soft glassy systems
have witnessed exciting scaling relations that are believed to underlie the flow of glasses under appliedstress. In particular, colloidal and granular systems have been powerful models to directly visualize andmeasure internal strain fields and their hierarchical organization. The emerging picture is that flow andflow instabilities are related to non-equilibrium phase transitions from a reversible elastic-like toirreversible plastic response of the material. At yielding of the material, plastic regions percolate acrossthe sample, mediating the flow in the otherwise elastic matrix. Flow is therefore neither strongly localized
nor homogeneous across the material; instead, system-spanning correlations of plastic activity occur,
revealing a novel kind of criticality of the slowly flowing material. Similar long-range correlated flow
phenomena have been observed in the deformation of crystals, as bursts of dislocations. Also in thesecrystalline materials, the internal elastic strain field is believed to be responsible for the highly correlated
dislocation activity. I will elucidate this underlying mechanism by detailed investigation of the strain fieldin colloidal crystals and glasses. Furthermore, I will show that the underlying general mechanism links oursoft materials to a far wider range of materials and phenomena including conventional material plasticity,geological flow phenomena, and earth quakes.

590

MMM 2018

(L1 invited)
Reversibility and Criticality in Amorphous and Crystalline Solids*Charles Reichhardt1
1. Los Alamos National Laboratory

The physical processes governing the onset of yield, where a material changes its shape permanently
under external deformation, are not yet understood for amorphous solids that are intrinsically disordered.
Here, using molecular dynamics simulations and mean-field theory, we show that at a critical strain
amplitude the sizes of clusters of atoms undergoing cooperative rearrangements of displacements(avalanches) diverges. We compare this non-equilibrium critical behaviour to the prevailing concept of a‘front depinning’ transition that has been used to describe steady-state avalanche behaviour indifferent materials. We explain why a depinning-like process can result in a transition from periodic tochaotic behaviour and why chaotic motion is not possible in pinned systems. These findings suggest that,at least for highly jammed amorphous systems, the irreversibility transition may be a side effect ofdepinning that occurs in systems where the disorder is not quenched.

MMM 2018

591

(L1 invited)
Exploring Crystal-plastic Constitutive Rules with the OOF Tool
*Andrew Reid1, Stephen Langer1, Shahiryar Keshavarz1,2
1. NIST, 2. Theiss Research

A focus of the US National Institute of Standards and Technology effort associated with the Materials
Genome Initiative is the construction of software tools for exploring structure-property relationships. Oneof these is the OOF Object-oriented finite-element tool, which is an integrated materials-focused image
segmentation, mesh construction, and modeling system, allowing interactive structure-property
explorations on meshes modeling realistic microstructures. A crystal-plastic modeling capability has
recently been added to the OOF finite-element tool, bringing together OOF's ability to easily create
meshes corresponding to real microstructures, and the capability to quickly and easily explore a wide
variety of constitutive rules derived from various dislocation activity mechanisms derived. This provides alength-scale bridge from continuum descriptions of dislocations in the bulk to representative volume
elements relevant to particular classes of microstructures. Early results of this exploration will bepresented. The promise of this effort is that, coupled with suitable reference experiments on small
systems, high-fidelity continuum crystal-plastic constitituve rules can be identified, and then employed inlarger-scale material models.

592

MMM 2018

(L1 oral)
Objective fusion of multiscale experiments and multiscale models
using Bayesian inference*Surya Raju Kalidindi1
1. Georgia Tech

There is currently no formal framework for fusing the information gathered from multiscale materials
modeling and measurement efforts in ways that optimally inform each other. This is mainly because thespace of governing physics in any selected multiscale materials phenomenon is extremely large (thisincludes all potential model forms and the ranges of parameter values needed to identify the governing
physics as accurately as possible), and the amount of the relevant experimental data is typically limited,
incomplete, and uncertain. Consequently, a direct calibration of the governing physics based on theavailable measurements using standard regression techniques usually does not produce reliable results.
In this paper, we explore the benefits of applying Bayesian inference techniques combined with
reduced-order models and higher-throughput experimental assays in establishing a mathematicallyrigorous framework for addressing the challenge identified above. More specifically, the new framework
will be demonstrated with a very simple case study - the identification of the intrinsic single crystal
material properties from spherical indentation stress-strain measured on a polycrystalline sample.

MMM 2018

593

(L2 invited)
The role of system size, internal disorder, and dislocation patterningon the nature of plastic fluctuations*Jerome Weiss1
1. CNRS/University of Grenoble-Alpes

In classical plasticity theory, plastic flow is assumed to be smooth and homogeneous, as illustrated by the
Orowan’s relation linking the strain-rate to an average mobile dislocation density. However, it has beenlong recognized that in HCP materials plastic deformation can occur through bursts of activity involving
the coordinated motion of numerous dislocations, i.e. dislocation avalanches. More recently, acoustic
emission (AE) measurements showed that these avalanches are associated with scale-free properties suchas intermittency, power-law statistics and fractal patterns. In the words of Mandelbrot, these plastic
fluctuations are wild, thus making tricky the definition of a representative volume element for plasticity.
On the other hand, in multi-slip systems such as FCC materials, short-range interactions between
dislocations lead to the emergence of dislocation patterns characterized by a well-defined internal lengthscale, which can be related with the dislocation density and strain-hardening from the similitude principle
and Taylor’s relation. AE measurements have shown that this patterning frustrate the development of
dislocation avalanches. Consequently, plastic deformation occurs essentially through small anduncorrelated, i.e. mild, fluctuations, coexisting however with few larger and intermittent events leading tofundamental rearrangements of the dislocation substructure. When the system size becomes comparable
or smaller than the internal length scale mentioned above, dislocation patterning can no longer emerge,leading to a dominance of wild (scale-free) fluctuations even for FCC or BCC materials, and a change inthe mechanisms of hardening. In other words, there is size effect on the nature of plastic fluctuations.
Such wild fluctuations might be unwelcomed in the context of nanotechnology. Recent works showed thatthe introduction of disorder (alloying) allows controlling (decreasing) the internal length scale, hence
mitigating wild plastic fluctuations at small scales. These different aspects of plastic fluctuations will be
reviewed.

594

MMM 2018

(L2 oral)
Dislocations associated with stick-slip friction of lubricants inboundary lubrication
Rong-Guang Xu1,2, Hengxu Song2, Stefanos Papanikolaou2, *Yongsheng Leng1
1. George Washington Univ, 2. West Virginia Univ

Improved understanding of squeezing and frictional behaviors of lubricant films under extreme
confinement at nanometer scales can lead to strategies for preventing surface failure and efficient energy
usage. Shearing of a solidified simple nonpolar film under nanoconfinement is studied by using aliquid-vapor molecular dynamics simulation method. We find that, in contrast with the shear melting andrecrystallization behavior of the solidlike phase during the stick-slip motion, interlayer slips within the film
and wall slips at the wall-film interface are often observed. The ordered solidified film is well maintained
during the slip. However, repeated film dilation and collapse of the lubricant film during the stick-slip
friction are observed, which is associated with the nucleation, propagation and annihilation of
dislocations found in the solidlike film. These novel observations may provide new insights into themechanical behaviors of lubricant films and thus improved lubricant design.

MMM 2018

595

(L2 oral)
Comparison of surface-indentation and pillar-compression at thenanoscale of FCC metals: Unification of size effects using 3D Discrete
Dislocation Dynamics*Hengxu Song1,2, Stefanos Papanikolaou1,2
1. west virginia university, 2. johns hopkins university

Plasticity of FCC materials at the nanoscale is size dependent, abrupt and stochastic. These features havehindered the predictability of nanoscale plasticity and consequently, engineering at small scales. Surface
indentation and pillar compression have been the two major pathways towards developing predictive
theories. While surface indentation has strong relevance to engineering, the development of theories has
been more efficient for pillar compression studies. In this work, we connect the flat-punchnanoindentation and uniaxial pillar compression size effects under one unified framework. Throughthree-dimensional Discrete Dislocation Dynamics (3D DDD) simulations, we investigate uniaxial
compression of pillars and also flat-punch nanoindentation, for identical pre-existing dislocation
densities. We study various pillar/punch sizes (0.25 –8 μm) and experimentally relevant initial dislocation
densities. We demonstrate that Tabor’s law, which is valid at the macroscale, also extends at thenanoscale, given that statistical averaging is appropriately considered. We develop the statistical theory
behind this nanoscale “Tabor’s law” using probabilistic concepts and we confirm its validity using
3D-DDD simulations.

596

MMM 2018

(L2 oral)
Statistical models for cross slip and reaction rate processes incontinuum dislocation dynamics*Vignesh Vivekanandan1, Peng Lin 1, Anter El-Azab1
1. Purdue University

We tackle the question of temporal coarse graining in dislocation dynamics with a special attention tocross slip and dislocation reaction rates. The time series approach was used to analyze the statisticalproperties of these processes from discrete dislocation dynamics. The statistical data required to perform
this analysis is obtained using the method of dislocation dynamics simulation. The temporal correlations
and correlation times of cross-slip and short-range reactions were computed. The correlation time wasused as a coarse graining time-scale in continuum dislocation dynamics. Using this mesoscopic
time-scale, a coarse grained stochastic representation of cross slip and dislocation reactions has beenachieved and implemented in continuum dislocation dynamics. We compare the performance of two
discrete dislocation dynamics approaches, the microMegas model and ParaDiS, on the results of thecoarse grained cross slip and junction formation rates, and, in turn, on the dislocation microstructure
predicted by continuum dislocation dynamics.

MMM 2018

597

(L2 oral)
Designing Interfaces: a combinatorial approach to geometricalmanipulation of interfaces*Hakan Yavas1, Alberto Fraile1, Ondrej Man3, Tomas Polcar1,2
1. Czech Technical University, 2. University of Southampton, 3. CEITEC

The selection and design of modern high-performance engineering materials are driven bycontrolling and optimizing varieties of mechanical and thermal properties such as strength,
ductility, plasticity, and toughness. Nanoscale metallic multilayers (NMMs) are the relatively
new class of materials with a high potential of changing their conventional counterparts thatare using in the high-end applications, i.e. nuclear, space and aerospace. The superior properties of
the NMMs are mostly correlated with the advanced design and fabrication of the interfaces ata very confined space (2- 20 nm). In this presentation, we will represent a novel nanoscaleinterface architecture of the Zr-Nb NMMs that are fabricated by following epitaxial and physicalvapor deposition based film-growth techniques. Then, we will discuss tslip transition at different
characters boundaries to shed a light into transition between various deformation mechanisms at aconfined volume. Additionally, we will also present the dynamic response maps of the molecular dynamics
simulations and correlate them to dislocation activity. Finally, we will utilize a machine learning based
approach to reveal hidden features of the slip and deformation mechanisms that might guide us toachieve theoretical limit of the strength.

598

MMM 2018

(L3 invited)
Intermittent micro-plasticity and its relation to dislocation structure - alinear stability analysis.
*Peter M Derlet1, Gábor Péterffy2, Péter Dusán Ispánovity2
1. Paul Scherrer Institut, 2. Department of Materials Physics, Eötvös University

The relationship between dislocation network stucture and plasticity is studied via a linear stability
analysis of the evolving dislocation configuration. This is done for a simplified one dimensional model ofinteracting dislocation pile-ups, which exhibits a mean-field depinning transition at yield, and for a twodimensional dislocation dynamics model always in a state of criticality that is well described by a jamming
transition. We find that the obtained eigen-modes prior to a plastic event play an important role incharacterizing the discrete micro-plastic regime of simulation, reflecting the degree of criticality thesystem is in, the nature of how a plastic event occurs and its eventual evolution into an avalanche.

MMM 2018

599

(L3 invited)
Discrete dislocation dynamics simulations of complexity in crystal
plasticity: strain burst statistics and machine learning*Lasse Laurson1
1. Aalto University

First, I will present an overview of our recent studies focusing on the statistical properties of strain
avalanches in crystal plasticity as observed in discrete dislocation dynamics (DDD) simulations. These
encompass various scenarios, including two and three dimensional DDD models, and considering systems
with and without an additional quenched pinning field (due to, e.g., precipitates) interacting with thedislocations. I discuss the results from the perspective of two main mechanisms affecting the nature of
dislocation dynamics and hence the deformation process: dislocation (de)pinning and dislocation(un)jamming.
Second, I will briefly present our very recent efforts to apply machine learning to predict the properties ofthe stress-strain curves of individual microscale samples using features of the initial, pre-exisiting
dislocation network as input. The resulting predictability of the deformation process is found to evolvewith strain in a non-monotonic fashion, something we attribute to the stochastic nature of thedeformation avalanches.

600

MMM 2018

(L3 oral)
Nanoindentation in the ultra-nano scale: Microstructure-property
relationships using statistical approaches
Hengxu Song1,2, Ryder Bolin1, Michael Tzimas1, *Stefanos Papanikolaou1,2
1. west virginia university, 2. johns hopkins university

Due to the difficulties of tensile/compressive tests at small length scales, nanoindentation is widely used
towards unveiling crystalline mechanical properties. However, crystal plasticity limits the understanding of
nanoindentation results at depths below 500nm: the Indentation Size Effect (ISE) in these scales leads tovery noisy and unclear data, with the measured hardness/stiffness being difficult to ‘translate’ intofeatures of the material microstructure. In this work, we demonstrate two statistical approaches toinvestigate the ultra-nano regime of FCC metals towards unveiling crystalline properties: First, we noticethat indentation together with in-plane tension consist of a phase diagram of the sample elasto-plastic
property. The elastic-plastic transition during indentation is naturally continuous for large dislocation
densities. In the large dislocation density regime, through the development of scaling functions for an
appropriately defined plasticity order parameter, we connect statistically the bulk crystal plasticity
transition with nanoindentation. Second, for low dislocation densities, we utilize the noise of theload-depth nanoindentation curves and the post-indent nanoindentation images to statistically connect
experiments with 3D discrete dislocation dynamics simulations. We develop a machine-learning approachthat can be used for nanoindentation and predict experimentally relevant pre-existing dislocation
densities in single crystalline FCC grains.

MMM 2018

601

(L4 invited)
Dynamic phases, pinning, and pattern formation for driven dislocationassemblies*Cynthia Reichhardt1, Caizhi Zhou2, Charles Reichhardt1, Irene Beyerlein3
1. Los Alamos National Laboratory, 2. Missouri University of Science and Technology, 3. University of California, Santa
BarbaraWe examine driven dislocation assemblies and show that they can exhibit a set of dynamical phases
remarkably similar to those of driven systems with quenched disorder such as vortices insuperconductors, magnetic domain walls, and charge density wave materials. These phases include
pinned-jammed, fluctuating, and dynamically ordered states, and each produces distinct dislocation
patterns as well as specific features in the noise fluctuations and transport properties. Our work suggeststhat many of the results established for systems with quenched disorder undergoing plastic depinningtransitions can be applied to dislocation systems, providing a new approach for understanding patternformation and dynamics in these systems.

602

MMM 2018

(L4 oral)
Effect of solute atoms and Peierls stress on the critical behaviour of
discrete dislocations*Peter Dusan Ispanovity1, Gabor Peterffy1, Peter M. Derlet2
1. Eotvos Universty, 2. Paul Scherrer Institut

It is well-known from micropillar and acoustic emission experiments that plastic strain accumulates insudden avalanche-like events. Based on the statistical analysis of these bursts it is now apparent thatplastic deformation can be described as a critical phenomenon. A lot of subsequent attention has been
paid to understand the fundamental nature of this criticality: whereas the analysis of experiments suggeststhat scale-free behavior is characteristic only to the onset of yield, discrete dislocation dynamic (DDD)
simulations hint at a more involved picture. Namely, the dynamics of the system is of glassy nature, where
power-law distributions arise irrespective of the distance to the yielding threshold. These DDD simulations
represent pure systems where neither Peierls stress nor any kind of impurities impede dislocation motion.
In the talk we will discuss how addition of these realistic ingredients affect the critical behavior in terms ofrelaxation properties, avalanche statistics, dynamic correlations and system size dependence.

MMM 2018

603

(L4 invited)
Temporal and spatial plastic instability of micrometer-scaled materials
1

1

1*Yinan Cui , Giacomo Po , Nasr Ghoniem

1. Mechanical and Aerospace Engineering Department, University of California, Los Angeles

Suppressing plastic instability is a critical goal to improve deformability and ductility in a wide variety ofengineering applications. At temporal scale, plastic instability is manifested as intermittent burst anddislocation avalanches, leading to uncontrollable deformation. At spatial scale, plastic instability is
manifested as the onset of flow localization, leading to sudden failure. While insight in the physical originof plastic instability accumulates, the need for a quantitative description of temporal and spatial plastic
instability remains challenged due to the lack of clear quantitative relation correlating deformation
behavior with microstructural features. Here, we will discuss the quantitative description method of
temporal and spatial plastic instability, and present a very simple model to predict them based on the ideaof stochastic activation of dislocations sources. Combined with systematic discrete dislocation dynamics
simulations, we will present that even though power law scaling is widely used to describe the statistical
distribution of strain burst magnitude, power law scaling does not tell the whole story at temporal scale. At
spatial scale, highly-irradiated materials will be taken as an example. We will unravel the mystery of how
and why irradiation-induced defects enhance or inhibit plastic instability, and how and why dislocation
channels arise, and what governs their width.

604

MMM 2018

(P1-66)
Molecular dynamics study on temperature and loading ratedependence of nano-indentation pop-in load
*Yuji Sato1, Shuhei Shinzato1, Takahito Ohmura2, Shigenobu Ogata1,3
1. Department of Mechanical Science and Bioengineering, Osaka University, 2. Research Center for Structural
Materials, National Institute for Materials Science, 3. Center for Elements Strategy Initiative for Structural Materials(ESISM), Kyoto University

Nano-indentation test is commonly used to investigate mechanical properties of materials in nano-scale.
In nano-indentation test, a displacement burst behavior of indenter, “pop-in”, can be usually observed.
Pop-in is considered to be attributable to dislocation nucleation phenomena, thus pop-in can beassociated with incipient plasticity in materials. It is well-known that the pop-in occurs when a shear stressalong a slip system beneath the indenter reaches near theoretical shear strength. However, thetemperature and loading rate dependency of pop-in load is still unclear. In this study, using atomistic
simulation with EAM interatomic potentials, we computed stress dependent activation energy ofhomogeneous dislocation nucleation event in BCC Fe and Ta under actual complex stress condition
beneath a spherical indenter. Based on the computed activation energy and stochastic modeling method,temperature and loading-rate dependent probability distribution of the critical indentation load was
successfully predicted. Eventually, we compared the atomistic predictions with experiments, andconfirmed validity of our atomistic modeling.

MMM 2018

605

(P1-67)
Geometry of curved surface and energetics of in graphene withdefects*Ako Kihara1, Xiao-Wen Lei1, Akihiro Nakatani2
1. Dept. of Mechanical Engineering, Univ.of Fukui, 2. Dept. of Adaptive Machine Systems, Osaka Univ.

Two-dimensional (2D) materials have attracted attentions as unique functional materials.
Among them, graphene is well-known as a fundamental structure of 2D materials of nano-carbon.
In 2D materials, lattice defects, such as dislocations and disclinations, cause out-of-plane deformation.
For example, carbon nano-cone or nano-horn is formed geometrically by the wedge disclination ofgraphene sheet (GS).
In this study, we focus on the fundamental mechanism which can explain how the shape of 2D materials
with defects is determined.
Typical four structure models of GS with defects are studied, i.e. positive perfect wedge disclination,negative perfect wedge disclination, positive partial wedge disclination, and negative partial wedge
disclination.
The partial wedge disclinations are implemented by the array of edge dislocations in which the local
structure consists of pentagon-heptagon atomic bonds.
Then the equilibrium configuration is calculated by using large-scale atomic/molecular massively parallel
simulator (LAMMPS).
The obtained surfaces are examined by fitting to analytical test functions.
All results of out-of-plane displacement z are organized by a universal form of z=rf(θ), in a cylindricalcoordinate (r, θ, z), in which f(θ) is an appropriate function of θ.
This result means that the all models of GS are represented as conical surfaces in a broad sense.
From a local viewpoint, according to the distribution of atomic site potential energy, it is observed that theenergy values at atoms in pentagon ring are relatively high, but the energy values at atoms in heptagon
ring are relatively low.
From a global viewpoint, the energy values decrease with increasing distance r from the core ofdisclination.
After a detail examination, we found the site potential energy is proportional to the square of curvature.
The fundamental knowledge obtained would be applicable to desgin/control the shape of 2D materials.

606

MMM 2018

(P1-68)
Collective motion of dislocation associated with local plasticity
initiation and subsequent behavior in bcc metals*Takahito Ohmura1
1. National Institute for Materials Science

Nanoindentation-induced mechanical behavior was investigated for bcc metals. Pop-in event that
corresponds presumably to local plasticity initiation was detected on load-displacement curves withmajor parameters of critical load Pc and corresponding excursion depth deltah. In a plot of Pc vs deltah,
the Pc increases monotonically with the deltah, and the slope depends on alloy systems. On the other
hand, brittle-to-ductile transition temperature (BDTT) also depends on the alloy systems, and the alloy
with higher BDTT shows higher slope in the Pc vs deltah plot. Dislocation structures underneath theindenter were observed through TEM before and after a pop-in event. No dislocations were observed
before initiation while considerable dislocations were generated right after the event. These results
suggest that dislocation nucleation and multiplication occur drastically upon plasticity initiation based oncollective dislocation motion. Since the BDTT is closely consistent with pop-in event, crack propagation
might be related with plasticity initiation. Statistical analysis of the events suggests physical models of thephenomena. In the case of the first pop-in event, the probability of the event to the Pc is Gaussian
distribution, which indicates that a thermally activated process dominates the event. For the second orlater cases, the probability to the magnitude of the event shows power law function like
Gutenberg-Richter model, which suggests a dislocation avalanche.

MMM 2018

607

Symposium M(M1 invited)
Structure-property Relations in Sheared Dense Flocculated
Suspensions*Jan Vermant1
1. ETH Zurich

Rheological properties of dense attractive colloidal suspensions are characterized by time anddeformation history dependent phenomena, which when reversible, are know as thixotropy. In the present
work we will use a series of advanced characterization tools to elucidate the structure property relations
in such systems. High frequency rheology is used to investigate the details of the colloidal interactions
betweenthe particles and aggregates. Superposition rheology and 2D small amplitude oscilatory strain areused to investigate the mechanical properties, combined with stress jumps. Finally, high speed confocal
rheoscopy is used to provide structural information on length scales of up to several 100 particles. The
experiments show the role of microstructural anisotropy and and heterogeneity of the structure arecaused by hydrodynamic stresses as particles and aggregates go on a "colloidal merry go round" during
shear flow. The evolution of microstructural descriptors as a function of rate and strain is discussed.
Comparison with recent simulation result on large scale structures will be included.
the goal is to give a hollistic view of the structure property relations in such systems.

608

MMM 2018

(M1 invited)
Soft deformable colloids make strong liquids with stress-driven
relaxation
Nicoletta Gnan1, *Emanuela Zaccarelli1
1. CNR Institute for Complex Systems, Rome

Fragility is a key concept in the glass transition which describes the dependence of the structural
relaxation time on the control parameter (packing fraction $\phi$ or temperature) upon approaching
dynamical arrest. Recently, Mattson et al[1] showed a connection between softness and fragility incolloidal systems, but a deep understanding of its microscopic origin still remains elusive. Numericalsimulations represents a strong ally to elucidate this problem, but in most cases softness is tuned bymodifying the pair-potential parameters allowing particles to overlap to a certain extent, while neglecting
crucial aspects that contribute to the elastic properties of realistic particles. On the other hand, morerefined numerical models of soft particles are challenging to simulate due to the presence of internal
degrees of freedom of high computational cost. As a consequence simple models like the Hertzian
potential cannot capture mechanisms such as particle deformations, making the concept of "softness'' insimulations and experiments very different.
To fill this gap, I will discuss a new model of 2D polymer rings with tunable softness which undergo
substantial deformation at high densities. The ability to deform has a strong impact on the dependence ofthe relaxation time on $\phi$ which change from fragile-to-strong behavior. In addition, at high packing
fractions, dynamics is controlled by an intermittent particle motion which gives rise to a compressed
exponential decay of the self-intermediate scattering function. This behavior can be rationalized in termsof deformed rings that act as stress dipoles[2,3]. This simple model thus provides microscopic insights
into two mechanisms which are of a deep interest in soft matter: the fragility dependence on softness andthe occurrence of a compressed exponential decay in dynamical correlation functions.
[1] J. Mattsson, H. M. Wyss, A. Fernandez-Nieves, K. Miyazaki, Z. Hu, D. R. Reichman and D. A. Weitz,
Nature 462, 83 (2009).
[2] L. Cipelletti, S. Manley, R. C. Ball and D. A. Weitz, PRL 84, 2275 (2000).
[3] E. E. Ferrero, K. Martens and J.-L. Barrat, PRL 113, 248301 (2014).

MMM 2018

609

(M1 oral)
Linear viscoelasticity on matter out of equilibrium*Leticia Lopez-Flores1, Magdaleno Medina-Noyola1, Jose Manuel Olais-Govea1,2, Martin
Chavez-Paez1
1. Universidad Autonoma de San Luis Potosi, 2. Instituto Tecnologico y de Estudios Superiores de Monterrey

The recent non equilibrium self consistent generalized Langevin equation (NE-SCGLE) theory ofirreversible process in liquids has permitted to obtain a description of non- equilibrium processes
involved in the arrested spinodal decomposition due to sudden and deep quenches inside the spinodal
region. For a simple model liquid, where the system could be modeling by a hard sphere plus an attractive
Yukawa tail, this theoretical approach predicts that the spinodal line is the borderline between theergodic and the arrested states. Also, by means of this approach has been determined a border between
phase separation and gelation, besides providing the corresponding dynamic properties to each phase.
This work addresses a general method to obtain the linear viscoelastic properties of non- equilibrium
processes involved in the spinodal decomposition when the system has been quenching inside thespinodal region. We show an example of the normalized shear viscosity as a function of the waiting time.
This scheme offers the opportunity to describe the linear viscoelasticity and the diffusion mechanics as
the waiting time elapses. Furthermore this approach is able to describe gelation effects, it leads naturally
to a diverging shear viscosity at glass and gelation transition points.

610

MMM 2018

(M1 oral)
Memory effects in functional polymers: The interplay between
entropic elasticity and kinetic arrest*Fathollah Varnik1, Elias M. Zirdehi1
1. Ruhr-University Bochum

In contrast to metallic alloys, the shape memory effect in polymers is not based on a change of crystalline
phases but on the interplay between entropic elasticity and kinetic arrest. Consequently, the shaperecovery process, during which chains start to recover their entropically most favorable conformation,strongly depends on preparation protocol, frozen stresses and the selected recovery temperature. In thiswork, we study these issues via molecular dynamics simulations with a special focus on aging processes
and the effects arising from the presence of small molecules. It is shown that aging leads to irreversible
plastic rearrangements and a resulting increase of residual strain during shape recovery. At a fixed
temperature, this process is enhanced if small molecules are added to the system. Interestingly, thetriggering temperature depends in a non-monotonic way on the size of added molecules. This observation
is rationalized in terms of diffusion coefficient of added molecules and the strength of their coupling to
the polymer matrix.

MMM 2018

611

(M1 oral)
Modelling and Experimental Verified Coupled Visco hyper
electro-elastic Behaviour of Dielectric Elastomer Circular Actuator*ARPIT SRIVASTAVA1, Sumit Basu1
1. IIT KANPUR, INDIA -208016

Soft dielectric elastomers, with very low stiffness and high permittivity and electric breakdown strength,
hold promise as candidate materials for a variety of applications including, in energy harvesting, as
actuators and biological muscles. As actuators in particular, a number of applications have emerged
where, utilising coupled electro-hyperelasticity under very high electric fields in thin, membrane-likestructures, large actuation has been achieved. Moreover, the theoretical framework forelectro-hyperelasticity of these materials has also been established. But long time durability of thesedevices, is still a matter of concern. The concern arises primarily from the fact that these soft elastomers
not only physically age with time, but are also highly rate dependent. We have used an equi-biaxiallypre-stretched circular dielectric elastomer membrane attached to a rigid frame with a load hung at thecentre to demonstrate the effects of viscoelasticity. The membrane is then loaded with oscillating voltage
and the motion of the center of the membrane is tracked with a laser displacement sensor, over manytime periods. As the membrane is taken through a large number of cycles,the response slowly drifts. For avery soft elastomer like VHB, the drift can be sometimes discerned in as few as 20-30 cycles of operation.
To model the deformation and the drift with time, a coupled electrostatic, visco-hyperelastic large
deformation model for the elastomer has been incorporated into an explicit Finite Element framework. We
have been able to reproduce the experimental response of the VHB membrane fixed to a rigid frame very
accurately. Though the modelling has been verified for VHB only, the framework is general enough to be
used to assess the effectiveness of any dielectric elastomeric material, used as a membrane under any
three dimensional electro-mechanically loaded configuration.

612

MMM 2018

(M2 oral)
MMM in aircraft industries: use cases for simulation of additive
manufacturing*Annett Seide1, Thomas Goehler1, Roman Sowa2
1. MTU Aero Engines AG, 2. MTU Aero Engines Polska Sp. z o. o.

MTU Aero Engines is Germany's leading engine manufacturer and an established global player in theindustry. Computational Materials Engineering and Additive Manufacturing (AM) of high temperaturealloys are only two of the forward-looking techniques MTU is working with. To represent industrial
application of MMM it will be displayed how MTU uses Materials Simulation techniques for AM process.
For a materials engineer some of the key aspects of the AM process are texture, microstructure (e.g.
precipitate size and phase fraction) and mechanical properties (e.g. yield stress). Consequentially, the AM
simulation includes laser/electron beam scanning strategy, interaction of beam and powder,microstructure evolution and crystal plasticity.
AM simulation at MTU covers amongst others the following typical use cases: (1) description of theinfluence of chemical composition on microstructure and mechanical properties, (2) determination of
surface roughness depending on scanning strategy and (3) heat treatment optimization with regard toyield strength and texture. All of those show the strong coupling between material/manufacturing history
and resulting material properties.
The main focus will be on the use case regarding determination of surface roughness as a function of
scanning strategy. The goal will be to present requirements and methods related to the specific use caseas well as an insight into the industrial application itself. We will give one example of successful
integration of materials modeling and simulation as tools to tailor materials properties through processparameter optimization.

MMM 2018

613

(M2 oral)
VISCOELASTIC BEHAVIOUR OF HETEROGENEOUS MATERIALSSTUDIED THANKS TO AN EXTENSION OF CRAFT SOFTWARE IN
HARMONIC REGIME*Julien Boisse Boisse1, Stéphane André1, Laurent Farge1
1. University of Lorraine, France

Following the routes opened by the resort to spectral solvers applied on real composite microstructures
to analyse the homogenization problem in elasticity, we extended a FFT approach implemented in the
CRAFT solver [1] to viscoelastic materials. The idea is to propose a virtual Dynamical Mechanical Analysis
experiment applied on heterogeneous microstructures. DMA performs a frequency analysis of the transfer
function of the material by applying a sinusoidal harmonic steady-state regime. The transfer function(modulus, relaxation, compliance... quantities) is complex with classical storage and loss components (realand imaginary parts) [2]. It offers a full frequency characterization of the material constitutive law which
can be applied afterwards in all cases of temporal excitations. CRAFT code and its central
Lippmann-Schwinger equation are then solved in complex variables.
Examples will be given of various microstructures made of two individual viscoelastic constituentsassumed to behave according to a standard 3-parameter Voigt rheological model (spring connected inseries with a Voigt unit [2]). As already shown [3], the key resulting effect on the homogenized effective
material is the appearance of an additional fading memory term i.e. of a transfer function with broadened
spectrum of relaxation times. Following this fact and connections established with fractional rheologicalmodels, we will show that a very efficient effective model can describe the mesoscopic behaviour of agreat variety of microstructures.
REFERENCES[1] H. Moulinec, P. Suquet, A numerical method for computing the overall response of nonlinear
composites with complex microstructure, Comput. Methods Appl. Mech. Engrg., 157, 69-94, 1998.
[2] N.W. Tschoegl, The Phenomenological Theory of Linear Viscoelastic Behavior, An Introduction,
Springer Verlag, 1989.
[3] R. Brenner, P. Suquet, Overall response of viscoelastic composites and polycrystals: exact asymptotic
relations and approximate estimates. Int. J. Sol. Struct., 50(10), 1824-1838, 2013.
[4] S. Andre, Y. Meshaka, C.Cunat, Rheological constitutive equation of solids: a link between models
based on irreversible thermodynamics and on fractional order derivativeequations, Rheologica Acta, 42, 500-515.

614

MMM 2018

(M2 oral)
Multi-scale modelling of Zener Pinning during the solid solution
treatment of a Nickel-based Superalloy*Magnus Jack Anderson1, Jonathan Benson1, Christos Argyrakis2, Jeffery William Brooks1, Hector
Christian Basoalto1
1. The University of Birmingham, 2. Rolls-Royce plc

The ability to manufacture components to provide location specific properties is needed to improve
component performance, increase component life and reduce cost. In Ni-base superalloys, the grain size
and the strengthening precipitate dispersion have a large impact upon creep and fatigue behaviour. It is
possible to obtain dual microstructures across turbine discs through careful design of the solid solution
treatment process, optimising mechanical properties where needed.
A simulation tool has been developed to assist in the design of such heat treatments, modelling grain
growth with Zener pinning precipitates that evolve during thermal processing. The work focuses onmodelling grain growth in the Ni-based Superalloy RR1000, containing a tri-modal γ’particle dispersion
where the largest particles pin grain boundary movement. A multi-scale approach has been developed tocapture the evolution of the precipitates, using a statistical approach to capture the kinetics of thesecondary and tertiary particle populations.
The grain growth is described using a two-dimensional model, including the evolution of primaryγ’precipitates. The level-set method is used to model the grain boundaries and precipitate-matrix
interfaces implicitly. A multi-component mean-field description has been applied to simulate the kineticsof the precipitates. A morphologically explicit mean-field growth rate has been developed to describe thekinetics of the primary particles, consistent with the description of the secondary and tertiary particles.
The proposed model shows good potential in capturing grain growth kinetics and may serve as a useful
tool for simulating solid solution treatments in multi-modal nickel-based superalloys.

MMM 2018

615

(M2 oral)
Assessment of residual stresses in welds made of multiphase alloys*Victor De rancourt1, Sylvain Flouriot1, Benjamin Sarre2, Benoit Panicaud2, Guillaume Geandier3
1. Commissariat a l'energie atomique, 2. Universite de Technologie de Troyes, 3. Universite de Lorraine - CNRS

Welding can induce strong changes in the parent metal so that the mechanical behavior of a welded
structure often remains unclear. Mechanical tests are therefore directly performed on welds to assess themechanical performance of a welded structure. The purpose of such mechanical tests consists in showing
evidences of safety margins. In the future, the growing knowledge on the physics of welding accompaniedwith better welding monitoring tools may replace the need for such tests. Such developments needenhanced multiphysics and multiscale models validated against experimental measurements.
Nevertheless, the complexity of such models is highly increased in the case of multiphase materials
The present work introduces a modelling framework accounting for coupled phase transformations andstresses. The modelling framework is weakly coupled with temperature which is introduced as an externalparameter in the simulations. The mechanical behavior features an homogenization scheme, chosen as
the β-rule, which accounts for the multiphase nature of the material. The mechanical behavior of eachphase accounts for viscoplasticity. Finally, plasticity is coupled with phase transformation to account fordislocation inheritance. The model is then calibrated on a two-phase α-β titanium alloy, namely
Ti-6Al-4V.
Finite element simulations of welding are then performed. Such simulations are compared withcharacterizations of residual stress fields obtained from X-ray diffraction experiments, which have been
performed recently at the European Synchrontron Radiation Facility. The qualitative comparison between
the experimental characterization and the numerical results is quite good as well as the quantitave
comparison of stress extrema. A more detailed comparison is then made including a comprehensive
parameter analysis. Finally, the discussion will underline the main role of the β-rule homogenization
method on the assessment of residual stresses.

616

MMM 2018

(M2 oral)
Atomic analysis of crystalline nucleation and growth in the
supercooled liquid of glass-forming binary alloy*Masato Wakeda1, Shigenobu Ogata2,3
1. Research Center for Structural Materials, National Institute for Materials Science, 2. Graduate school of engineeringscience, Osaka Univ., 3. Center for Elements Strategy Initiative for Structural Materials (ESISM), Kyoto Univ.

Microstructures of solid metals is significantly affected by solidification process from liquid melts. In
special, nucleation and growth of crystalline nucleus in supercooled liquid dominates the glass forming
ability of amorphous metals or microstructures of crystalline metals. However, key factors affecting
nucleation and growth of crystalline nucleus are still under discussion. In experiments, unveiling thenucleation and growth processes is difficult due to limitation of observable time- and space-scale
resolution. On the other hand, atomic simulation such as molecular dynamics (MD) has a difficulty toinvestigate the crystallization of alloys from liquid state, because the time-scale of the crystallization ofalloys is generally beyond the time-scale of atomic simulation. In order to provide atomistic knowledge ofthe solidification of liquid metal, we in this study focus on the feature of crystalline nucleus in the
supercooled liquid metals. First, we prepared atomic models of glass-forming binary alloy, in which a
spherical crystal nucleus is embedded in advance. The size of embedded nucleus is smaller than thecritical nucleus size. Then we conducted MD simulation under the NPT condition at supercooled liquid
temperature under the constraint of nucleus radius: the constraint is realized by adding a boost potential
to the liquid-crystal composite model. During MD simulations, we evaluated the crystal nucleus fromgeometrical and statistical aspects. The obtained results provide the atomistic insight into the early stageof crystallization of metal.

MMM 2018

617

(M2 oral)
Crystal growth in fluid flow: Nonlinear response effects*Hailong Peng1,2, Dieter Herlach1, Thomas Voigtmann1
1. German Aerospace Center, 2. Central South University

Mechanism of solidification is of great interest both to experimentalist and theorist, as it determines themechanical and thermophysical properties of formed crystalline structures. Many materials, for example,
most polymeric and metallic materials of daily life, are produced from the liquid state as their parent
phase, in the presence of strong flow (e.g., in extrusion or casting processes). Since crystal growth governs
the evolution of the microstructure, detailed knowledge of how crystallization is affected by theprocessing conditions offers an effective way to design and control material properties in applications.
We investigate crystal-growth kinetics in the presence of strong shear flow in the liquid, using
molecular-dynamics simulations of a binary-alloy model. Close to the equilibrium melting point, shear flow
always suppresses the growth of the crystal-liquid interface. For lower temperatures, we find that thegrowth velocity of the crystal depends nonmonotonically on the shear rate. Slow enough flow enhances
the crystal growth, due to an increased particle mobility in the liquid. Stronger flow causes a growth
regime that is nearly temperature-independent, in striking contrast to what one expects from thethermodynamic and equilibrium kinetic properties of the system, which both depend strongly on
temperature. We rationalize these effects of flow on crystal growth as resulting from the nonlinear
response of the fluid to strong shearing forces.

618

MMM 2018

(M2 invited)
Time-dependent active microrheology in dilute colloidal suspensions
Sebastian Leitmann1, Suvendu Mandal4,1, Matthias Fuchs2, Antonio M. Puertas3, *Thomas
Franosch1
1. Innsbruck Univ., 2. Konstanz Univ. , 3. University of Almeria , 4. Heinrich-Heine University Düsseldorf

In a microrheological set-up a single probe particle immersed in a complex fluid is exposed toa strong external force driving the system out of equilibrium. Here, we elaborate analytically thetime-dependent response of a probe particle in a dilute suspension of Brownian particles to a large
step-force, exact in first order of the density of the bath particles. The time-dependent drift velocity
approaches its stationary state value exponentially fast for arbitrarily small driving
in striking contrast to the power-law prediction of linear response encoded in the long-time tails of thevelocity autocorrelation function. We show that the stationary-state behavior depends nonanalytically on
the driving force and connect this behavior to the persistent correlations in the equilibrium state. We
argue that this relation holds generically. Furthermore, we elaborate thatthe fluctuations in the direction of the force display transient superdiffusive behavior.

MMM 2018

619

(M3 invited)
Heterogeneous thermal properties in a glass from molecular dynamics
calculations*Jean-Louis BARRAT1
1. Univ. Grenoble Alpes

It is well known that elastic properties of amorphous systems are heterogeneous, leading to peculiarities
in vibrational spectra and thermal properties. In this work, we investigate the heterogeneity inthermoelastic properties, and more precisely in the thermal expansion coefficient, in a model metallic
glass. We find heterogeneities that are similar - in terms of length scales - to those in elastic constants. It
has been suggested that such heterogeneities could be the reason for "cryogenic rejuvenation" processes
observed under thermal cycling in several experiments. We investigate this hypothesis by comparing thevalues of the local yield stresses with the stresses generated by heterogeneities in thermal dilation.

This work was performed in collaboration with Dr Baoshuang Zhang (University Grenoble Alpes andBeijing Computational Science Research Center).

620

MMM 2018

(M3 oral)
Structural-dynamical phase transition in the phase space of histories
of a polydisperse hard sphere liquid*Matteo Campo1,2, Thomas Speck1
1. Johannes Gutenberg University Mainz, Germany, 2. Graduate School Materials Science in Mainz, Staudingerweg 9,55128 Mainz, Germany

The glass transition is a longstanding problem in condensed matter physics. One of the main points ofdiscussion is whether it is possible to characterize the glass transition using structural arguments,
together with dynamical ones. In fact, it is known that supercooled liquids do not show major changes intheir global (two-point) structure upon cooling while displaying a dramatic change in their dynamical
properties. Recent insights have brought attention to the local structure instead. For a wide range ofmodel glassformers, it is possible to identify certain geometric motifs that minimize the local free energy.
These motifs are referred to as Locally Favourite Structures (LFS) and are found to be growing incorrelation with the slowing-down of the dynamics. Using tools from large deviation theory and statisticalmechanics of histories, it was recently possible to identify for some model glassformers astructural-dynamical transition between the supercooled liquid state and a state composed of trajectories
rich in LFS with very slow dynamics. We present here a study of this transition for a fundamental atomistic
model glassformer, the polydisperse hard sphere liquid. We reveal the first-order nature of the transitionby employing finite size scaling on the length of the histories of the system, and a phase diagram isconstructed as a function of the density versus the local order, and the density versus the degree ofmobility of the system.

MMM 2018

621

(M3 oral)
Time- and History-dependent Structure and Morphology ofvan-der-Waals Liquids forming Physical Gels and Porous Glasses*Magdaleno Medina-Noyola1, Leticia Lopez-Flores1, Jose Manuel Olais-Govea1, Benigno
Zepeda-Lopez1
1. Instituto de Fisica (Laboratorio Nacional de Ingenieria de la Materia Fuera de Equilibrio), Universidad Autonoma de
San Luis Potosi

In spite of its relevance, no universal principle seems to exist that explains how Boltzmann’s postulate S= kB ln W operates for non-equilibrium conditions, predicting, for example, the transformation of liquids
into non-equilibrium amorphous solids (glasses, gels, etc.) in terms of molecular interactions. Here,however, we present evidences that the missing fundamental principle to understand non-equilibrium
states of matter is provided by Onsager’s description of irreversible processes and thermal fluctuations,adequately combined with Boltzmann’s postulate and extended to genuine non-equilibrium conditions[J. Phys.: Cond. Matter 21: 504103 (2009)]. Formatted as the non-equilibrium self-consistent generalized
Langevin equation (NE-SCGLE) theory of irreversible processes in liquids [Phys. Rev. E 82, 061503(2010)], this approach has been shown to provide a fundamental tool for the understanding of the mostessential fingerprints of the transformation of liquids into amorphous solids, such as their aging kinetics ortheir dependence on the protocol of fabrication [J. Chem Phys. 143, 174505 (2015); Phys. Rev. E 96,
022608 (2017)]. In this work we focus on the NE-SCGLE–predicted scenario of the structural andmorphological transformation of van-der-Waals (or "Lennard-Jones--like") simple fluids into hard-sphereglasses at hight densities and temperatures, into physica gels at intermediate densities and low
temperatures, into porous glasses at intermediate densities and even lower teperatures, and into
cluster-cluster aggregates al very low densities and temperatures. As an illustration, we present thevisualization of the non-equilibrium development and arrest of sponge-like structures by arrested spinodal
decomposition. The comparison of the theoretical predictions (based on a simple specific model system),with simulation and experimental data measured on similar but more complex materials, suggests theuniversality of the predicted scenari.

622

MMM 2018

(M4 invited)
Mechanical behavior and emerging morphologies in active matter*Ignacio Pagonabarraga1
1. CECAM, EPFL

Flocks of birds, schools of fishes, or bacterial colonies constitute examples of living systems thatcoordinate their motion. In all these systems their constituent elements generate motion due to energy
consumption and can exchange information or react sensitively to chemical cues in order to move
together or to react collectively to external signals. Artificial systems, such as nanorobots, exploit theheterogeneous compositions of their surface to displace as a result of the heterogeneous chemical
processes that take place in the presence of appropriate chemical substances.
All these systems are intrinsically out of equilibrium in the absence of any external driving. Their collective
properties result as a balance between their direct interactions and the indirect coupling to the medium
in which they displace, and a self-consistent dynamical approach is required to analyze their evolution.
The mechanical balance that determines the states they develop spontaneously make these systems very
versatile and have a natural tendency to for large scale aggregates.
I will consider simple statistical models to address fundamental questions associated to these systems andwill analyze the implications the generic self-propulsion has in the emergence of structures in suspensionsof model self-propelled particles. I will discuss the potential of schematic models to address fundamental
questions, such as the connection of the effective phase diagram and pressure with effective equilibrium
concepts. I will analyze the collective behavior of these emerging morphologues and their response toexternal forcings, as well as how we can understand the resistance to deformation in this type of systems.

MMM 2018

623

(M4 oral)
Time dependent interaction between intruders in granular media*Hisao Hayakawa Hayakawa1, Takahiro Tanabe2
1. Yukawa Institute for Theoretical Physics, Kyoto Univeristy, 2. Graduate School of Advanced Mathematical Sciences,
Meiji University

In this presentation, we will discuss the interactions between two intruders in two dimensional granularenvironment from DEM (discrete element method) simulation and phenomenological theory. Through the
DEM simulation, we found that the interaction is repulsive if there exists a steady flow or the amplitude ofan external oscillation is small enough, but the interaction becomes attractive if the amplitude of theoscillation is large. Such an attractive interaction is known as depletion effect or Casimir effect nearly
equilibrium environments, but it is interesting that both attractive and repulsive interactions can beobserved in out-of-equilibrium systems such as granular media. We also develop a phenomenology basedon an analogous equation to Boltzmann-Enskog equation. The phenomelogy can explain the drag force
acting on one intruder in granular environment and the repulsive interaction between two intruders undera steady flow, but is still far away to describe the attractive interaction between two intruders in highly
oscillated situations.

624

MMM 2018

(M4 oral)
Hystory-dependent shear jamming of granular materials underoscillatory shear*Michio Otsuki1, Hisao Hayakawa2
1. Graduate School of Engineering Science, Osaka Univ., 2. Yukawa Institute for Theoretical Physics, Kyoto Univ.

Granular materials have rigidity above a critical density [1]. Such rigidity transition, known as the jamming
transition, has attracted much attention among researchers in these days. It is well-known that frictionlessgrains under small strain exhibit a continuous transition of the shear modulus G, while recent studies haverevealed that G of frictional grains with harmonic repulsive interaction discontinuously emerges at thecritical density [2].
In this talk, we present our recent numerical results on the shear modulus of frictional grains underoscillatory shear. It is confirmed that the shear modulus depends on the amplitude of the initial oscillatory
shear before the mesurament. Even at densities below the transition point, where isotropic jamming
occurs without shear, the initial oscillatory shear can induce the finite shear modulus. This behavior isconsistent with a transition known as shear jamming [3]. We also discuss the evolution of the force chain
network under the initial oscillatory shear and the conection with the discontinuous shear thickenning.
[1] M. van Hecke, J. Phys.: Condens. Matter 22, 033101 (2009).
[2] M. Otsuki and H. Hayakawa, Phys. Rev. E 95, 062902 (2017).
[3] D. Bi, J. Zhang, B. Chakraborty, and R. Behringer, Nature (London)
480, 355 (2011).

MMM 2018

625

(M4 oral)
Structural predictor for nonlinear sheared dynamics in simple
glass-forming liquids*Trond S. Ingebrigtsen1, Hajime Tanaka1
1. University of Tokyo

Glass-forming liquids subjected to sufficiently strong shear universally exhibit striking nonlinear behavior;for example, a power-law decrease of the viscosity with increasing shear rate. This phenomenon has
attracted considerable attention over the years from both fundamental and applicational viewpoints [1, 2,
3, 4]. However,
the out-of-equilibrium and nonlinear nature of sheared fluids have made theoretical understanding of thisphenomenon very challenging and thus slower to progress. We find here [5] that the structural relaxation
time as a function of the two-body excess entropy, calculated for the extensional axis of the shear flow,
collapses onto
the corresponding equilibrium curve for a wide range of pair potentials ranging from harsh repulsive tosoft and finite. This two-body excess entropy collapse provides a powerful approach to predicting the
dynamics of nonequilibrium liquids from their equilibrium counterparts. Furthermore, the two-bodyexcess entropy
scaling suggests that sheared dynamics is controlled purely by the liquid structure captured in the form of
the two-body excess entropy along the extensional direction, shedding light on the perplexing mechanism
behind shear thinning.
References[1] D. J. Evans, H. J. M. Hanley, and S. Hess. Phys Today 37, 26 (1984).
[2] R. Yamamoto, and A. Onuki. Phys. Rev. E 58, 3515 (1998).
[3] V. Lubchenko. Proc. Natl. Acad. Sci. U.S.A. 106, 11506 (2009).
[4] X. Cheng, J. H. McCoy, J. N. Israelachvili, and I. Cohen. Science 333, 1276
(2011).
[5] T. S. Ingebrigtsen, and H. Tanaka. Proc. Natl. Acad. Sci. U.S.A. 115, 87 (2018).

626

MMM 2018

(P1-69)
FORMATION OF PHYSICAL GELS BY ARRESTED SPINODALDECOMPOSITION IN CHARGED COLLOIDS*Jose Manuel Olais-Govea1, Alonso Gomez-Canales1, Leticia Lopez-Flores2, Martin Chavez-Paez2, Magdaleno Medina-Noyola2
1. Instituto Tecnologico y de Estudios Superiores de Monterrey, 2. Universidad Autonoma de San Luis Potosi

The general understanding of gelation processes is recognized as a major challenge in soft matter field.
Particularly, attractive liquids that suffer a suddenly instantaneous isochoric quench an homogeneousstate to thermodynamic instability could form a physical gel. Although, both experimental and simulation
advances in understanding gelation phenomenon had led to build a general comprehension about
physical gel properties and even had discussed the perform of novel materials as amorphous solids, there
is no a clear theoretical approach to explain these class of ubiquitous materials. In recent work, however,
the general theory referred to as the non-equilibrium self-consistent generalized Langevin equation(NESCGLE) theory, based on a non-equilibrium extension of Onsager’s canonical theory of thermal
fluctuations, was adequately adapted to describe memory effects, protocol-dependent preparation andirreversible aging processes associated with the glass and the gel transitions in attractive simple liquids. In
particular, this theory was capable of predict a glass-glass transition line and propose a glass-gel line in a
monocomponent attractive Yukawa simple fluid (HSAY) by arested spinodal decomposition when thatsystem is instantaneously quenched inside spinodal region. In the present work, we extend this analisys in
the context of a very specific model system, namely, let us consider an screened restricted primitivemodel (YRPM), represented as an electroneutral mixture of charged hard spheres embedded in adielectric medium of uniform dielectric constant. Additionaly we show pertinent comparisons between
our theoretical results and the gel formation in a mixture of equally-sized oppositely charged colloids bothexperimentally and by means of computer simulations.

MMM 2018

627

(P1-70)
How to improve the ductility of CuZr BMGs based on cyclic
pre-straining: MD simulations and mechanical testing*Jonathan Amodeo1, Oriane Baulin1, Damien Fabregue1, David Rodney2
1. MATEIS, Univ. Lyon 1, 2. ILM, Univ. Lyon 1

Bulk metallic glasses are currently used as technical materials(e.g.magnetic transmitters, golf clubs,
brazing materials) for their high elastic and strength properties, significantly larger than their crystalline
counterparts. However, their use is generally limited to elastic engineering applications due to their
quasi-brittle behaviour caused by the early initiation of localized shear bands upon plastic deformation.
In this study, we used a combination of atomistic simulations and compressive mechanical testing toinvestigate the role of mechanical pre-cycling on the stress response and elementary deformation
processes of a CuZr glass. In the experiments, unprestrained Zr52.5Cu17.9Ni14.6Al10Ti5(Vitreloy 105) samples
show a plastic strain before failure of about 3% strain and a yield strength of about 1900 MPa. After
applying a limited number of cycles with a maximum amplitude of 400 MPa (n=2, 4 or 6), the plastic strain
increases up to 15%. It remains however in the same low range for higher pre-cycling amplitudes of 700
and 1200 MPa. MD simulations performed on Cu64.5Zr35.5thin films show that (i) unprestrained samples
deform due to a local shear banding process, (ii) pre-cycled samples are characterized by the occurrenceof homogeneous deformation zones with a concentration that depends on the cyclic stress amplitude i.e.
the lower the stress amplitude of the pre-cycles, the higher the proportion of homogeneous deformation.
This emphasizes the existence of a critical stress under which the main shearing process of CuZr BMGs
changes due to pre-cycling, reducing shear localization and the early occurrence of cracks. These results
are confirmed by the statistical analysis of experimental vein patterns on fracture surfaces that show larger2

cusps (φ>3500 μm ), attributed to highly localized plastic events, only in the cases of samples
pre-cycled at high stress amplitude.

628

MMM 2018

(P1-71)
Modeling plastic deformation of amorphous solids from atomic scale
mechanisms*Francesca Boioli1, Tristan Albaret2, David Rodney2
1. LEM, CNRS-ONERA, Chatillon, France, 2. ILM, University of Lyon 1, France

Amorphous solids are characterized by high strength and low ductility. The latter property is aconsequence of the localization of the plastic deformation in shear bands, which leads to catastrophic
failure. As a consequence, understanding the localization of plastic deformation and the formation ofshear bands is of utmost importance. Generally, it has been accepted that local irreversible
rearrangements of small clusters of atoms, Shear Transformations (STs), are the elementary processes
involved in the deformation of amorphous systems and several mesoscale models based on STs havebeen proposed. Still the fundamental mechanisms underlying ST occurrence and shear bands formation
are not yet clear. In this context, atomistic simulations can provide significant details that would otherwise
be unavailable.
In this work we characterize shear transformations (STs) at the atomic scale in a model of amorphoussilicon using a mapping on Eshelby inclusions. First, by using Nudged Elastic Band calculations, wemeasure the energy barrier against ST activation. Analyzing different paths leading to either an isolated ST
or an avalanche, we show that the barrier is systematically controlled by the first ST with an activation
volume equal to the effective volume of the ST at the activated state, which represents only a fraction of
the complete ST volume. The activation volume is also found smaller for avalanches, presumably becauseof accumulated local damage. Furthermore, we investigate the dynamic process of ST formation,
determining the characteristic time involved in the development of STs and the influence of the strain rateon the STs distribution and organization. This work provides essential information to build reliable
mesoscale models of plasticity.

MMM 2018

629

(P1-72)
Thermally Activated Creep and Constant Shear Rate Deformation in
Amorphous Materials*Samy MERABIA1, Julien LAM2, François DETCHEVERRY1
1. CNRS and Universite Lyon 1, 2. Université de Bruxelles

In this contribution, we shall discuss two issues related to the deformation of amorphous materials.
The first issue concerns transient creep also called Andrade creep, characterized by strain slowly
increasing algebraically with time, a regime interrupted by fluidization and eventually steady flow. Here wecharacterize creep and fluidization on the basis of a mesoscopic viscoplastic model that includes
thermally activated yielding events and a broad distribution of energy barriers, which may be lowered
under the effect of a local deformation. We relate the creep exponent observed before fluidization to thewidth of barrier distribution and to the specific form of stress redistribution following yielding events. We
show that Andrade creep is accompanied by local strain hardening driven by stress redistribution and findthat the fluidization time depends exponentiallyon the applied stress, in qualitative agreement with experiments.
The second issue to be discussed concerns constant shear rate deformation of molecular glasses. Recent
experiments on polymer glasses [2] have demonstrated enhanced mobility in the preyield regime
accompanied by a narrowing of the distribution of segmental relaxation times. Yet, the mechanisms atplay remain partially understood.
In the light of the mesoscopic model [1], we characterized the evolution of the distribution of energy
barriers during constant shear rate deformation. Our simulation results account for enhanced mobility
and the narrowing of the distribution of relaxation times. Good agreement is obtained with theexperimental observations of Bending et al., without any adjustable parameter.
All the simulation results are interpreted in the light of a mean-field analysis, and should help inrationalizing the creep and deformation phenomenology as observed in disordered materials.
References[1] S. Merabia and F. Detcheverry, Thermally activated creep and fluidization in flowing disorderedmaterials, EPL (2016)[2] B. Bending, K. Christison, J. Ricci, and M. D. Ediger, Measurement of Segmental Mobility during
Constant Strain Rate
Deformation of a Poly(methyl methacrylate) Glass, Macromolecules (2014)

630

MMM 2018

(P1-73)
Numerical analysis of shrinkage process based on the experimental
data*Barbara Kucharczykova1, Hana Simonova2, Petr Frantik2
1. Brno University of Technology, Faculty of Civil Engineering, Institute of Building Testing, 2. Brno University ofTechnology, Faculty of Civil Engineering, Institute of Structural Mechanics

The shrinkage process in cement based materials is still an actual problem of civil engineers andmanufacturers of cement based composites, such as concrete, mortars, floor compounds etc. There aremany standardized and non-standardized testing techniques which are used for estimation/determinationof real value of shrinkage process during the whole time of material ageing, unfortunately the results areoften inconsistent, especially because of different measurement equipment, dimensions of the test
specimens and initial time of the start or evaluation of measurement. There are also incomplete
experimental data of early-age volume changes, which can be used for numerical prediction models. The
article focuses on the implementation of the experimentally obtained data to the shrinkage model B4designed by research group of prof. Z. P. Bazant. The results show that especially the early-age part ofreal-measured shrinkage process does not correspond with the prediction model. In order to improve thecorrelation between the measured process and the prediction model, it is necessary to adjust the existingcorrection factors. The published results were obtained within the implementation of the research project
granted by the Czech Science Foundation - project No. GA17-14302S.

MMM 2018

631

Symposium N(N1 invited)
Increasing the power of accelerated molecular dynamics methods andplans to exploit the coming exascale*Arthur F Voter1
1. Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico

Many important materials processes take place on time scales that far exceed the roughly one
microsecond accessible to molecular dynamics simulation. Typically, this long-time evolution ischaracterized by a succession of thermally activated infrequent events involving defects in the material. In
the accelerated molecular dynamics (AMD) methodology, known characteristics of infrequent-event
systems are exploited to make reactive events take place more frequently, in a dynamically correct way.
For certain processes, this approach has been remarkably successful, offering a view of complex
dynamical evolution on time scales of microseconds, milliseconds, and sometimes beyond. We haverecently made advances in all three of the basic AMD methods (hyperdynamics, parallel replica dynamics,
and temperature accelerated dynamics (TAD)), exploiting both algorithmic advances and novel
parallelization approaches. I will describe these advances, present some examples of our latest results,
and discuss what should be possible when exascale computing arrives in roughly four years.

632

MMM 2018

(N1 invited)
Atomistic processes at interfaces on extended timescales*Jutta Rogal1
1. Ruhr University Bochum

Obtaining atomistic insight into the fundamental processes during phase transformations and theirdynamical evolution up to experimental timescales remains one of the great challenges in materials
modelling. In particular, if the mechanisms of the phase transformations are governed by so-called rare
events the timescales of interest will reach far beyond the applicability of regular molecular dynamics
simulations. In addition to the timescale problem the simulations provide a vast amount of data in thehigh-dimensional phase space. A physical interpretation of these data requires the projection into a
low-dimensional space and the identification of suitable reaction coordinates.
In this presentation, I will give an example of our analysis of the atomistic processes at a complex phaseboundary during a solid-solid phase transformation. The migration of the phase boundary proceeds via
concerted multi-atom processes on a complex energy landscape. Here, we employ an adaptive kinetic
Monte Carlo (AKMC) approach together with driven adiabatic free energy dynamics (d-AFED) toinvestigate such processes at the interface between the body-centered cubic and A15 phase inmolybdenum.

MMM 2018

633

(N2 invited)

Modeling Microstructure Evolution in Rapid Solidification Phenomena
Using Structural Phase Field Crystal Models*Nikolas Provatas1
1. McGill University

This talk will study several closely connected density functional type theories that employ both short andlong range, rotationally invariant, multi-point particle interactions. Collectively, these models give rise to aclass of structural phase field crystal (XPFC) models. These XPFC models allow for numerous
microstructural phenomena to be studied that couple important physics emergent at the atomic scalewith phase transformation kinetics occurring on diffusional time scales. Results from recent solidification
studies conducted using XPFC modelling in pure materials and alloys will be presented and compared toexperiments and other theory. These include multi-step nucleation in solidification, void formation in
nano-confined liquid pools during rapid cooling, solute drag, and dislocation-assisted nucleation insolid-state precipitation in alloys. Coupling of the base phase field theory with heat transfer will also bedemonstrated for the study of latent heat effects in solidification. We close by discussing coarse graining
methods for deriving practical phase field theories from phase field crystal models for meso-scalemodelling applications.

634

MMM 2018

(N2 oral)
The Phase Field Method: Crystal Structures and Facets*Peter Voorhees1, Eli Alster1, Nana Ofuri-Opoku1,3, David Montiel2, Katsuyo Thornton2, James
Warren3
1. Northwestern University, 2. University of Michigan, 3. National Institute for Standards and Technology

Phase field crystal (PFC) method allows the atomic scale motion and defect formation to be determinedon diffusive timescales. A major challenge with the method is to devise free energy functions that canyield complicated crystal structures. We introduce a phase-field crystal model that creates an array ofcomplex three- and two-dimensional crystal structures via a numerically tractable three-point correlation
function. This approach successfully yields energetically stable simple cubic, diamond cubic, simple
hexagonal, graphene layers, and CaF2 crystals, as well as the particularly complex and technologicallyimportant perovskite crystal structure. Highly anisotropic interfaces play an important role in thedevelopment of material microstructure. We examine the capability of the PFC model to quantitatively
describe faceted interfaces by coarse graining the PFC model to attain both its complex amplitude
formulation, and its corresponding phase field limit. Using this formulation, we find that the model yields
Wulff shapes with missing orientations, the transition to missing orientations, and facet formation. We
demonstrate, in two dimensions, how the resultant model can be used to study the growth of crystals withvarying degrees of anisotropy in the phase-field limit.

MMM 2018

635

(N2 invited)
Using free energy calculations and statistical mechanics to probe thebrittle to ductile transition of bcc metals*Thomas Swinburne1
1. CINaM, CNRS/Aix-Marseille Univ.

The fracture response of bcc metals is known to be controlled by dislocation mobility, which in turn isdependent on the slow, thermally activated nucleation of kink pairs on <111> screw dislocations. We
present a general, efficient scheme to calculate free energy barriers in large crystalline systems[1] which
we apply to the kink nucleation process, finding significant anharmonic contributions at low homologous
temperatures. A statistical mechanical approach is then used to study kink-limited dislocation motion
through a field of obstacles[2]. We identify a crossover obstacle density below which the activation energy
for plastic flow is half the free energy barrier for kink pair nucleation. Our results show striking agreementwith fracture experiments across a wide range of bcc metals and are applied to interpret experimental
data on irradiation induced embrittlement.
[1] TD Swinburne and M-C Marinica, Physical Review Letters 120 (13), 135503[2] TD Swinburne and SL Dudarev, Physical Review Materials 2 (7), 073608

636

MMM 2018

(N3 invited)
Kinetics of Fivefold-Twinned Nanowire Growth*Kristen Fichthorn1
1. Penn State University

There has been significant emphasis recently on the synthesis of fivefold-twinned Ag and Cu nanowires,
which are considered to be excellent candidates for transparent conductors in flexible and stretchable
electronic devices. A fundamental understanding of nanowire growth is important in achieving optimal
syntheses. Nanowires grow from fivefold-twinned seeds and our work shows that likely shapes for thesestructures include {111} end facets and “notches”, {100} side facets, and {110} facets between thenotches and the ends. We find that the density of islands on the {111} facets of growing wires is lower
than that on {100} facets and that islands are likely to nucleate on {111} facets near {110}-{111} facetboundaries. Our climbing-image nudged-elastic band calculations of diffusion barriers based onembedded-atom method potentials indicate that diffusion in the {111} notches and on the {110}“steps” is significantly faster than diffusion on {100} facets. Thus, these structures become“superhighways” that channel atom diffusion to the wire ends to increase wire aspect ratios. Smallislands facilitate trapping of atoms on {111} facets. We use finite Markov chains to model nanowire growth
and to predict net atom fluxes from nanowire sides to the ends. These simulations predict anisotropic
nanowires similar to those seen experimentally.

MMM 2018

637

(N3 oral)
Accelerated quantum molecular dynamics simulations of chemistry
under extreme conditions*Romain Perriot1, Marc Cawkwell1, Enrique Martinez1
1. Los Alamos National Laboratory

Atomistic simulations are a key component to understand reaction chemistry in materials; for instance,
the detonation chemistry of energetic materials under pressure, which is characterized by rapid breaking
and remaking of covalent bonding, and where intermediate products and reaction rates are difficult tocharacterize in experiments.
While accurate methods exist to study systems of relevant size with explicit electronic contributions
critical to the description of bonds, e.g. density functional tight-binding (DFTB), the timescale of thesimulation is often limited to a few hundreds of picoseconds. This restrains the study to systems in which
reactions occur relatively quickly. In the case of detonation chemistry, this implies high pressure andtemperature. In order to consider less extreme conditions, one can benefit from the rare occurrence ofchemical reactions to use accelerated molecular dynamics methods such as parallel replica dynamics(PRD).
In this work, we describe efforts to combine the DFTB code LATTE, developed at LANL, with the PRD
method, in order to perform accelerated QMD (AQMD) simulations of reactive chemistry. AQMD was first
applied to the study of liquid benzene, a prototypical reactive hydrocarbon that has been studied bothexperimentally and theoretically. AQMD allowed to perform simulations reaching several nanoseconds,
and thus the observation of reactions at pressures below 20 GPa. Importantly, these simulations
unraveled the precursor reaction to polymerization: the formation of Diels-Alder dimers.

638

MMM 2018

(N3 oral)
Hydrogen diffusion in TiHx: insights from PRD accelerated QMD*Ivan Novoselov1,2, Alexey Yanilkin1
1. Dukhov Research Institute of Automatics, 2. Moscow Institute of Physics and Technology

TiHx is of practical interest due to high volume concentration of hydrogen, and for many applications its
diffusion properties should be established with confidence. We investigate the mechanisms of diffusion,and calculate corresponding rates in the framework of quantum molecular dynamics (QMD). However,time scales accessible to conventional QMD are very limited. It therefore can be employed only at very
high temperatures, where no experimental data is available. In order to explore diffusion at lower
temperatures, we accelerate QMD simulations via parallel replica dynamics algorithm (PRD). Applicationof PRD allows to reveal different mechanisms of hydrogen diffusion in TiHx, and also makes it possible toperform direct comparison with experiment

MMM 2018

639

(N3 invited)
Shape fluctuation of metallic nanoclusters: observations fromlong-timescale simulations*Rao Huang1, Li-Ta Lo2, Arthur F. Voter2, Danny Perez2
1. Xiamen University, 2. Los Alamos National Lab

Metallic nanoclusters are functional materials with many applications owing to their unique physical andchemical properties, which are sensitively controlled by their shapes and structures. An in-depth
understanding of their morphology stability is therefore of crucial importance. It has been well
documented by transmission electron microscopy (TEM) studies that metallic nanoclusters caninterconvert between different isomers. However, the relevant mechanisms remain elusive because thetimescales of such shape fluctuations are too short to be resolved experimentally and yet too long forconventional atomistic simulations. By employing a recently introduced Accelerated Molecular Dynamics
method, Parallel Trajectory Splicing, we present simulations that reached timescales of milliseconds andthus provide a clear description of the dynamic process of the experimentally observed shape fluctuation
in metallic nanoclusters. We observe transformations back and forth between face-centered-cubic (fcc)
and structures with five-fold symmetry (decahedron or icosahedron). These transitions occur following
either by a partial-dislocation-mediated twinning mechanism or by a surface-reconstruction driven
process. The identified pathway is in remarkable agreement with the existing microscopy results andserves as further evidence that shape fluctuation can occur directly through thermal activation, without
involving melting or other external factors.

640

MMM 2018

(N4 invited)
Multiscale diffusion method for simulations of long-time defectevolution with application to dislocation climb*Kristopher Baker1, William Curtin2
1. Knolls Atomic Power Laboratory, 2. Swiss Federal Institute of Technology

In many problems of interest to materials scientists and engineers, the evolution of crystalline extended
defects (dislocations, cracks, grain boundaries, interfaces, voids, precipitates) is controlled by the flow ofpoint defects (interstitials/substitutional atoms and/or vacancies) through the crystal into the extended
defect. Accurate modeling of this process requires atomistic methods in and around the extended defect,
but the flow of point defects into and out of the extended defect region can be treated by coarse-grained
methods. This talk presents a multiscale algorithm to provide this coupling, which was first documented in
the manuscript "Multiscale diffusion method for simulations of long-time defect evolution with application
to dislocation climb," published in 2016 in the Journal of the Mechanics and Physics of Solids, Number
92, pages 297-312. Specifically, direct accelerated molecluar dynamics (AMD) of extended defectevolution is coupled to a diffusing point defect concentration field that captures the long spatial andtemporal scales of point defect motion in the presence of the internal stress fields generated by the
evolving extended defect. The algorithm is applied to study vacancy absorption into an edge dislocationin aluminum where vacancy accumulation in the core leads to nucleation of a double-jog that thenoperates as a sink for additional vacancies; this corresponds to the intial stages of dislocation climb
modeled with explicit atomistic resolution. The method is general, so it can be applied to many otherproblems associated with nucleation, growth, and reactions due to accumulation of point defects incrystalline materials.

MMM 2018641

(N4 oral)
Accelerated Quantum Molecular Dynamics*Enrique Martinez Saez1, Christian Negre2, Romain Perriot2, Danny Perez2, Eduard Kober2, Marc
Cawkwell2, Arthur F. Voter2, Anders Niklasson2
1. Material Science and Technology Division, MST-8,Los Alamos National Laboratory, Los Alamos, 87545 NM, USA, 2.
Theoretical Division, T-1, Los Alamos National Laboratory, Los Alamos, 87545 NM, USA

The accurate study of the long-term evolution of rare events is extraordinarily challenging as
computations are arduous and quantum-based molecular dynamics simulation times are limited to, at
most, hundreds of ps. Here, the Extended Lagrangian Born-Oppenheimer molecular dynamics formalism
is used in conjunction with Parallel Replica Dynamics to obtain an accurate tool to describe the long-term
dynamics of reactive benzene. Langevin dynamics has been employed at different temperatures tocalculate the first reaction times in a periodic benzene sample at different pressures. We have alsostudied the long-term behavior of vacancy complexes, which compare satisfactorily with experimental
observations. Our coupled engine runs for times on the order of ns (two to three orders of magnitude
longer than traditional techniques) and is capable of detecting reactions characterized by rates
significantly lower than we could study before.

642

MMM 2018

(N4 oral)
On the effect of hydrogen on vacancy diffusion*Sebastian Echeverri Restrepo1, Anthony T Paxton2
1. Department of Metallic Materials & Ceramics, SKF, 2. Department of Physics, King's College London

Although it is widely recognised that even small amounts of hydrogen (H) can cause embrittlement in iron(Fe) and in (high strength) steels, the fundamental mechanisms that cause it are not yet completely
understood. To contribute to a better understanding of the fundamental behaviour of hydrogen in metals,
in the present work we used parallel replica dynamics (PRD) to study the effect that different amounts ofatomic H have in the diffusion of a single vacancy in body centred cubic (BCC) Fe. Using PRD we
calculated, for the first time, the diffusion of hydrogen vacancy complexes at temperatures and timescales that are not reachable using classical methods. These calculations showed that the diffusivity of avacancy-H complex is not only dependent of the migration barriers, but also that the addition of one H
atom reduces the diffusivity of a vacancy in BCC Fe and the subsequent addition of more H atoms has the
opposite effect.

MMM 2018

643

(N5 invited)
Understanding the impact of extended defects on the behaviour of C
atoms: a multi technique approach*christophe Domain1, charlotte S becquart2
1. EDF, 2. Univ.Lille, CNRS, INRA, ENSCL, UMR 8207, UMET, Unité Matériaux et Transformations

The interaction at the atomistic scale of interstitial solutes such as carbon or nitrogen with extended point
defects has consequences at the macroscopic level. Typical examples are the yield peak and Lüders
plateau, related to static strain aging or the Portevin-Le Chatelier effect (the presence of serrations on theplastic part of the stress-strain curve during tensile test) due to dynamic strain aging. In ferritic steels,these atypical behaviors are due to the interaction between dislocations and solute atoms (mainly C).
Depending on aging time and temperature, solute atoms diffuse towards dislocations, forming Cottrell
atmospheres, and reduce their mobility. Because of the time scale of C diffusion and the stresses creatednear the extended defects, typical atomistic approaches such as molecular dynamics (MD) or on-latticeatomistic kinetic Monte Carlo (on-lattice AKMC) approaches are not appropriate to investigate, at theatomistic level, these phenomena. In this talk, we will thus present the approach we have pursued toinvestigate the behavior of C atoms in the vicinity of extended defects (dislocations, dislocation loops …)
in Fe. We have applied different complementary techniques. Calculations based on the density functional
theory (DFT) as well as with an empirical FeC atomistic potential have been used extensively to determine
the strength of the interaction between carbon atoms and the various defects in stable or metastableconfigurations. On-lattice static Monte Carlo as well as off-lattice kinetic Monte Carlo approaches havebeen applied to build Cottrell atmospheres and study dynamically the behavior of C atoms in the vicinityof extended defects (dislocations, …). Finally molecular dynamics (MD) simulations have been deployed
to estimate the stress necessary to release the dislocations from the C atmospheres.

This work has been realized in collaboration with R. Candela (U. Lille), N. Mousseau (U. Montreal), M.
Perez (INSA Lyon), R. Veiga (U. Federal do ABC), O. Waseda (INSA Lyon)644

MMM 2018

(N5 oral)
Localised on-the-fly Kinetic Monte Carlo*Johannes Bulin1
1. Fraunhofer-Institut SCAI

Various on-the-fly Kinetic Monte Carlo algorithms have been used to simulate the long-term evolution of
atomistic systems. Unlike classical Kinetic Monte Carlo methods, these algorithms search for possible
transitions/reactions only when necessary (hence on-the-fly), rather than relying on a previously-known
set of transitions. Unfortunately, their performance deteriorates when large systems or systems with many
similar transition mechanisms are investigated.
We will present a localised on-the-fly Kinetic Monte Carlo variant that is based on the k-ART algorithm by
El-Mellouhi, Lewis, and Mousseau. This algorithm is capable of localising transition searches, enabling thesimulation of significantly larger systems, and can also recycle known transition mechanisms. Unlike
k-ART, which uses a graph-based technique to identify similar local environments, we use an RMSD (rootmean square deviation) measure that is invariant under index permutations and rotations of local
environments. This allows our algorithm to deal not only with highly-structured materials like crystals, butalso with amorphous materials, which are difficult to classify with graphs.
We will conclude by demonstrating computational results of our algorithm, and discuss potential areas ofapplication.

MMM 2018645

(N5 oral)
A Preconditioning scheme for Minimum Energy Path finding methods*Stela Makri1, James Kermode1, Christoph Ortner1
1. University of Warwick

In transition state theory, the study of thermally activated transitions between energy minima is achieved
by finding transition paths connecting the minima. These paths provide information on the energy barrier
and reaction rates of the system without going through long and expensive simulations. To find them,current techniques use steepest descent-like minimisation to relax a discretised initial guess. However,steepest descent typically gives slow convergence rates in the presence of ill-conditioned potentials. In
this talk I will be discussing how to reduce the condition number of the potential of an arbitrary system
and improve the convergence speed and robustness of transition path finding methods, using apreconditioning scheme. Our key assumption is that the cost of constructing a preconditioner is muchsmaller than the cost of computing the potential; for density functional theory the cost of single point
evaluations is much more expensive than the computation of a preconditioner and thus the proposed
approach improves computing times significantly. We have developed a preconditioning scheme, where
the preconditioner acts as a coordinate transformation of the discrete images along the path to aid theill-conditioning in the transverse direction and a preconditioning scheme that considers the interactions
between the images is currently in development. Finally, we are working towards a preconditioning
scheme for finding energy barriers in hybrid quantum mechanical –molecular mechanical models.

646

MMM 2018

(N5 invited)
Temperature Programmed Molecular Dynamics - Accessing rare
events using a combination of finite time sampling and bias potentials*Abhijit Chatterjee1
1. Dept. of Chemical Engineering, Indian Institute of Technology Bombay

Rates of physical and chemical processes often obey Arrhenius law. Recently, we developed thetemperature-programmed molecular dynamics (TPMD) method1,2,3 that provides a convenient way of
estimating the Arrhenius parameters of kinetic pathways even in situations where the underlying
landscape is rugged. The TPMD method employs a temperature program with finite temperature MD inorder to accelerate thermally activated events from a particular state of the system. Kinetic pathways aresought from a collection of states without any prior knowledge of these pathways. Since kinetic pathways
are selected with a probability that is proportional to their rate constants, we find that slow pathways withsmall pre-exponential factors and large activation barriers are rarely sampled with TPMD. We introduce a
procedure to overcome this limitation by bias potentials. This additional feature in the TPMD method
dramatically improves its ability to estimate Arrhenius parameters. Examples of the variation of the TPMD
method are provided for metal surface diffusion in presence of solvent.
1. V Imandi, A Chatterjee, Estimating Arrhenius parameters using temperature programmed molecular
dynamics, The Journal of Chemical Physics 145, 034104, 2016.
2. S Divi, A Chatterjee, Accelerating rare events while overcoming the low-barrier problem using atemperature program, The Journal of Chemical Physics 140 (18), 184116, 2014
3. A Chatterjee, Accelerating Rare Events and Building Kinetic Monte Carlo Models Using Temperature
Programmed Molecular Dynamics, Journal of Materials Research,
http://dx.doi.org/10.1557/jmr.2017.460

MMM 2018

647

(N6 invited)
Modelling metals, alloys and cement paste across length and timescales*Laurent Karim Beland1, Roger E Stoller3, Yuri N Osetsky3, Tingtao Zhou2, Katerina Katerina
Ioannidou2, Franz-Josef Ulm2, Roland Pellenq2
1. Queen's University, 2. Massachusetts Institute of Technology, 3. Oak Ridge National Laboratory

Fundamentally, structural materials are bound by atomistic cohesive interactions, while our societies use
these to design systems at scales of meters—or more. Likewise, the material’s microstructural evolution
takes place on scales as short as nanoseconds—e.g. diffusion of radiation-induced defects clusters inmetallic alloys—all the way up to decades—e.g. creep of concrete under load.

This presentation will focus on applying multi-scale methods to address these challenges for two classesof systems: (1) metals and alloys under irradiation, and (2) calcium silicate hydrates (C-S-H), the mainbinding phase of cement paste.

In metals and alloys, the kinetics of collision cascade induced defects was modelled using adaptive kinetic
Monte Carlo. Namely, the kinetic Activation Relaxation Technique —a self-learning, fully atomistic
algorithm able to handle off-lattice defects and handle long-range elastic interactions—was used tocapture the time-evolution of cascade debris after neutron or ion irradiation. Furthermore, a detailed
investigation of point-defect diffusion in NixFe(1-x) concentrated alloys revealed a non-monotonic
dependence of diffusion coefficients as a function of Fe concentration. This was explained by coupling
percolation effects with the composition dependence of point-defect vacancy migration energies.

In C-S-H, the adsorption of alkali ions—Na, K, and Cs—was studied, in the context of the alkali-silica
reaction and spent fuel storage. By applying molecular dynamics, semi-grand canonical Monte Carlo, and
the Activation Relaxation Technique nouveau, adsorption of alkali ions in the hydrated layer of C-S-H wascharacterized. The effect of alkali uptake on mesoscale mechanical properties was calculated using acoarse-grained model of C-S-H. While alkali uptake leads to significant expansion of individual C-S-H
grains, it leads to modest—less than 5 MPa—mesoscopic expansive pressure, well within the elastic
regime of concrete.

648

MMM 2018

(N6 oral)
BCA-MD-KMC hybrid simulation for long time helium plasma
irradiation inducing fuzzy nanostructure on tungsten*Atsushi M Ito1,2, Arimichi Takayama1, Hiroaki Nakamura1,3
1. National Institute for Fusion Science, National Institutes of Natural Sciences, 2. The Graduate University for
Advanced Studies, 3. Department of Electrical Engineering, Graduate School of Engineering, Nagoya University

In this paper, we developed the triple hybrid simulation method for plasma-material interaction (PMI).
Although the present method is composed only of atomistic simulations, an elapse time achieved
experimentally relevant time scale.
PMI occurs in the processing of semiconductors, thin film depositions, applications into nanomaterial
formations, and the inside wall of magnetic confinement nuclear fusion reactors. In atomistic viewpoint,
PMI is the surface reaction of a target material due to ion particles continuously injected from plasma. The
PMI, which is not an exception, follows the multi-scale multi-physics mechanisms.
Moreover, an important factor in PMI is competition between an irradiation flux and the diffusion speed of
the injected particles in the target material. The irradiation flux in laboratory experiments is 1020 to 1024 m-2 -1

s , and irradiation time is 102 s to 104 s. Although molecular dynamics (MD) is often used for PMI, theexperimental irradiation time is too long for MD. Therefore, the irradiation flux was generally set 104 to 10
8times of the experimental flux. However, since the diffusion speed in MD becomes realistic, thecompetition between the irradiation flux and the diffusion speed becomes unrealistic. Thus, theexpansion of time scale is necessary for PMI simulation.
Here, the present target phenomenon is fuzzy nanostructure formation on the tungsten surface by
exposure to helium plasma. Injected helium atoms agglomerated in the tungsten material, and thenanoscale helium bubbles are formed. After that, the fuzzy nanostructure of tungsten measuring several
ten nanometers in width is grown on a surface. To represent the fuzzy nanostructure, we developed the
BCA-MD-KMC hybrid simulation method. In this method, the injection process of helium ions is solved bybinary collision approximation (BCA), the deformation process of a target material due to the pressure
from the helium bubbles is solved by MD, and the diffusion process of the helium atoms in a tungsten
material is solved by kinetic Monte-Carlo (KMC). In the hybrid simulation, the injection flux is kept at 1022
m-2s-1 same as an experimental flux and then the irradiation time achieved 100 s. As a result, the fuzzy
nanostructure growth was successfully represented.

MMM 2018

649

(N6 oral)
Atomistic Modelling of Pipe Diffusion: a Direct Comparison of MD,
KMC, aKMC and DMD
Frederic Houlle1, Jutta Rogal2, *Erik Bitzek1
1. FAU Erlangen-Nuernberg, 2. ICAMS Ruhr University Bochum

Understanding and influencing self-diffusion in complex microstructures is of fundamental importance for
improving the high-temperature mechanical properties of materials. Modelling diffusion in a quantitatively
predictive way requires information about the underlying atomic-scale processes and corresponding
activation energies. While various atomic-scale simulation approaches are routinely employed to study
diffusion in homogeneous environments, the presence of strain gradients and defects still posessignificant challenges to predictive atomic-scale models. In particular, currently only few detailed studies
that compare the application of different simulation methods to diffusion at extended defects exist.
Here we present our recent atomistic simulations results of vacancy-mediated diffusion in the vicinity ofan edge dislocations in aluminum modelled by an EAM potential. The direct high-temperature molecular
dynamics (MD), adaptive kinetic Monte Carlo (aKMC) and diffusive molecular dynamics (DMD)
simulations all use the identical atomistic starting configuration. The activation energies of allnearest-neighbor vacancy jumps in this configuration were determined by the nudged elastic band (NEB)
method and used to build the event catalogue for the kinetic Monte Carlo (KMC) simulations and to
parametrize the DMD model. Significant differences between the direct MD simulations and the (a)KMC
simulations are observed, which could be attributed to the coupling of the diffusive vacancy motion withthe thermally-induced fluctuations of the dislocation.

650

MMM 2018

(N6 oral)
Strategies for optimal construction of Markov chain representations ofatomistic dynamics*Danny Perez1, Thomas Swinburne1
1. Los Alamos National Laboratory

A common way of representing the long-time dynamics of materials is in terms of a Markov chain thatspecifies the transition rates for transitions between metastable states. This chain can either be used togenerate trajectories using kinetic Monte Carlo, or analyzed directly, e.g., in terms of first passage times
between distant states. While a number of approaches have been proposed to infer such a representationfrom direct molecular dynamics (MD) simulations, challenges remain. For example, as chains inferred froma finite amount of MD will in general be incomplete, quantifying their completeness is extremely desirable.
In addition, making the construction of the chain as computationally affordable as possible is paramount.
In this work, we simultaneously address these two questions. We first quantify the local completeness ofthe chain in terms of Bayesian estimators of the yet-unobserved rate, and its global completeness in termsof the residence time of trajectories within the explored subspace. We then systematically reduce the costof creating the chain by maximizing the increase in residence time against the distribution of states inwhich additional MD is carried out and the temperature at which these are respectively carried out. Using
as example the behavior of vacancy and interstitial clusters in materials, we demonstrate that this is anefficient, fully automated, and massively-parallel scheme to efficiently explore the long-time behavior ofmaterials.

MMM 2018

651

(N6 oral)
Simulating the collective diffusion mechanism of amorphous solids at
experimentally relevant time scales*Yunjiang Wang1,2, Shigenobu Ogata3
1. Institute of Mechanics, Chinese Academy of Sciences, 2. School of Engineering Science, University of Chinese
Academy of Sciences, 3. Graduate School of Engineering Science, Osaka University

The nature of collective diffusion in amorphous solids is in strong contrast with diffusion in crystals.
However, the atomic-scale mechanism and physics of collective motion remains elusive in disordermaterials. Here the free energy landscape of collective diffusion triggered by single atom jump in a
prototypical CuZr model metallic glass is explored with the recently advanced well-tempered
metadynamics which significantly expands the observation time-scale of diffusion process at atomic-scale.
Metadynamics samplings clarify a long-standing experimentally suggested collective diffusion mechanismin the deep glassy state. The collective nature is strongly temperature-dependent. It evolves fromstring-like motion with participation of only several atoms to be large size collective diffusion at hightemperature, which would remarkably promote the atomic transport upon glass transition. The
temperature and pressure dependence of collective diffusion are further quantified with big activation
entropy and small activation volume of half atom volume, which both agree quantitatively withexperiments. Direct atomic-scale simulations of diffusion at laboratory time-scale brings several physicalinsights into the nature of collective diffusion in amorphous solids which is beyond the knowledge
established in diffusion of crystals.

652

MMM 2018

(N7 oral)
Bridging Time Scales with Variationally Enhanced Sampling*Omar Valsson1
1. Max Planck Institute for Polymer Research

The usefulness of atomistic simulations is generally hampered by the presence of several metastablestates separated by high barriers leading to kinetic bottlenecks. Transitions between metastable states
thus occur on much longer time scales than one can simulate. Numerous enhanced sampling methods
have been introduced to alleviate this time scale problem, including methods based on identifying a few
crucial order parameters and enhancing their sampling through the introduction of an external biasing
potential.
Here will we discuss Variationally Enhanced Sampling (Valsson and Parrinello, PRL 113 090601, 2014), a
generally applicable enhanced sampling method where an external bias potential is constructed byminimizing a convex functional. We present numerous examples from physics and chemistry which show
the flexibility and practicality of the method. We will furthermore show how the variational property of themethod can be used to extend the method in various innovative ways, e.g.: to obtain kinetic informationfrom atomistic simulation; to accelerate nucleation events by employing models from classial nucleation
theory; and to incorporate experimental information into molecular simulations.
We will also introduce the VES code (http://www.ves-code.org), an open-source library for the PLUMED 2
plugin that implements methods based on Variationally Enhanced Sampling

MMM 2018

653

(N7 oral)
Simulations of Branched Polyelectrolytes*Filip Uhlik1
1. Charles University

Polyelectrolytes (PEs) are polymers with ionizable groups. Under suitable conditions, PEs are soluble inaqueous media. This property makes them attractive for biological, medicinal and environment-friendly
applications that require organization of matter at nanoscale. The applications of PEs range from drug
carriers, water pollutants removal, oil recovery to sea water desalination. In some of them, e.g. as
thickening agents or super-absorbents in diapers, they have already reached mass production.
In this contribution a general and efficient simulation technique based on hybrid Monte Carlo (HMC)
method will be described. HMC uses dynamics for evolution, but dynamics of a long polymer with N beads
is itself slow (Rouse time ∝N2). For this reason we use a faster unphysical evolution with a modified
Hamiltonian while still efficiently Monte Carlo sampling the Boltzmann distribution with the original one.
The resulting simulation is much faster than molecular dynamics and other methods. It isalso well suited for simulations of coarse-grained models of PEs and allows for using reaction and otherensemble [F. Uhlik et al. Macromolecules 47 (2014) 4004].
Results for several types of branched PEs (e.g., stars and combs) for different conditions of solvent quality,
Bjerrum and Debye lengths will be given. While strong PEs remain fully ionized, the degree of ionization of
weak PEs can be influenced by both changing pH and ionic strength. This results in a responsiveness toexternal stimuli needed in many applications. The conformations of weak PEs are coupled with charge
redistribution and this can lead to unexpectedly complex behavior. For example, strong PE stars inmarginally poor solvents form bundles while weak PE stars redistribute charge and form core-shellstructures with some arms uncharged and collapsed and others charged and extended. Behavior of otherbranching types as well as formation of inter-polyelectrolyte complexes will be also discussed.

654

MMM 2018

(N7 oral)
Adaptive resolution simulations coupling molecular dynamics todissipative particle dynamic*Matej Praprotnik1
1. National Institute of Chemistry

We present the hybrid coupling of the molecular dynamics (MD) and dissipative particle dynamics (DPD)
methods, bridging the micro- and mesoscopic levels of detail. The coupling is achieved using the adaptive
resolution scheme (AdResS), which is a linear momentum conserving multiscale method. Our
methodology is thus suitable for simulations of liquids on the micro/mesoscopic scale, wherehydrodynamics is crucial. The presented approach is showcased for water at ambient conditions. The
supramolecular coupling is carried out by our clustering algorithm SWINGER that assembles,
disassembles, and reassembles water clusters on demand during the course of the simulation. This allows
for a seamless coupling between standard atomistic MD and DPD models.

MMM 2018655

(N7 oral)
Using Diffusive Molecular Dynamics Simulations to Investigate Grain
Boundary Segregation and Grain Boundary Structural Transformations
Louis Hebrard1, *Chad W Sinclair1
1. Dept. of Materials Engineering, University of British Columbia

We have investigated the time evolution of segregation on both grain boundary structural and chemicaltransitions in Al-Mg alloys by means of alloy diffusive molecular dynamics (a-DMD) calculations. The
predictions with regard to segregation are compared with classic continuum approaches including the
calculation of grain boundary free energy curves. The role that chemical segregation plays in grainboundary structural transitions (complexions) is studied and used to constructtemperature-pressure-composition stability plots for one particular grain boundary.

656

MMM 2018

(P2-75)
Why the structure-property relationship in metallic glasses should beestablished beyond short-range order: Insight from potential energy
landscape*Dan Wei1,2, Yunjiang Wang1,2, Lanhong Dai1,2
1. University of Chinese Academy of Sciences, 2. Institute of Mechanics, Chinese Academy of Sciences

For crystals, structures provide all information needed for predicting material properties. However, what
determines non-crystalline solids properties remains elusive for many years. Extensive work has been
performed to identify structures playing an important role in glass, but a key question arises that what is
the hidden rule of structural feature that can predict properties. Here we calculate an atom’s activationenergy (the system’s long-time property) for thermally activated relaxation with the Activation-Relaxation
Technique (ART) and correlate the searched local potential energy landscape with several of thesuccessful structural predictors. We find a common nature in the successful structural predictors thatspatial correlation of structural information matters a lot once they tend to determine an atom’s
properties. There exists a critical correlation length of about sub-nanometer which is corresponding to thesecond shell of the pair correlation function of glassy structures. We further demonstrate this concept bymanipulating the cutoff distance of local structural entropy –one of the successful structural feature –that
only if this local structure is defined beyond the short-range order it can predict activation of local atom
rearrangement in the model metallic glass. In this way, we question the prevailing approach of materials
science aimed at identifying simple structural motifs responsible for metallic glass properties.

MMM 2018

657

(P2-76)
Vacancy segregation analysis near grain boundaries in metallic
systems by long-time atomistic simulations*Shotaro Hara1
1. Dept. of Mechanical Engineering, Chiba Institute of Technology

Long-term degradation phenomena in metallic systems under a high temperature, such as creep voiding,are governed by the vacancy diffusion, accumulation and growth processes at an atomistic scale aroundthe material heterogeneities like grain boundaries. However, the basic properties such as the equilibrium
vacancy concentrations and the kinetics near grain boundaries are not still understood because amolecular dynamics simulation often suffers from tracking thermally-activated processes due to its limited
time scale. In this study, the vacancy segregation behavior at grain boundary has been analyzed using
diffusive molecular dynamics simulations, which is a novel approach for exploring the atomic level massaction along the chemical potential gradient at diffusive time scale. The equilibrium vacancy
concentrations and the chemical potential distributions at grain boundaries are computed for thedifferent grain boundary character. The correlation between the grain boundary energies and theirconcentrations have also been considered. Furthermore, the kinetic nature of grain boundary sliding wasinvestigated using this new scheme and the effect of stress on the sliding was discussed.

658

MMM 2018

(P2-77)
EXPERIMENTAL AND DENSITY FUNCTIONAL THEORY STUDIES OFPRECIPITATE INTERFACES IN ALUMINIUM ALLOYS, WITH FOCUS ON
β" & β*Haris Rudianto1, Deni Hariadi1, Andriansyah Andriansyah1
1. Gunadarma University

Aluminium is the third most abundant element in the Earth’s crust, after oxygen and silicon. Before thestart of the last century, aluminium started to come into wide use as a structural material. Since then, it
has reached the position of being the second most used metal in the world, with only steel beating it to
the throne. One important structural limitation of aluminium is their fatigue strength. Precipitation
hardening is utilized to strengthen a wide variety of alloy systems especially for Aluminum Alloys. An
example is the class of commercially important Al–Mg–Si based alloys which are strengthened by anumber of metastable precipitate phases, where the needle-shaped β’’-Mg5Si6 precipitates are often
the main contributor to hardening. Beginning with the supersaturated solid solution (SSS), the genericprecipitation sequence in Al–Mg–Si alloys is generally believed to be :SSS -> Mg/Si clusters -> Guinier-Preston zones -> β’’ -> β’ ->β

In practice, the sequence can be even more complex and a number of other metastable phases,
depending on alloy composition and the heat treatment time and temperature. In this research, interface
energy was calculated by Quantum Espresso with super cells designed on VESTA. For comparison, in thisresearch, experimental was also carried out to determine effects of strengthening precipitates onmechanical properties. T6 heat treatment was done starting from solid solution treatment, quenching andfinished my artificial aging. Hardness was done to determine mechanical properties and SEM-EDS andXRD were done to characterize the materials.

Keywords; Aluminum Alloys, DFT, Strengthening Precipitates, Heat Treatment

MMM 2018

659

Symposium O(O1 oral)A numerical insight into third body flow regimes within dry contacts*Guilhem Mollon1
1. INSA Lyon - LaMCoS

In this communication we present new results related to the mechanical behavior of the solid interfacial
material that is present within a dry sliding interface, and that transmits the load while accommodating
the relative displacement between the surfaces –the so-called third body. To reach this purpose, aninnovative numerical method is employed, based on the multibody meshfree framework. In this approach,the solid matter composing the third body is represented as a large collection of individual grainsinteracting by contact and adhesion, much like in the Discrete Element Modelling (DEM) framework,commonly applied in this case for the last fifteen years. However, in contrast with DEM, this newframework allows to consider each grain as highly deformable, which relaxes the assumption of a purely
granular third body. In the simulations we present, 2000 such grains are placed between two rigid roughsurfaces with periodic lateral boundary conditions, and these surfaces are submitted to typicaltribological loadings (pressure and shear). While playing on a limited number of parameters of the third
body (deformability, adhesion, damping), we observe the emergence of a very wide range of mechanicalbehaviors in the interface, such as granular flow, quasi-fluid Couette regime, fragile cracking,agglomeration, rolling, wall-slip, etc. The consequences of these regimes on the local friction coefficientand on the fluctuations of the loading on the surfaces in space and time (which ultimately control the
degradation and the wear of these surfaces) are evaluated. Finally, comments are addressed on thepossible application of such knowledge in multi-scale approaches in order to relate these results toexperimental measurements.

660

MMM 2018

(O1 oral)
A novel multiscale framework for modeling of diamond tools wearA
novel multiscale framework for modeling of diamond tools wear
*Adriana Quacquarelli1, Nicolas Fillot1, Guilhem Mollon1, Thierry Commeau2, Aurelie Nouveau2
1. Université de Lyon, LaMCoS, INSA-Lyon, CNRS UMR5259, F-69621, 2. Umicore Specialty Powders France

Stone cutting involves diamond impregnated tools (DIT) consisting in microdiamonds embedded into a
metallic binder. Diamond Tools lifetime is influenced by the wear mechanism of the metallic matrix due tothe abrasive debris flow generated during cutting process. Both diamonds, stone debris and the metallic
matrix have different dimensions. Therefore, different mechanical and physical process, somehow related,
are involved at different scales. Experimental tests are necessary but not sufficient to predict the overallbehaviour of diamond tools. A numerical multiscale approach is then initiated to extend the experimental
approach.
As DIT performance is influenced by the microstructure properties of the binders (porosity, grains size,
bulk properties and composition of metallic powder), a microscopic 2D model representative of metal
microstructure has been built. A multibody meshfree technique coupled with a Discrete Element Method(DEM) approach is used. The granular swarf is described by rigid grains with realistic shape, while themetallic matrix is represented by a collection of degradable grains which fails by fatigue due to thecontinuous generation and flow of rock debris. Cracking initialization and propagation can be monitored.
In typical sections far from diamond, an empirical local wear law can be written, proposed and compared
to experiments.
At another scale, the flow of stone debris is studied and assimilated to a viscous flow of continuous
material.
Then, A 3D continuous model is implemented to monitor wear close to diamond. The pressure field isobtained by solving Reynolds equation and used in a global wear law. The microgeometry evolution isthen compared to experiments.
Finally, the whole set of numerical models at the different scale can be used as a tool to anticipate theperformance of the metallic matrix of diamond tools.

MMM 2018

661

(O1 oral)
Thermodynamics of sliding contact: Joule–Thomson effect*Vera Deeva1, Stepan Slobodyan2
1. Tomsk Polytechnic University, 2. Tver State Technical University

The structure and properties (physical, thermal, mechanical, etc.) of complex materials during its
formation process have strong correlation with the surrounding medium. Each of the material particles
interacts with surroundings randomly, independently of each other, resulting from a probabilistic nature.
This fact leads to the rich material properties with small changes in parameters.
An electrical contact pair in the commutator and brush assembly of the electric machine is of interest
from the point of view of its structure due to material multivariance (carbon, metal, nanostructure, andothers), which have a correlation with environment formed by wear particles or lubricant under sliding
interaction. These elements exemplify the strong correlations between mechanical fluctuations of thewear particles and temperature behavior of the contact area (air with wear particles or lubricant), in
particular, the non-equilibrium thermodynamics. In our paper we consider the abundance of wear
particles as the porous plug. To study this throttling process, we shall concern with the stream as a
two-phase system, in particular, gas medium and wear particles in the sliding contact. After due
calculation, we receive the expression showing that measure of the strength of association between
Joule-Thompson effect and the surrounding medium following a parabolic law, likewise under atemperature gradient, the temperature in contact area is not constant, it varies linearly with thecoordinate of the entry end of the contact area. Furthermore, we consider two cases: kc = 1 and kc = 0.2.
The comparison indicated a maximum difference of 30 percent of the thermal effects between the entry
and outlet ends of the contact area. This means that the strong correlations that are responsible formechanical fluctuation leads to the increase in the dispersion of the temperature fluctuations in the outlet
end of the contact area caused by the corresponding susceptibility to the surrounding medium. The
Joule-Thompson effect is probably one of the most important factors determining the different interaction
conditions and accordingly material properties of the entry and outlet ends of the contact area between
commutator and brush.

662

MMM 2018

(O1 oral)
Molecular Simulation of adsorption process of anti-corrosion additives*Kohei Nishikawa1, Hirotoshi Akiyama1, Kazuhiro Yagishita2, Hitoshi Washizu1
1. University of Hyogo, 2. JX Nippon Oil & Energy Corporation

Newly formed metal surface is often unstable and becomes stable when it is terminated with anothermolecule, but the original color and properties may be diminished when it is covered with oxygen or
gasses in atmosphere. To prevent this phenomenon, anti-copper-corrosion additives adsorb onto thesurface of cupper and save copper`s color and properties from oxygen or other substances. In spite of themany property, there are few molecular findings about anti-copper-corrosion additive and the mechanismof adsorption onto the surface of copper and prevent corrosion. For anti-copper-corrosion, we use
benzotriazole C6H5N3 (BTA) which used for a long time.
The method of simulation is molecular dynamics method and for parameter we use reax force field(reaxFF) potential which involves chemical reaction to analyze the mechanism of the adsorption and theproperties of the anti-copper-corrosion additive of copper (Cu) and oxidized copper (Cu2O) surface. For
this large-scale computing, we use molecular dynamics calculations software LAMMPS which is good atparallelized efficiency. Outline for this simulation model, we make a slab composed with copper (Cu) andoxidized copper (Cu2O), and put 60 anti-copper-corrosion additives foreside of it, randomly. Thencalculate time development of this system to replicate the real system.
We analyze destination, orientation and direction, and charge transfer of anti-copper-corrosion additives
BTA to the slab. When we compare the adsorption destination of BTA molecules on copper (Cu) tooxidized cupper (Cu2O), 5 times as many BTA molecules adsorbed onto the copper (Cu) than the oxide
copper (Cu2O). Orientation and direction also show a difference, BTA molecules adhesion on oxidized
copper (Cu2O) were horizontal and that adhesion on copper (Cu) were slightly verticality. In addition, a lotof BTA molecules adsorbed on copper (Cu) toward N to bottom of it, and transfer the charge from Cu
atoms of top layer of the copper (Cu) part. This selective deposition is thought that the mechanism that
few of the anti-copper-corrosion additives are able to protect copper`s new surface.

MMM 2018

663

(O1 oral)
The adhesive behavior of elastic contacts in the presence of interfacial
shear stresses
Nicola Menga1,2, *Giuseppe Carbone1,2, Daniele Dini2
1. Department of Mechanics, Mathematics and Management - Polytechnic University of Bari, 2. Department of
Mechanical Engineering, Imperial College London

Among the many factors influencing the contact behavior of two mating surfaces, the interplay between
shear stresses (and associated frictional response) and adhesion in elastic contacts is still a long-standing
tribological problem. Most of the theoretical models focusing on this phenomenon seem to indicate thatthe presence of frictional stresses at the sliding interface tend to mask adhesion, thus leading to a
reduction of the contact area. This is usually explained by invoking the only theory available in theliterature (by Savkoor and Briggs in 1977) to study the interrelation between contact tangential stresses
and adhesion, which, however, holds true only in the case of full stick conditions between the mating
surfaces, i.e. when slip at the contact interface is totally prevented from taking place. Moreover, on thecontrary, some experimental investigations have shown that, under gross slip conditions between almost
perfectly smooth surfaces no contact area reduction is observed as long as the sliding velocity ismoderate, and in some cases, even an increase of the contact size is reported. Aiming at shedding light on
this behaviour, we focus on the adhesive sliding contact between two perfectly smooth surfaces under thecondition that gross slip takes place at moderate velocities. We treat the exemplar case of a smooth rigid
sphere sliding on a soft elastic half-space. We developed a theoretical model, which, by relying on thetheory of contact mechanics and on arguments borrowed from thermodynamics, shows that an increaseof the contact area, compared to the classical JKR case, may be caused by the presence of constantuniform shear stress at the interface. This is specifically true at low velocity, before the onset of stick-slip.
In fact, at low-speed sliding, the shear stress fluctuations at the interface, which produce an apparent
repulsive surface energy term, are negligible compared to the average stress. However, when the contact
moves into the stick-slip regime the shear stress fluctuations may become comparable to the averageinterfacial stress leading to a strong repulsive surface energy, which may also justify why adhesion isinstead almost completely masked at relatively large sliding velocities.

664

MMM 2018

(O1 oral)
Soft Matter Mechanics: numerical and experimental methodologies fordry and lubricated tribological problems*Carmine Putignano Putignano1,2, Giuseppe Carbone1,2
1. Polytechnic University of Bari , 2. Imperial College London

Assessing the mechanics of soft matter is a crucial point in modern research, involving, at the same time,engineering, physics and material science. Indeed, modelling soft materials is particularly challenging
given the strongly time-dependent and usually nonlinear constitutive stress-strain relations that govern
their response. Further complexity is embedded in the modelling analysis when soft solids are into
contact and the problem is exacerbated by the geometry of the intimately mating surfaces. In this work,
we focus specifically on the contact mechanics of linear viscoelastic materials and we present a variety ofBoundary elements methods developed to determine the mechanical solution in terms of stresses, strains
and, ultimately, friction. In particular, we describe the main features of viscoelastic contact mechanics
under different contact conditions, involving steady-state, reciprocating and generalized motion laws.
Each configuration is different since the solution is dramatically influenced by the relaxation of thedifferent regions into contact. In all these analyses, a fundamental role is played by the surface roughness,
which introduces a huge number of space and, consequently, time scales. Such a scenario is furtherly
complicated when the presence of a fluid is considered at the contact interface: indeed, the fluid
viscosity, coupled with the material viscoelasticity, determines tremendous variations in comparison withthe classical elasto-hydrodynamic theory. In detail, the pressure and the film thickness distributions show
strongly non-symmetrical trends at the contact inlet and outlet. All this entails a friction curve strongly
different from the Stribeck curve, usually predicted for elastic solids, and demonstrates the necessity of ad
hoc developed modelling strategies for soft materials.

MMM 2018

665

(O1 oral)
Two simple models for pull-off decay of self-affine rough surfaces*Antonio Papangelo1,2, Michele Ciavarella1,2
1. Politecnico di Bari, 2. Hamburg University of Technology

Predicting the adhesive behaviour of randomly rough multiscale surfaces nowadays remains a tough task.
The classical asperity model of Fuller and Tabor (1975) reduces the rough surface to a set of independent
asperities, which behave accordingly to the JKR model for adhesion of spheres (Jonshon et al. 1971).
Fuller and Tabor showed that the pull-off force is strongly affected by the height root mean square (rms),
so that a tiny variation in height rms leads to order of magnitudes reduction in surface stickiness.
Nevertheless, the present understanding of rough contact as “fractals” poses serious questions about
the validity of asperity models. Recent large numerical calculations by Pastewka and Robbins show that“slopes and curvatures” may play an important role, which is in contrast with asperity model
predictions. We propose and discuss here two simple models for pull-off decay, namely the Bearing Area
Model (BAM, Ciavarella, 2017) belonging to a DMT class of models, and Generalized Johnson Parameter(GJP, Ciavarella & Papangelo, 2018) model. BAM starts from the observation that the entire DMT solution
for “hard” spheres (Tabor parameter tending to zero) assuming the Maugis law of attraction, is very
easily obtained using the Hertzian load-indentation law and estimating the area of attraction as theincrease of the bearing area geometrical intersection when the indentation is increased by the Maugis
range of attraction. GJP instead postulates that stickiness of randomly rough multiscale surfaces depends
on a generalization of the classical Johnson parameter valid for the single sinusoid. The GJP is obtained as
the ratio between the adhesive energy and the elastic energy needed to flatten the surface. We makeextensive comparisons of GJP and BAM predictions with respect to Pastewka and Robbins (2014) and
Persson and Scaraggi (2014) numerical calculations showing reasonable agreement. We show that for low
fractal dimensions, BAM and GJP are insensitive to rms slopes and curvatures, so being independent on“small-scale features”, which are difficult to define for fractal surfaces.

666

MMM 2018

(O2 oral)
Droplet Spreading on a Surface Exhibiting Solid-liquid Interfacial
Premelting*Yang Yang1, Brian B Laird2
1. East China Normal University, 2. University of Kansas

We study the spreading kinetics of Pb(l) droplets on an Al(111) substrate using MD simulation. The Al-Pb
solid-liquid interface was found [Phys. Rev. Lett, 110, 096102 (2013)] to exhibit premelting below the Tm(Al). Because the Al(111) free surface does not premelt, the spreading of the droplet is accompanied by atransition from a faceted to a premelted surface. We examine the effect of this premelting on the droplet
spreading kinetics and compare the results to two standard mechanisms of droplet spreading:hydrodynamic spreading, in which energy dissipates through viscous relaxation, and kinetic spreading,where friction dominates. When premelting is present, kinetic spreading is observed at intermediatetimes, with exponential relaxation at long times. For two non-premelting Al/Pb interfaces (one facetedsurface at 625K and one where premelting was suppressed) hydrodynamic spreading is seen instead atintermediate times. We also examine the effect of premelting on the droplet equilibrium contact angle.

MMM 2018

667

(O2 oral)
Comparative Study on the Adsorption of Violate Organic Compounds
on the Surfaces of Two-Dimensional Materials: Toward the Early Lung
Cancer Detection*Van An Dinh1,2
1. Nanotechnology Program, Vietnam Japan University, 2. Center for Atomic and Molecular Technology, Graduate
School of Engineering, Osaka University

Every year, the large number of death due to lung cancer is reported. 5 years after diagnosis, there is only
small number (13% ~ 48% depending on country) of the lung cancer patients are alive. Screening tests todetect lung cancer at an early stage is very important for localizing the cancer cells and significantly
improving the possibility of the curability of the disease. Breath contains clinically useful markers [2] suchas the violate organic compounds (VOCs), which can be detected by electronic sensors. The
two-dimensional (2D) materials such as graphene, silicone, germanene, etc. are expected to be promisingmaterials for the electronic sensors for detecting VOCs because these materials exhibit a very highsensitivity in adsorption of gases.
In this talk, we present a comparative study on the adsorption of various VOCs on the surfaces of thesubstrate MoS2 and other 2D materials such as silicene, graphene, borophene and germanene by using
the quantum simulation method based on Density Functional Theory (DFT). Scanning images of theadsorption possibility are shown for the six types of VOCs in breath of lung cancer patients on thesesubstrate materials by using computational DFT-base Nanoscope [2] to determine the potential
adsorption areas and the path of VOCs diffusion on the surfaces of substrates. The adsorption energy is
calculated by DFT method combining with the five approaches of van der Waals dispersion: revPBE,
optPBE, optB88, optB86b and DFT-D2. Charge transfer between the substrates and VOCs is explored by
calculating the Bader charges. In addition, the effect of electric field on the adsorption is also
investigated.
[1] Phillips, M. et al. Lancet. 353, 1930–1933 (1999), ib. Chest. 123, 2115–2123 (2003).
[2] Developed by author (Vietnam Japan University).

668

MMM 2018

(O2 oral)
Adsorption of the Volatile Organic Compounds on Graphene including
Van de Waals Interaction*Thi Viet Bac Phung1, Trong Lam Pham1, Yoji Shibutani1,2, Van An Dinh 1,2
1. Nanotechnology Program, Vietnam Japan University –Vietnam National University, 2. Center for Atomic andMolecular Technologies, Graduate School of Engineering, Osaka University

The interactions of some volatile organic compounds (VOCs) such as ethanol, acetone, benzene, toluene,
diclomethane with graphene are systematically investigated by using first principles calculations. The
VOCs are chosen as selected examples of main VOCs in exhaled breath in lung cancer patients [1, 2]. To
evaluate the adsorption sites of VOCs on graphene, we have performed simulation including physicaladsorption under the different van de Waals functionals. The global minimum energy configurations andbinding energies for VOCs molecules adsorbed on graphene are determined by using Computational

DFT-based Nanoscope [3] for imaging the binding possibility of the adsorbed molecules on graphene. It isshown that the adsorption energy is highly sensitive to the wDW potentials. We explore the fundamental
insights of the interactions between VOCs molecules and graphene. Furthermore, the effects of theexternal electric field on the charge transfer between the adsorbed molecules and graphene are also
discussed.

Keywords: Graphene; VOCs adsorption; Charge transfer; Electric field; First-principles calculations
References[1] P. Fuchs, C. Loeseken, J. K. Schubert, and W. Miekisch, International Journal of Cancer 126, 11,2663–2670, 2010.
[2] X. Sun, K. Shao, T. Wang, Anal. Bioanal. Chem. 408, 11, 2759-80, 2016.
[3] Developed by V. A. Dinh, Vietnam Japan University.

MMM 2018

669

(O2 oral)
Diffusion of a Cu nanodroplet on an amorphous carbon surface
Yong-Long Chen1, *Yu-Chen Chiu1, Kai Chen2, Yu-Chieh Lo1
1. National Chiao Tung University, 2. Xi'an Jiaotong University

Due to better electrical and thermal conductivity, copper (Cu) nanowires are promising material for theapplication in the next-generation transparent conductors, integrated circuits. Recently, there have been
few reports on the discovery of single-crystalline metal nanowires growing on a carbon substrate. It isnoteworthy that on amorphous carbon substrates the most productive growth of Cu nanowire can beachieved. The development of such a direct process is important and attractive for the production of Cu
nanowires. Amorphous carbon films play an important role in the production of Cu nanowires. However,
the influence of amorphous carbon films on Cu still remains unclear. According to our research, becauseof the roughness of amorphous carbon surface, Cu atoms are difficult to diffuse on an amorphous carbon
film. Nevertheless, Cu can diffuse on amorphous carbon surface with the form of cluster. To confirm this,we conducted molecular dynamics (MD) simulations to explore cluster diffusion behaviors of nanoscale
Cu on various carbon films with different morphology. The surface effects, including surface roughness
and sp2/sp3 bond ratio of carbon films. The aim of this work is to understand cluster diffusion ofnanoscale Cu on the amorphous carbon films in order to find out the optimal controlling factors for theformation of Cu nanowires.

670

MMM 2018

(O2 oral)First principal modeling of oxygen and carbon adsorption on Fe (110)
surface with symmetrical tilt Sigma3(111) grain boundary*Ivan Lobzenko1, Yuki Uchiyama1, Yoshinori Shiihara1
1. Toyota Technological Institute

Iron is material of great interest for vast majority of industries. Being a good catalyst, Fe plays an importantrole in chemical reactions with carbon and oxygen atoms involved. Despite extensive experimental andtheoretical studies a complicated process of oxidation and carburization of iron surfaces is not wellunderstood yet [1,2]. In particular, low attention is paid to chemisorption on iron surfaces with grainboundaries. Nowadays with extensive development of methods for production of metals with nano-hetero
structure, studies of adsorption on surfaces with high concentration of grain boundaries became an
important issue.
In the present work the study of oxygen and carbon atoms adsorption on iron surfaces have been made bymeans of density functional method. Adsorption on Fe (110) surface with symmetrical tilt Sigma3(111)
grain boundary is compared to that on “clean” Fe (100), (110) and (111) surfaces. It is revealed thatgrain boundary enhances adsorption properties of iron for both oxygen and carbon atoms. In order toexplain such effect, modeling of adsorption on clean surfaces under various strains was made.
Comparison of electronic properties of structures under study shows correlation of adsorption energies
with the electronic density of states at Fermi level.
References[1] T.Ossowski, A. Kiejna, Surface Science. 637-638, p. 35 (2015);[2] H. Grabke, Materials at High Temperatures 17, p. 483 (2000).

MMM 2018

671

(O2 oral)
Theoretical study of the effects of boron doping on the electronic
structure of g-C3N4/TiO2(001) heterojunction*Jianhong Dai1, Yan Song1
1. Harbin Institute of Technology at weihai

The formation of g-C3N4/TiO2(001) heterojunction between anatase TiO2 crystal and graphite-like carbon
nitride can decrease the recombination of photo-generated electron-hole pairs, and improve the chargetransfer properties. The influence of boron doping on the stability and electronic structures of g-C3N4/TiO
2(001) interfaces were studied by first principles calculations. The stable g-C3N4/TiO2(001) interface andpossible occupations of boron dopant in the interface were studied by comparing the formation energies
of different configurations. The boron was found to be easily doped in the g-C3N4/TiO2(001) interfacialzone. The electronic structures of g-C3N4/TiO2(001) interface and boron doped systems were evaluated.
Strong bonding interactions between the g-C3N4 and TiO2(001) were found in the g-C3N4/TiO2(001)interface, and the boron dopant can improve the charge transfers between g-C3N4 and TiO2(001).
Therefore, the boron may improve the photocatalytic properties of g-C3N4/TiO2(001) heterojunction.

672

MMM 2018

(O2 invited)
Atomistic Simulations that Reach Anthropological Timescale andBeyond*Ju Li1
1. Massachusetts Institute of Technology

Atomistic and first-principles modeling, which describe the world as assembly of atoms and electrons,
provide the most fundamental answer to problems of materials. However, they also suffer the most severe
timescale limitations. For instance, in molecular dynamics (MD) simulations, in order to resolve atomic
vibrations, the integration time step is limited to hundredth of a picosecond, and therefore the simulation
duration is limited to sub-microsecond due to computational cost. Although a nanosecond simulation isoften enough (surprisingly) for many physical and chemical properties, it is usually insufficient forpredicting microstructural evolution and thermo-mechanical properties of materials. In this invited talk I
will discuss recent attempts at overcoming the timescale challenges of atomic-resolution simulations: (a)strain-boost hyperdynamics [Phys. Rev. B 82 (2010) 184114] for simulating primarily displacive eventsand associated issues of activation entropy and the Meyer-Neldel compensation rule, (b) diffusive
molecular dynamics (DMD) [Phys. Rev. B 84 (2011) 054103] for microstructural evolution driven byrepetitive diffusion events and coupled displacive-diffusive processes.

MMM 2018

673

(O3 invited)
Scale Dependence of Friction: How Elasticity Destroys Superlubricity
Joseph Monti1, Lars Pastewka2, *Mark Owen Robbins1
1. Dept. of Physics and Astronomy, Johns Hopkins University, 2. University of Freiburg

Friction in single-asperity contacts is studied as a function of contact radius a, substrate stiffness G,
atomic structure, adhesive strength and adsorbed layers. Friction between bare, rigid surfaces can beobtained by a simple sum over atomic forces. When surfaces are aligned and have the same periodicity,
the forces add in phase and the friction force F rises linearly with area A. When the two surfaces are
misaligned or disordered, so that there is no common periodicity, forces add out of phase and F~Ax with x
less than or equal to 1/2. This is known as structural superubricity and implies that friction vanishes in thelimit of large contact sizes. Most surfaces do not share a common period but friction is almost always
observed at macroscopic scales. We use an efficient Greens function technique to study contacts withdimensions of micrometers while resolving atomistic interactions at the surface. A new formulation that
includes dynamic effects explicitly will be described. For small tips and high loads the contact area follows
predictions for contact of rigid surfaces, x=1 for identical aligned surfaces, x=1/2 for random surfaces and

x=1/4 for incommensurate crystals. Elasticity becomes important when a exceeds the core width bcore ofinterfacial dislocations. For a>bcore parts of the contact can advance independently. The friction for
identical aligned surfaces drops as a power law and then saturates at the Peierls stress for edge
dislocations. The friction on incommensurate and disordered surfaces saturates at nearly the same value.
Thus for all geometries x=1 in large contacts. While this means that elasticity destroys superlubricity, thefriction between bare surfaces drops exponentially with the ratio of substrate stiffness to local interfacial
shear stress. In contrast, when adsorbed layers are included between surfaces, all geometries have x=1
with a large prefactor.

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Coarse-Grain Simulations of Polymer Solutions with Hydrodynamics
and Long-range Interactions*Hitoshi Washizu1,2, Hiroaki Yoshida3, Soma Usui1, Taiki Kawate1
1. Univ. Hyogo, 2. Kyoto Univ., 3. Toyota Central R&D Labs., Inc.

Polymer solution is used as lubricant in Tribological purpose. Viscosity index improver is used tonormalize temperature dependence of viscosity of the lubrication oil. Here we show our numericalsimulation approach to investigate the dynamics of polymer solution. The numerical scheme is for
simulating the dynamics of suspensions of Brownian particles, coupling molecular motion treated byLangevin equation and hydro-dynamics treated by lattice Boltzmann method.
In order to simulate bulk properties of polymers under shear, we apply periodic external field so that to
make shear field. The external force is applied to the fluid dynamics part. Polymer is dragged to the region
where the shear force is strongest, and changes its shape. The structure change is described by the radiusof gyration which decreases with time.
In order to describe confined system, non-slip boundary is adopted in bottom line and moving wall is set
in top line. During the simulation, the polymer are pull to upper layer where the shear field is large. To
show the orientation of the molecule, order parameter is calculated. Due to the effect of external field, theorientation of the polymers changed to the direction of the shear force.
In above simulation, the chemical properties of the polymers are not discussed. We simulate theequilibrium structure of polymer using point dipole interactions. We use Monte Carlo Brownian Dynamics
method for time integration.The radius of gyration of non-polar polymers are same as analytical theory.
The structure of the polymers with functional groups is calculated. The interaction between functionalgroups make tight structure in low temperature and show high viscosity index than non-polar polymers.
The polar polymer shows strong initial distribution dependence.

MMM 2018

675

(O3 oral)
A Multi-Scale Approach for the Design of Novel Lubricants*Georgios Bletsos1, Konstantinos Gkagkas2, Varvara Asouti1, Evaggelos Papoutsis-Kiachagias1,
Daniele Savio3, Kyriakos C Giannakoglou1
1. National Technical University of Athens, 2. Toyota Motor Europe NV/SA, 3. Fraunhofer Institute for Mechanics ofMaterials IWM

Friction accounts for ~15% of fuel energy losses in conventional vehicles. A deeper understanding of
lubrication mechanisms in engineering systems and how to accurately model them is therefore necessary.
Atomistic simulations can provide fundamental insights in the fluid behaviour under extreme contact
conditions, at a high computational cost. On the other hand, continuum methods, while capable ofefficiently solving macroscopic problems, cannot resolve features and flow patterns at the nanometerscale, due to the breakdown of the continuum assumption [1]. In this work, we employ a multi-scale
approach that combines continuum and particle-based descriptions for simulating hydrodynamic
lubrication systems, while seeking ideal designs of virtual lubricants that maximise load carrying capacityand minimize friction.
Inspired by computational [2] and experimental [3] studies of ionic liquids as lubricants, we emulate theirlayering behaviour and near-wall solidification [2] on the continuum domain by introducing an
inhomogeneous viscosity field in the Navier-Stokes equations. Using an Evolutionary Algorithm, optimal
viscosity profiles leading to the minimisation of specific friction are identified. By doing so, a potential
improvement in friction performance up to 65% was found for a converging hydrodynamic slider.
The study is then extended to nano-hydrodynamic lubrication using Coarse Grain Molecular Dynamics
simulations, which can help in selecting specific molecule typologies featuring the aforementioned
viscosity variations - and thus the potential for significant friction reduction - under severely loaded
contact conditions.
1. I. Cosden and J. Lukes, Comput. Phys. Commun. 184 (8) (2013)
2. K. Gkagkas, V. Ponnuchamy, M. Dašić, and I. Stanković, Tribol. Int. 113 (2016)
3. J. Qu, D. Bansal, B. Yu, J. Howe, H. Luo, S. Dai, H. Li, P.J. Blau, B. Bunting, G. Mordukhovich and D.
Smolenski, ACS Appl. Mater. Interfaces 4 (2) (2012)

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MMM 2018

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Impact of ionic liquid ordering on their triborheological properties*Konstantinos Gkagkas1, Andras Vernes2,3
1. Advanced Material Research Division, Toyota Motor Europe NV/SA, 2. AC2T research GmbH, 3. Institute of Applied
Physics, TU Wien

The development of novel lubrication concepts is a multi-scale challenge that requires the correlation of
nano- and meso-scale features, such as molecule structure and their ordering, with macro-scale
properties such as observed friction. Ionic Liquids (ILs) are interesting lubricants that are shown to have apositive impact on friction reduction. A feature that plays an important role in their behaviour is the
Coulombic interaction between ions leading to layering in the transverse direction to the sliding and to anear-wall solidification, as seen both experimentally [1] and numerically [2]. At the same time, in-plane
crystalline structures are also formed [3, 4, 5].
In this contribution, we quantify the impact of the internal structure on the triborheological properties ofILs. More precisely, both structure factors and bond-orientational order parameters will be calculated oncoarse grain MD models of ILs in order to describe the ion ordering and formation of crystalline structures
under bulk and confined conditions. Then an attempt will be made to correlate the detected orderingwith internal (ion shapes and sizes), external (wall structure, pressure and shearing velocity) andconstitutive (e.g., coefficient of friction) system parameters. Through such quantification, we expect an
improved understanding of the underlying physico-chemical mechanisms, guiding us towards the designof optimised tribological systems.
1. A. M. Smith, K. R. J. Lovelock, N. N. Gosvami, P. Licence, A. Dolan, T. Welton, and S. Perkin, J. Phys.
Chem. Lett. 4, 378 (2013).
2. K. Gkagkas, and V. Ponnuchamy, Model. Simul. Mater. Sci. Eng. 25 (6), 064004 (2017).
3. K. Gkagkas, V. Ponnuchamy, M. Dašić, and I. Stanković, Tribol. Int. 113, 83 (2016).
4. R. Capozza, A. Vanossi, A. Benassi and E. Tosatti, J. Chem. Phys. 142, 064707 (2015).
5. O. Fajardo, F. Bresme, A. Kornyshev, and M. Urbakh, Sci. Reports 5, 7689 (2015).

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677

(O4 invited)
Molecular simulation to better understand soot-detergent interactions
in engine oils
*Sophie LOEHLE1, Elias Gebremedhn2, Michael Mazarin1, Stephan Steinman2, Carine Michel2
1. Total M&S, 2. ENS Lyon

In the engine, soot is formed as a result of incomplete combustion. Some of the formed soot is absorbed
into the lubricating oil film present until a certain level is reached in which it precipitates. This lead not
only to the increase of the lubricant’s viscosity but also it adsorbs on the metallic surface, thusincreasing engine wear. Most automotive enhancement additives contain, among other additives,
detergents which help control the agglomeration and deposition of soot particles and other corrosive
contaminants. A typical detergent molecule features a head part with constituting polar functional groups
and a long hydrocarbon tail group [1]. The common perception is that the detergents do the job asdispersants and/or through formation of protective coating on steel surfaces. However, little is known
about the complex physicochemical and structural details of the soot-detergent interfaces, and it seems
that such details are too difficult to capture using conventional experimental techniques alone. In thisrespect, molecular modeling can fill the gap by providing atomistic level understandings on
soot-detergent interactions and interfacial structural features.Finding an atomistic representation of thesoot is the crucial step in modeling soot-detergent interactions. First, we show how we developed reliablesoot model using experimental data [2-4], we employed a deterministic structural elucidation method toautomatically generate representative soot structures. Following, we show how we assessed physisorptionenergies between the soot models and some polar detergent-head groups using the latest semi-empirical
PM7 method, and how the result can be used to complement experimental screening techniques.
References:
1- J.D. Burrington, J. K. Pudelski, J. P. Roski, Chapter 18: Challenges in detergents and dispersants –Practical Advances in Petroleum Processing 2007
2- Christine Esangbedo, Andre L. Boehman, Joseph M. Perez, Tribology International 2012, 47, 194
3- A. La Rocca, F. Bonatesta, M. W. Fay, F. Campanella, Tribology International 86 (2015) 77-84
4- I. Z. Jenei et al., Nanotechnology n°8, Vol 29 (2018)

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MMM 2018

(O4 oral)
A Molecular Dynamics Study on the Wear Mechanisms ofHydrogenated Diamond-like Carbon*Yang Wang1, Jingxiang Xu1, Yusuke Ootani1, Yuji Higuchi1, Nobuki Ozawa1, Koshi Adachi2,
Momoji Kubo1
1. Institute for Materials Research, Tohoku University, 2. Department of Mechanical System Engineering, Graduate
School of Engineering, Tohoku University

Diamond-like carbon (DLC) is a promising solid lubricant and widely used as a lubricant coating in various
industrial applications such as engine, hard-disk, and space instruments. Hydrogenated DLC coatingsshow the much better low-friction properties and lubricity than the non-hydrogenated DLC coatings.
However, wear of the hydrogenated DLC coatings causes the reduction of the durability of DLC coatingsand limits their utilization. For the further improvement of the hydrogenated DLC coatings, the deep andcomprehensive understandings of the wear mechanisms of the hydrogenated DLC are very essential;
nevertheless, these understandings are still lacking because the experiments are difficult to observe thecomplicated atomic-scale friction processes and tribochemical reactions at the interface. To handle these
difficulties, we employ a molecular dynamics method to investigate the wear mechanisms of thehydrogenated DLC. We perform friction simulations in the vacuum by using the DLC models with ahydrogen concentration of 0%, 20%, 30%, 40%, and 50%. From the simulation results, we observe twowear mechanisms of DLC: 1) chemical wear as the generation of hydrocarbons desorbing from DLC
surface and 2) mechanical wear as the transfer of carbon atoms to the counter substrates. The chemical
and mechanical wear co-decide the wear behaviors of DLC. Furthermore, with increasing the hydrogenconcentration in DLC, chemical wear increases and mechanical wear decreases, causing a parabolic-likehydrogen dependence of the total wear with a minimum value at the hydrogen concentration of 30%. Our
simulations successfully give fundamental understandings about the wear mechanisms of thehydrogenated DLC and provide the theoretical instructions to improve the anti-wear properties of DLC
coatings.

MMM 2018

679

(O4 oral)
Effect of Tribochemical Reactions on Diamond-like Carbon and Wearunder Water Lubrication: A Molecular Dynamics Simulation
Investigation*Jing Zhang1, Yang Wang1, Jingxiang Xu1, Yusuke Ootani1, Nobuki Ozawa1, Koshi Adachi2,
Momoji Kubo1
1. Inst. for Materials Research, Tohoku Univ., 2. Dept. of Mechanical Systems Engineering, Tohoku Univ.

Diamond-like carbon (DLC) coatings possess excellent frictional properties. However, the wear of DLC
coatings is still a problem. To improve the properties of DLC, clarification of the wear mechanisms isrequired. It is reported that the wear of DLC under the water lubrication is lower than that in the air. To
expound the wear mechanisms, clarification of this phenomenon is strongly demanded. However, thedetailed mechanism is still unclear due to the complicated tribochemical reactions. Therefore, we studied
the wear mechanisms of DLC under both vacuum and water environment by using the molecular
dynamics method. The DLC model is made of two hemispherical substrates, to simulate real roughsurfaces. The friction simulations were carried out by sliding two substrates. During the friction, underboth environments the transfer of carbon atoms to their counter substrates was observed, indicating theadhesive wear of DLC. The number of transferred carbon atoms under vacuum condition was much larger
than that under water lubrication, suggesting that the water environment suppresses the wear of DLC.
Considering that interfacial C-C bonds which connect two DLC substrates promotes the transfer of carbon
atoms, we investigated the number of interfacial C-C bonds. The number of interfacial C-C bonds underwater lubrication was less than that in vacuum, indicating that the formation of the interfacial C-C bonds
was suppressed by the water. To understand the cause of the suppression effect of water on theinterfacial C-C bonds formation, we investigated the chemical reactions during friction process. With
friction time going, the increase in the C-H and C-O bonds and the decrease in water molecules wereobserved, indicating the reaction between water molecules and carbon atoms. We suggest that thesechemical reactions increase H and OH terminations on the DLC substrates and suppress the formation of
the interfacial C-C bonds, and therefore the adhesive wear is inhibited.

680

MMM 2018

(O4 oral)
Formation Mechanism of Tribofilm of Silicon Carbide under Water
Lubrication: Molecular Dynamics Simulations*Fumiya Nakamura1, Yang Wang1, Jingxiang Xu1, Yusuke Ootani1, Nobuki Ozawa1, Koshi Adachi1, Momoji Kubo1
1. Tohoku Univ.

Water lubrication has the characteristic of low environmental burden. It is known that silicon carbide (SiC)
shows quite low friction coefficient by the formation of a tribofilm on the surface of SiC during water
lubrication. Understanding of its mechanism is essential to improve friction characteristic for practical use
and application. However, it is difficult to observe directly such a complicated phenomenon including
friction and chemical reaction by experiments. Therefore, in this study, molecular dynamics method which
can simulate chemical reaction was conducted to analyze the structure and formation mechanism oftribofilm in the friction process of amorphous SiC under water environment.
In the simulation, we prepared the model consisting of two amorphous SiC substrates sandwiching watermolecules. The substrates have rough surfaces and the two of substrate surfaces can be in contact during
the friction. The substrates were given a constant pressure (3 GPa) and slid for 1000 ps at 100 m/s.
We performed the sliding simulation of SiC under water lubrication. In the initial structure, the surface ofthe substrates was corrugated. However, the rough surface of the substrates was smoothed while Si atoms
from the a-SiC substrate reacted with water molecules and formed bonds with O atoms of H2O during
friction. Firstly, we investigated the change in number of water molecules and Si-O-Si bonds during
friction. It was found that the number of water molecules decreased constantly while the number of
Si-O-Si bonds increase correspondingly. This result shows that Si reacts with H2O to form a SiO2 layer.
Next, we investigated the change in the distribution ratio of C and Si before and after friction. The ratio of
C was increased around the substrate surfaces and that of Si was increased on the substrates indicatingthat the bilayer tribofilm of the SiO2 and C rich SiC was generated on the substrate by the tribo-chemical
reaction.

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681

(O5 invited)On the formation of superlubricious layers during boundary
lubrication of diamond and diamond-like carbon*Michael Moseler1,2
1. Fraunhofer Institute for Mechanics of Materials IWM, 2. University of Freiburg

Ultralubricity and superlubricity has been observed for diamond and tetrahedral amorphous carbon (ta-C)
coatings lubricated with water and unsaturated organic friction modifiers, respectively. Although
spectroscopic characterization suggests that lubricity is connected to the formation of oxygen-containing
surface layers, the underlying tribo-chemical mechanisms and the detailed structures of these layers
remain elusive. This talk presents atomistic calculations on various scales that elucidate themechano-chemical processes for the formation of oxygen functional groups and aromatic protective
layers [1,2].
In the first part of this talk, quantum molecular dynamcis calculations are presented that shed light onfriction regimes for diamond lubricated by water and the different characteristic layers in the different
regimes [1]. We find four universal friction regimes that are active on all low-index diamond surfaces:
cold-welding&amorphisation, ether cold-welding without amorphisation, H/OH-termination,
H/OH-termination with water layer coverage. In addition four special regimes are found: aromatic Pandey
surface reconstruction for (111), ether/keto passivation for (110) and (001) as well as two mild
cold-welding regimes for (110).

In the second part, combined experiments and multiscale simulations are presented that unveil thetribochemical mechanism leading to superlubricity of ta-C/ta-C tribopairs lubricated by unsaturated fatty
acids [2]. Experiments show that superlow friction can be achieved by lubrication with unsaturated fatty
acids. Atomistic simulations reveal that, due to the simultaneous presence of a carboxylic group and a
C=C double bond, unsaturated fatty acids can concurrently chemisorb on both ta-C surfaces and bridge
the tribogap. The resulting, sliding-induced mechanical strain triggers a cascade of molecularfragmentation reactions that release passivating hydroxyl, keto, hydrogen and olefinic groups to the ta-C
surfaces. These findings provide design principles for novel organic friction modifiers that promote theformation of superlubricious amorphous carbon layers.

[1] T.Kuwahara, G.Moras, M.Moseler, Phys. Rev. Lett. 119 096101 (2017)[2] T.Kuwahara, P.A.Romero, S.Makowski, V.Weihnacht, G.Moras, M.Moseler, under consideration (2018)

682

MMM 2018

(O5 oral)
Unveiling the chemical reactions involved in moisture-induced
weakening of adhesion between aluminum and epoxy resin: a hybrid
quantum-classical simulation study*Shuji Ogata1, Masayuki Uranagase1
1. Dept. of Physical Science and Engineering, Nagoya Institute of Technology

Epoxy resin is commonly used as an adhesive for bonding metals. Recent industrial demands, e.g.,light-weight multi-material cars and highly reliable pressure sensors, motivate researchers to improve thereliability and durability of bonded materials. One critical issue regarding adhesive bonding is theweakening of its bonding strength by the water that migrates from a moist environment along theinterface. When bonding naturally surface-oxidized aluminum with epoxy resin, the adhesion strength
reduces by about 40 % in a moist environment. Surface treatment techniques, such as sealing with
polyvinyl chloride, only delay such adhesion weakening.

Despite the importance of this issue, adhesion weakening is not well understood. To understand themechanisms at the electronic structure level, we perform atomic dynamics simulations on various Al andepoxy resin interface systems with water molecules inserted in the contact region. In accordance withexperimental conditions, the Al layer is surface oxidized to a depth of 10 Åwhile the bisphenol-A typeepoxy molecule has both OH and ether groups. Shear deformations are simulated using the hybrid
quantum-classical method in which about 1,500 atoms at the contact region are treated withdensity-functional theory.

For the first time, calculated adhesion strengths compare well with the experimental values. Former
simulations gave one order of magnitude larger adhesion strength than the experimental value. Threetypes of chemical reactions that affect the adhesion strength are found to occur depending on theterminal functional groups of the Al oxide surface and the water layer formation. Separate calculations
confirm small barrier energies for all the reaction processes.

MMM 2018

683

(O5 oral)
Efficient evaluation of adhesion free energy between a liquid andpolymer-grafted substrate*Masayuki Uranagase1, Shuji Ogata1
1. Nagoya Institute of Technology

Relating to recent rapid advancement in substrate-decoration technique such as the one using the
Self-Assembled Monolayers, there exist strong demands to predict the adhesion free energy between acomplex liquid and polymer-grafted substrate. In principle, the adhesion free energy can be evaluatedthrough the molecular simulation by integrating the work required to separate the liquid from thesubstrate. In former methods for the work of adhesion, a planar shape potential is introduced and thework of adhesion is calculated through integrating the force exerted on the potential from liquid
molecules when liquid molecules are separated from the solid surface by gradually shifting the potential
origin. These methods work well for sufficiently flat solid substrate. However, they are inefficient for apolymer-grafted substrate because it becomes necessary to shift the potential over a long distance.
Motivated by that, we propose a novel method to efficiently evaluate the work of adhesion of the interface
between a liquid and a polymer-grafted surface. In the present method, a set of spherical potentials
whose centers are set at the atomic positions of either polymer or substrate are introduced instead of aplanar potential. It enables efficient separation of the liquid molecules from the polymer-grafted
substrate. Moreover, potential-parameter update schemes are carefully chosen to suppress sharp
variation in the force exerted on the potential and thereby to minimize the number of integration points.
The present method is applied to the interface between a water and a gold substrate on whichpoly(ehtylene oxide)s (PEOs) are grafted. Then, we find that the work of adhesion tends to become large
at the intermediate density of PEO. This tendency is related to the number of water molecules accessible
to oxygen atoms in PEO.

684

MMM 2018

(O5 oral)
Ultimate response of confined fluids under extreme conditions: a
Molecular Dynamics analysis*Alejandro Porras-Vazquez1, Laetitia Martinie1, Philippe Vergne1, Nicolas Fillot1
1. INSA Lyon

In order to control energy losses in mechanical systems, the study of friction in lubricated contacts has tobe approached. Depending on the operating conditions and its chemical nature, the confined fluid can
exhibit a limiting shear stress at high pressure [1, 2]. The physical origins of this phenomenon and its
implications to friction are currently not completely understood [3]. In this study, an extensive Molecular
Dynamics analysis of the shearing behavior of different types of molecules is performed. The behavior of
the confined sheared system is explored and compared to the fluid bulk state. One of the main conclusion
is that flow profile results from two competing forces in the molecular system, which is linked to theweakest interfaces being sheared. Additionally, the saturated shear stress response is found to be related
to the concept of atomic mobility. With increasing pressure, the relaxation time of the material isincreased. Thus, atomic mobility can be saturated for a range of high pressures, diffusion would not beallowed, and a solid-like behavior is displayed. Finally, a link between flow and friction behavior is
discussed.

[1] Martinie, L. and Vergne, P., “Lubrication at extreme conditions: a discussion about the limiting shear
stress concept”, Tribol. Lett., 63, 2016.
[2] Ewen, J. P., Gattinoni, C., Zhang, J., Heyes, D. M., Spikes, H. A. and Dini D., “On the effect of confined
fluid molecular structure on nonequilibrium phase behaviour and friction”, Phys. Chem. Chem. Phys.,
2017.
[3] Ndiaye, S. N., Martinie, L., Philippon, D., Devaux, N. and Vergne, P., “A Quantitative Friction-BasedApproach of the Limiting Shear Stress Pressure and Temperature Dependence”, Tribol. Lett., 65, 2017.

MMM 2018

685

(O6 invited)
Modeling the plastic deformation of a metal crystal induced by contact
with a rough rigid surface*Lucia Nicola1,2
1. Delft University of Technology, 2. University of Padova

When rough bodies are pressed against each other, a small contact area has to support all the load. This
is why plastic deformation occurs even at moderate nominal contact pressure. Although indentation ofelastic bodies by self-affine rough indenters has been studied extensively, little attention has so far been
devoted to plasticity. This is mostly because capturing plasticity as well as contact with a self-affine roughsurface is computationally quite challanging. Here, we succeed in achieving this goal by using Green's
function dislocation dynamics, a modeling technique that combines discrete dislocation plasticity (DDP)
with Green's function molecular dynamics (GFMD). As usual in DDP, plasticity is studied by tracking thenucleation and glide of each dislocation in the metal crystal. The dislocation image fields are instead
calculated with GFMD, which allows to describe the self-affine rough surface using wavelenghts spanningfrom 5 nanometers to 100 micrometers.
Simulations are performed varying various parameters of the surface roughness, including the Hurst
exponent and the root-mean-square height. Results show that the plastic response is size-dependent. An
important implication of the size-dependence is that, when bodies deform plastically, it is not possible toscale observables such as contact pressure and contact area with crystal size or root-mean-square height,
as typically done for elastic contact problems.

686

MMM 2018

(O6 oral)
On the potential use of liquid crystals as ‘smart’ lubricants –an MD
simulation study*Kerstin Falk1, Konstantinos Gkagkas2, Gianpietro Moras1, Michael Moseler 1,3
1. Dept. of Tribology, Fraunhofer IWM, Freiburg, 2. Toyota Motor Europe, Technical Center, Zaventem, 3. Dept. ofPhysics, University of Freiburg

Liquid crystals (LC) –well-known for their application in LC-displays - have recently raised interest also for
lubrication applications: favorable friction properties (specific friction of the order 0.005) have been
experimentally observed in different systems [1]. The special behavior is attributed to the anisotropic
viscosity and compressibility, which LC exhibit in semi-ordered phases (i.e. nematic or smectic phase).
The LC molecules' elongated shape allows them to align with the shear, which could simultaneously
reduce friction and increase load support. Moreover, the complex phase behavior and dynamic properties
might also be exploited to design novel tribological systems with tunable or self-regulating friction,so-called ‘smart’ lubricants [2,3].
Here, we present a molecular dynamics simulations study of different LC lubricated model tribo-systems
with the aim to characterize the LC rheological properties and investigate possible tuning mechanisms on
a microscopic level. Shearing simulations of LC lubricating films with different structure characteristics,
i.e. preferential molecule orientation, were performed for varying loads and shear rates. The response of
the systems (velocity profiles, local viscosity and shear force) was evaluated and correlated to the fluid
film structure, which is influenced by the flow. Of particular interest are situations, where the inherent
equilibrium structure of the LC film (induced by surface boundary conditions or external fields) is different
from the shear direction, due to the competing forces for aligning the LC molecules in different directions.
[1] T. Amann, C. Dold and A. Kailer, Tribology International 65 (2013), p. 3
[2] C. Tadokoro, T. Nihira and K. Nakano, Tribol Lett 56 (2014), p. 239[3] S. Itoh, Y. Imura, K. Fukuzawa andH. Zhang, Langmuir 31 (2015), p. 11360

MMM 2018

687

(O6 oral)
Sliding on physisorbed cetyltrimethylammonium bromide (CTAB)*Johannes Laurin Hoermann1, Lars Pastewka1
1. University of Freiburg

In solution, amphiphilic surfactants can form physisorbed boundary films on immersed surfaces. A film’s
structure and thereby its lubrication performance depend on numerous properties of all involved
substances, such as the surface’s potential, as well as surfactant and background electrolyte
concentration. We investigate the dependency of the coefficient of friction on adsorption film topography
in exemplary contact systems by means of classical all-atom molecular dynamics. For this purpose, thepresent study focuses on cationic CTAB at idealized aqueous solution–gold interfaces. Recent
parametrizations of CTAB, originally intended for micelle modeling in bulk solution, are combined withrepresentations of simple gold surfaces. The stability of differently shaped pre-assembled surface
aggregates at various effective surfactant concentrations are compared against expected behavior as
documented by abundant experimental contributions. We finally present friction tests in simple shear
geometries of surfaces covered with pairwise identical CTAB film morphologies.

688

MMM 2018

(O6 oral)
Atomistic simulations of amines as organic friction modifiers
Rafael Pereira de Matos1, Sophie Loehlé2, Clotilde Minfray1, *Manuel Cobian1
1. LTDS ECL, Universite de Lyon, 2. TOTAL Marketing & Services, Solaize

Amines molecules can be used as organic friction modifiers OFM in a
lubricated contact working under in mixed/boundary regime.
Several experimental studies have been already done using steel against
steel contact. Our aim is to propose
a computational simulation protocol to have a better understanding ofwhat happens at atomic scale in the contact.
We study some physical properties (density, diffusivity) of amines
blended in Poly Alpha Olephin PAO using
molecular dynamics MD simulations employing the OPLS force field.
Green-Kubo formalism gives us access to diffusion coeficients of aminesin the oil.
Then, we present ab-initio (DFT+U based) study of the adsorption ofthis OFM on alpha-hematite Fe2O3 surface {01-12}.
The interactions between the amine molecule and the surface will beanalysed and used to fit Lennard-Jones potential
to model the interaction and open the way to new simulations using
molecular dynamics.
We will conclude presenting simulations of the friction process
applying stress and shear and computing a
numerical coefficient of friction.

MMM 2018

689

(O7 invited)Toward Exascale Atomistic Simulations of Interfaces*Aiichiro Nakano1
1. Univ. of Southern California

Under the U.S. Department of Energy, Aurora/A21 early science program, our group will be one of the 10
initial users of the Nation's first exaflop/s computer that can perform 1018 mathematical operations per
second, when it is introduced in 2021. To make quantum molecular dynamics (QMD) and reactive
molecular dynamics (RMD) simulations exa-scalable, we have developed an extension of thedivide-and-conquer algorithmic framework called divide-conquer-recombine. On today's supercomputing
platform, for instance, the framework has achieved over 98% of the perfect speedup on 786,432 IBM Blue
Gene/Q processors for 40 trillion electronic degrees-of-freedom QMD in the framework of density
functional theory and 68 billion-atom RMD. Production simulations include (1) 16,616-atom QMD
simulation of rapid hydrogen production from water using metallic alloy nanoparticles, (2) 6,400-atom
nonadiabatic QMD simulation of photoexcitation dynamics for efficient solar cells, and (3) 112
million-atom RMD simulation of metacarbon synthesis by high temperature oxidation of SiC nanoparticles.
We will discuss tribology and interfacial applications such as frictional heating of energetic molecular
crystals.

690

MMM 2018

(O7 oral)
Molecular Dynamics Simulation Study on the Structure, Role, andFormation Mechanism of Tribofilms of Silicon-Based Materials in
Water*Yusuke Ootani1, Jingxiang Xu1, Naoki Takahashi1, Koshi Adachi2, Momoji Kubo1
1. Institute for Materials Research, Tohoku University, 2. Department of Mechanical Systems Engineering, Graduate
School of Engineering, Tohoku University

It is known that silicon-based materials such as silicon carbide have low friction coefficients in water
lubrication system. A tribofilm formed at a sliding interface is considered to reduce the friction coefficient,however detailed mechanism is still unclear because in-situ observation of the atomic scale sliding
interface is difficult. Therefore, in this work, we investigated the structure, role and formation mechanismof the tribofilm of silicon-based materials by using molecular dynamics method. We performed sliding
simulations of a SiO2 substrate, which is a model of a native oxide layer of silicon-based materials, in water
environment. We found that hydrolysis reaction of Si-O bond in SiO2 surfaces occurred at contact areas ofthe surfaces as, Si-O + H2O →Si-OH + OH. The hydrolysis reaction was mediated by proton transfer
process. Since the hydrolysis reaction dissociated Si-O bonds, Si-O bond network of the SiO2 surfaces
became sparse, and then, several SiO2 clusters were removed from the surfaces as wear debris. The wear
debris were dissolved in the water layer between two surfaces, forming a colloidal silica layer. On theother hand, the water molecules penetrated into the sparse SiO2 surfaces and hydrate the surfaces,
forming a hydrophilic silica gel layers on the SiO2 surfaces. Therefore, at the sliding interface, the colloidal
silica layer was sandwiched by two silica gel layers. The colloidal silica layer prevents the contact of thesurfaces and reduces a friction force, whereas the hydrophilic silica gel layer holds the colloidal silica
layer at the sliding interface. Thus, the colloidal silica and silica gel layer that formed by hydrolysis
reaction reduces the friction coefficient of silicon-based materials in water lubrication systems.

MMM 2018

691

(O7 oral)
Influence of Tribo-Film Structure Generated from MoDTC at DLC/DLC
Interface on Friction Behavior : A Molecular Dynamics Simulation*Masahiro Saito1, Jingxiang Xu1, Yusuke Ootani1, Nobuki Ozawa1, Koshi Adachi2, Momoji Kubo1
1. Institute for Materials Research, Tohoku Univ., 2. Graduate School of Engineering, Tohoku Univ.

Diamond-like carbon (DLC) has excellent friction properties such as low friction and high wear resistance.
Thus, it is used as a solid lubricant material for engine parts. For this application, it is expected that DLC is
used with the MoDTC friction modifier. When MoDTC is used as additives, a tribo-film of MoS2 layers isformed at a sliding interface during friction, leading to low friction. It is known that frictional property ofthis system depends on many factors such as composition of additives, a structure of DLC, and so on.
Thus, to improve the friction property, it is required to understand effects of these factors on friction
behavior of the tribo-film. However, the friction behavior is unknown because in situ observation of thesliding interface is difficult. Thus, in order to investigate the friction behavior, we performed molecular
dynamics simulation by using reactive force field, which takes into account the chemical reactions. In thisstudy, we analyzed the influence of the defects in the MoS2 layers on friction behavior of DLC by using a
DLC/MoS2 layers/DLC model. Here, we investigated the influence of the defects by comparing the friction
behavior of the MoS2 layers with and without grain boundaries. The friction simulation of the model
without grain boundaries shows that the DLC/MoS2 interface is the sliding interface. On the other hand,the simulation of the model with grain boundaries shows that the MoS2/MoS2 interface is the sliding
interface since C-S bonds is formed between DLC and MoS2 around grain boundaries. In addition, theMoS2 layers with grain boundaries show higher friction force because the sliding between the MoS2 layers
is inhibited when a grain boundary attacks another grain boundary in neighboring layers. Then, we foundthat these friction behaviors caused by grain boundaries in the MoS2 are major factors that increase
friction force in this system. We will report the influence of other defects at the conference.

692

MMM 2018

(O7 oral)
Shear-induced amorphization of silicon and diamond yields liquid-like
amorphous solids*Gianpietro Moras1, Andreas Klemenz1, Thomas Reichenbach1, Adrien Gola2, Hiroshi Uetsuka3,Michael Moseler1,4, Lars Pastewka1,5
1. Fraunhofer Institute for Mechanics of Materials IWM, 2. Institute for Applied Materials, Karlsruhe Institute ofTechnology, 3. Asahi Diamond Industrial Co. Ltd., 4. Institute of Physics, University of Freiburg, 5. Department ofMicrosystems Engineering, University of Freiburg

Silicon and diamond are brittle materials with the same crystal structure but opposite density change
upon melting. Using molecular dynamics simulations, we show that both crystals can respond to shear
deformation by undergoing mechanical amorphization. The resulting amorphous material has liquid-likestructure, is denser than the crystal in silicon and less dense than the crystal in diamond. As a result, asnormal pressure increases, amorphization, which is often related to nanoscale wear, is enhanced in silicon
but suppressed in diamond. Moreover, shear-induced amorphization of silicon yields a high-density,
ductile amorphous material at pressures much lower than the polyamorphic transition (~14 GPa)observed upon hydrostatic compression. These results are potentially relevant for the friction propertiesof other crystals that densify upon melting and show polyamorphism, like germanium and ice.

MMM 2018

693

(O7 oral)
A new damage implementation for Smooth Particle Hydrodynamics
and its application to simulating the wear response of metals.
*Alban de Vaucorbeil1, Christopher Hutchinson1
1. Monash University

Simulations of materials and structures submitted to wear, machining, forming, forging, and impacts, toname but a few, often involve large deformations, damage, and failure. These phenomena are challenges
for usual computational tools such as finite element modeling. When large deformations occur, finite
element methods have limitations due to the need to remesh which is computationally expensive andsometimes fails. Therefore, mesh-free methods such as Smooth Particle Hydrodynamics (SPH) are moresuitable for these applications.
The use of SPH for the simulations of wear, tribology, and impacts problems in solids has recently gained
momentum as large deformations and instability phenomena such as damage and fracture could be easily
handled. However, little work exists on the implementation of damage in SPH. This is thought to be due tothe approximation problems around surfaces. As damage and fracture develops in solids, this creates new
surfaces around which the SPH approximation breaks down.
In this presentation, we present the results of simulations of wear response to show case a new
implementation of damage in SPH that solves some of the problems due to the approximation
break-downs close to surfaces. This implementation is built as an upgrade of the SPH package for solids
available within LAMMPS called Smooth Mach Dynamics.

694

MMM 2018

(O8 oral)
Friction mechanism of nanostructured steel in lubricant: A
coarse-grained molecular dynamics study*Yoshitaka Umeno Umeno1, Atsushi Kubo1, Yuta Sudo1
1. The University of Tokyo

Intensive studies have been dedicated to the problem of boundary lubrication in recent years.
Experimental studies have demonstrated a significant reduction in friction coefficients of steel that has
nanostructured surfaces. It is conjectured that nanostructured metals have a large density of sites withstrong attraction to lubricant molecules, resulting in reduced friction. The detailed mechanism of thephenomenon however remains unclear.
To reveal the mechanism of lubrication between metal surfaces, atomic- or molecular-level investigations
are required for the effect of metal-molecule interaction on boundary lubrication. Coarse-grained
molecular dynamics (CGMD) is a useful and efficient simulation method to tackle such problems. Indeed,a CGMD study of shear flow of lubricant molecules between metal surfaces showed that the more
molecules were adsorbed on the surfaces, the lower the friction coefficient became. Nevertheless,molecular-level mechanisms are not fully understood regarding the experimentally presented effect of thesurface nanostructure on the friction coefficient or the nature of structural ordering in shear flow.
In this study, we performed CGMD simulations with the aim to reveal the molecular-level friction
mechanism in boundary lubrication between nanostructured steel surfaces. As lubricant molecules,
polyethylene (nonpolar) and fatty acid (polar) were modeled by the coarse-grained method representing
methylene (CH2) and carboxyl (COOH) groups by individual particles. The effect of the surface
nanostructure was mimicked by distributing atoms producing stronger attraction with the moleculargroups. The effect of alignment of the strong potentials on the friction coefficient is apparent in the caseof polar molecules. In both polar and nonpolar lubricants, the nanostructure enhances critical shear stressfor separation of adsorbed lubricants from the surface. These results suggest salient influence of thesurface nanostructure on the friction coefficient.

MMM 2018

695

(O8 oral)
Coarse-Grained Molecular Dynamic Simulation on the Wear
Mechanism of Polymer Brush with Different Chain Topologies*Zhongmin Liu1, Shuichi Uehara1, Jingxiang Xu1, Yusuke Ootani1, Nobuki Ozawa1, Momoji Kubo1
1. Institute for Material Research,Tohoku Univ.

As a low friction material, polymer brush has attracted attention in many areas because of its good
frictional properties and biological compatibility. The frictional properties of polymer brush are improvedby different chain topologies. However, the effects of the chain topologies on the wear of polymer brush
are still not clear. To reduce the wear of the polymer brush, it is required to understand the wear
behaviors and mechanism of different topological polymer brush. However, in-situ observation of thesliding interface at the molecular scale is difficult. Thus, we investigated the wear mechanism of polymer
brush with different topological chain by using coarse-grained molecular dynamics (CGMD) method.
We perform friction simulations of linear polymer brush, cyclic polymer brush where the polymer chains
have cyclic structure, and crosslinked polymer brush where the polymer chains are connected with theneighboring polymer chain with crosslinker to understand the effect of chain topologies on the wear. We
found that cyclic polymer brush and crosslinked polymer brush had smaller number of dissociation thanlinear polymer brush because the cyclic polymer brush and crosslinked polymer brush showed smaller
frictional forces than that of linear polymer brush. We found that the cyclic polymer brush and crosslinked
polymer brush had less interpenetrating beads than that of linear polymer brush. Therefore, the cyclic
polymer brush and crosslinked polymer brush have smaller entanglements, leading to the low frictional
force. We conclude that chain topologies of polymer brush can improve the wear resistance because they
suppress the interpenetration at the sliding interface.

696

MMM 2018

(O8 oral)
Molecular Dynamics Simulation Study on Friction of Bottlebrush
Polymer with a Cationic Anchor Block Adsorbed on Substrate*Shuichi Uehara1, Zhongmin Liu1, Yusuke Ootani1, Nobuki Ozawa1, Momoji Kubo1
1. Institute for Materials Research, Tohoku Univ.

In recent years, concentrated polymer brush (CPB), which is constructed by grafting polymer chains onto
a substrate with high density, has been developed. CPB has attracted much attention for application to
low friction materials as artificial joints because of its high wear resistance and biocompatibility. However,
the graft of polymer chains onto substrate is complicated and takes much cost. As an alternative method
analogous to CPB, bottlebrush polymer (BBP) known as macromolecules with polymeric side chains is
candidate for low friction materials. Previous studies have revealed that BBPs with a cationic anchor blockadsorbed on a negatively charged surface swell, which produces low friction force. However, the detail offriction mechanism of BBP is not clear. Furthermore, the effects of sliding speed and side chain length on
friction properties are also unknown because the in-situ observation of the sliding interface is difficult.
Thus, computational simulation is required. Herein, to reveal the mechanism during friction of BBP
consisting of a cationic anchor block, we developed a coarse-grained molecular dynamics code which canconsider chemical specificity of each monomer, and performed friction simulation in water solvent. The
low friction coefficient was observed at the low load, but friction coefficient was increased at high load.
We found that, at low load, water beads which coordinate to the side chain of BBP suppressed contact
between BBPs on the counter surfaces, because side chains of BBP prefer to interact with the water
beads. This swelling state of BBP facilitates shear between the opposite substrates. With high load, water
beads were removed from the sliding interface, which reduces the number of water beads around BBP.
Therefore, the contact between BBPs on a counter surface leads to high friction coefficient. The effects ofside chain length and sliding speed on the friction properties will be discussed on the day.

MMM 2018

697

(O8 oral)
Temperature dependent dynamics simulation of traction fluid bymolecular dynamics method*Eiji Tomiyama1,2, Takeshi Iwasaki3, Hitoshi Washizu2
1. Research Organization for Information Science and Technology, 2. Graduate School of Simulation Studies, Univ. of
Hyogo, 3. Lubricants Research Laboratory, Idemitsu Kosan Co., Ltd.

Traction drive is a mechanism that transmits power by a rotor and lubricating oil. In order to evaluate thecharacteristics of the fluid molecules in the traction drive, we construct a simulation method to apply
shear to the fluid confined between solids using LAMMPS, a molecular dynamics simulator.
In this simulation, conditions are set as follows. The boundary condition of the system is a periodic
boundary condition in the horizontal direction. Fluid is composed of a single hydrocarbon compound andthe number of atoms is about ten thousand. The solids are a model metal surface imitating (100) plane of
α-iron, and thier atomic motion is frozen. The force field parameter of fluid molecules uses the Dreiding
force field. Apply a pressure of 0.8 GPa to 1.24 GPa in the direction perpendicular to the solid atoms.
Furthermore, a constant slip of relative sliding speed 1 m/s to 50 m/s is applied. Then, the simulation
time on the order of nanoseconds is passed and the steady state is set. With the above setting, an
ensemble of constant pressure / constant shear state is created.
The temperature dependency of the coefficient of traction is evaluated by changing the temperature of
the fluid from -20 °C to 140 °C. The coefficient of traction also change with the change of temperature,and the tendency is at least qualitatively consistent with the experimental values.

698

MMM 2018

(O9 invited)
Combined experimental and computational study on the
superlubricity mechanism of 2D Materials at the microscale*Tianbao Ma1
1. Tsinghua University

With chemical inertness, atomically flatness and interlayer van der Waals interaction, graphene and other2D materials could act as potential ultra-thin protective coating to reduce adhesion, friction and wear.
Firstly, we predict a stable superlubricity state with vanishing friction between graphene and MoS2
heterostructure. This is attributed to the perpetual interfacial incommensurability with the large intrinsic
lattice mismatch, which leads to near-zero sliding-induced interfacial charge density fluctuation andultrasmooth potential energy surface. Theoretical prediction has found preliminary evidence by themeasurement of interlayer lateral force constant of 2D materials.
Secondly, we report a direct AFM measurement of sliding friction between graphene-coated microsphere(GMS) and graphene, and between GMS and hexagonal boron nitride (h-BN) hetero 2D layers. The
exceptionally low and robust friction coefficient of 0.003 is obtained in ambient atmosphere, under highlocal contact pressure. This sustainable ultralow friction is attributed to the overall incommensurability ofthe multi-asperity contact covered with randomly oriented graphene nanograins. Furthermore, the local
contact pressure fluctuations induced by the atomic roughness could be markedly suppressed by coating
few-layer graphene (layer number > 3) on the contacting asperities.
Thirdly, we report the preservation of ultra-low-friction state on graphene even under harsh chemical
modifications. By proper alignment of graphene on a Ge(111) substrate, friction of graphene could bewell preserved at an ultra-low level even after fluorination or oxidation. This behavior is experimentallyfound to be closely related to the suppression of molecular-level deformation of graphene within the
moiré superlattice structure. Furthermore, friction modulation with dual-scale stick-slip behavior isobserved on graphene/Ru(0001) substrate. The moiré superlattice-level slip instability could be
attributed to the large sliding energy barrier, which arises from the morphological corrugation of grapheneon Ru(0 0 0 1) surface.

MMM 2018

699

(O9 oral)
Quantum chemistry vs. rheology of some EMIM-based ionic liquids
Szerena Kisztina Ujvari1,2, Konstantinos Gkagkas1, *Andras Vernes2,3
1. Advanced Material Research Division, Toyota Motor Europe NV/SA, Technical Center, Hoge Wei 33B, 1930
Zaventem, 2. AC2T research GmbH, Viktor-Kaplan-Str. 2/C, 2700 Wiener Neustadt, 3. Institute of Applied Physics, TU
Wien, Wiedner Hauptstr. 8-10/134, 1040 Vienna

Ab-initio tribology is the part of computational tribology mainly dealing with tribological phenomena
where electrons matter. In a previous work, [1] for example, the electronic contribution to the real contact
area has been estimated by applying Bader’s quantum theory of atoms in molecules (AIM). [2]In this contribution, AIM is used in combination with the density-functional theory (DFT), i.e., for theaccordingly determined ab-initio electronic charge density in case of some1-ethyl-3-methyl-imidazolium-based ionic liquids (ILs) to topologically characterize the involved ions also
when the selected EMIM-based ILs are confined. Considering the calculated Bader’s manifolds for boththe free-standing and confined ions, and the separation area between the ions, an attempt will be made(i) to explain for ILs both experimentally [3] and computationally [4] observed layered structures, and (ii)to evaluate the relaxation time of ions. The latter will also be compared with that derived from themeasured viscosities, and in addition the measured viscosities will be correlated with some
DFT-determined quantum chemical indices [5].
1. M. Wolloch, G. Feldbauer, P. Mohn, J. Redinger, and A. Vernes, Ab-initio calculation of the real contact

area on the atomic scale, Phys. Rev. B 91, 195436 (2015)
2. Richard F. W. Bader, The density in density-functional theory, J. Mol. Struc –TheoChem 943 (1-3), 2(2010)
3. A. M. Smith, K. R. J. Lovelock, N. N. Gosvami, Peter Licence, A. Dolan, T. Welton, and S. Perkin,
Monolayer to bilayer structural transition in confined pyrrolidinium-based ionic liquids, J. Phys. Chem.
Lett. 4, 378 (2013)
4. K. Gkagkas, and V. Ponnuchamy, The impact of coulombic interactions among polar molecules andmetal substrates on flow and lubrication properties, Model. Simul. Mater. Sci. Eng. 25 (6), 064004 (2017)
5. M. Karelson, V. S. Lobanov, and A. R. Katritzky, Quantum chemical descriptors in QSAR/QSPR studies,
Chem. Rev. 96, 1017 (1996)

700

MMM 2018

(O9 oral)
Lubrication with a refrigerant : a challenge made possible thanks to
fluid/surface chemistry
Stéphane TROMP1, Laurent JOLY2, Manuel COBIAN3, *Nicolas FILLOT1
1. Univ Lyon, INSA-Lyon, , CNRS UMR5259, LaMCoS, F-69621 Villeurbanne, 2. Univ Lyon, Universite Claude Bernard
Lyon 1, CNRS, Institut Lumiere Matiere, F-69622 Villeurbanne, 3. Univ Lyon, Ecole Centrale de Lyon, ENISE, ENTPE,CNRS UMR5513, Laboratory of Tribology and System Dynamics, F-69134 Ecully

Lubrication is often seen as the major solution to avoid wear and high friction due to the relative motionof solids in contact. But this sometimes requires a heavy design (e.g. addition of lubricant pumps) while
the actual trend goes for weight reduction of mechanical systems. The use of working fluids (e.g.
refrigerant, gasoline) instead of classical lubricants (oil) allow to bypass this problem. Nonetheless,tribological contacts have to work with ultra low viscosity fluids (not designed to be good lubricants). The
major consequence is that the film thickness separating the surfaces reaches the same order ofmagnitude as surface roughness. At this stage, a minimum quantity of fluid needs to be trapped between
apserities to avoid direct contact. This is made possible if fluid molecules are adsorbed onto the surfaces.
The work presented here aims at explaining experimental observations of R-1233zd refrigerant ability tolubricate a highly loaded contact. Starting from a dedicated force field parametrization for the
refrigerant-hematite interaction (from DFT calculations [1]), Large-Scale Molecular Dynamics simulationsof extreme compression and shearing of R-1233zd show the resistance of the refrigerant adsorption, andits capability to undergo normal and tangential stresses.
[1] S. Tromp, L. Joly, M. Cobian, N. Fillot, Chemical Physics at Interfaces within a Refrigerant-LubricatedContact: From Electronic Structure to Large-Scale Molecular Dynamics Simulations. J. Phys. Chem. C,
2018, DOI: 10.1021/acs.jpcc.7b11267

MMM 2018

701

(O9 oral)
Meso-scale SPH simulation for friction and wear between
elastic-plastic solids with various asperities*Natsuko Nakagawa Sugimura1,2, Le Van Sang2, Yuji Mihara1, Hitoshi Washizu2
1. Dept. of Mechanical Engineering, Tokyo City Univ., 2. Grad. Sch. of Simulation Studies, Univ. of Hyogo,

We made the meso(micron-oder) scale model to simulate friction, wear and adsorption with heating
between elastic-plasic bodies. Constitutive equation is based on the continuum equation and calculatedwith SPH(Smoothed Particle Hydrodynamics). SPH is one of particle methods to solve continuum
equation with mesh free. SPH was initially developed for the hydrodynamics by Lucy et al., but it wasapplied to calculation of solids afterwards by Swegle et al. and has gained good results. In SPH, thephysical variables of the particles are reconstructed by the adding the physical variables of thesurrounding particles by an appropriate weighting called kernel. SPH particles are not truly particles likemolecules but only physical fields. However, we regard the particles on the friction surfaces as also coarse
grained molecules and add the interaction forces directly between such “particles”, which is mimicking
the particle approach. That is, our model is what continuum calculation and particle calculation are
mixed. It is just meso-scale that is situated in between the macro scale to demand continuum approachand the nano scale to demand particle approach. Although its physical discontinuity is not yet resolved, if
the using both approaches confirm the reproducing valid frictional phenomena, the mechanism of its
discontinuity may also be resolved. At this time, for the purpose to efficiently realize the calculation of thegreat amount of particles with parallel code, we use the FDPS open platform by Iwasawa & Makino et al. tocreate our simulation code.
We set the tens of micron system in which elastic-plastic bodies with various asperities are faced and one
body is sliding under the vertical load to cause shear friction. So far, in the system with interface withregular asperities, the elasticity, plasticity, friction cure, heat generation and conduction were to someextent qualitatively expressed, although the frictional results depend on the several parameters and SPH
boundary processing. Then, we will firstly show the characteristics of our model with regular asperity
system in detail. Moreover, we will discuss the frictional properties changing the shape of asperities, i.e.
with realistic asperities.

702

MMM 2018

(P1-74)
Designing Lubricant Additives for Titanium Carbide Surface:First-principles and Molecular Dynamics Investigations*Tasuku Onodera1, Jun Nunoshige1, Hiroshi Kanemoto1
1. Research & Development Group, Hitachi, Ltd.

In order to highly improving performance of industrial products such as automobile, the innovativetechnology should be strongly required, especially for the fields of surface and interface technology.
Several problems on surface fractures, i.e. fatigue, wear, corrosion and erosion have been arisen in a
metallic bearing or gear which typically work under the conditions of high temperature, extreme pressure
and shear.
One of the technologies for reducing surface fractures is a hard coating on surface. For example, in thefield of tribology, diamond-like carbon (DLC) coatings have been strenuously investigated so far. Covering
surface with the film leads to low friction and low wear performances, contributing to extending a productlifetime and to reducing a maintenance work of products. However, under the lubrication by engine oil,
DLC coating unexpectedly caused wear. This negative effect may be due to existence of organic
molybdenum compounds in engine oil as a friction modifier. A novel additive compound has beendeveloped by modifying adsorption property. This story tells us that tailor-made additives have to berequired for own hard coating applied to shear parts.
Titanium-based hard coatings are also forcused for extending lifetime of products. Especially, titanium
carbides show extremely high hardness and coincidently have anti-wear, anti-corrosion, and anti-oxidant
properties. This material has been applied to cutting tools used at high temperature, extreme pressure
and shear. Hence, titanium carbide seems to be a suitable surface coating material for extending lifetime.
In the present study reported here, for the purpose of extending lifetime of industrial products, a chemicalcomposition of lubricant oil was optimized for titanium carbide coatings. An optimum molecular structureof additive compounds and its interaction with titanium carbide were theoretically investigated by using amethod of density functional theory (DFT). An adsorption energy on TiC(100) was calculated for several
additive compounds. The simulation results showed that the amide-type molecule showed highadsorption energy, suggesting that these additives would exhibit a good lubrication performance.

MMM 2018

703

(P1-75)
Crystal Growth Molecular Dynamics Simulation of alpha-Al2O3 Cutting
Tools for Realizing Their Best Tribological Properties*Shandan BAI1, Hiroya Nakata1, Jingxiang Xu2, Nobuki Ozawa2, Momoji Kubo2
1. KYOCERA Cop., 2. Tohoku Univ.

Aluminum oxide (Al2O3) as coating materials has been used for cutting tools. It is widely applied onmachining metals, because Al2O3 films show the tribological properties of high hardness and heat
resistance. The Chemical Vapor Deposition (CVD) coating process for Al2O3 film is a very effective way to
get its excellent tribological properties. AlCl3/CO2/H2 gas system is used for the CVD process to obtain
the Al2O3 coating. At the beginning step of the CVD process, the surface reactions affect the orientation of
Al2O3 surface. However, the relationship between the surface reactions and Al2O3 orientation is difficult to
be clarified by the experiments. In this study, we investigate the details of surface reactions on thealpha-Al2O3 (0001) and (11-20) surfaces using computational simulation methods at the molecular scale.
The alpha-Al2O3 take place through the hydrolysis reactions of AlCl3 and H2O. In order to clarify thesurface reaction mechanism on alpha-Al2O3 (0001) and (11-20), we calculate the AlCl3 molecule
adsorption on these surfaces using the first principles calculation. GGA-PBE functional is employed. One
AlCl3 molecule is placed on various sites atom of alpha-Al2O3 (0001) and (11-20), and the system is
optimized. The highest adsorption energies are -107.63 and -60.87 kcal/mol, respectively. The result
indicates that AlCl3 molecule easily adsorbed on the alpha-Al2O3 (0001) surface. After the optimization of
the AlCl3 molecule, the 6-coordinated Al and 4-coordinated Al of the AlCl3 molecule are observed on thealpha-Al2O3 (0001) and (11-20) surfaces, respectively. It means that 6-coordinated Al shows the stable
adsorption state. Furthermore, molecular dynamics simulation is performed to clarify the surface reaction
dynamics on the alpha-Al2O3 (0001) and (11-20). The details will be discussed on our presentation.

704

MMM 2018

(P1-76)
Atomistic modeling of polymer friction*Robin Sam Vacher1
1. SINTEF-NTNU

Polymers are present in our daily life and we are using their tribological properties even without noticing
it, walking with shoes is just one example. However, the theoretical understanding of the friction and wearof polymers is still lacking and therefore, development of novel low friction material of solid polymers canonly be achieved by trial and error procedures including expensive testing. In this work, the focus is puton the influence of structural properties of solid polymers on the friction and wear. Molecular dynamic
simulation is used to capture the crystallization process of PolyVinyl Alcohol (PVA). The coarse grainpotential for PVA (CG-PVA) developed by Meyer and Muller-Plathe (”Formation of chain-folded

structures in supercooled polymer melts”, J. Chem. Phys. 2001, 115, 7807) has been used in order todrastically speed up the simulation. Depending on the cooling rate and the number of monomers per
chain, the crystalline domains will have different structural properties (eg. crystallinity level). We create
samples of polymers with different crystalline structure, and investigate numerically by simulating theeffect of rubbing the surface with an AFM tip the relation between friction, wear, and crystallinity.
Keywords : molecular dynamic, polymers, friction and wear

MMM 2018

705

(P1-77)
A two degree-of-freedom extension to the Prandtl Tomlinson-model
for friction strengthening in layered materials*David Andersson1,2, Astrid de Wijn2
1. Department of Physics, Stockholm University, 2. Department of Mechanical and Industrial Engineering, Norwegian
University of Science and Technology

The Prandtl Tomlinson-model model has been exceedingly successful in describing the stick-slip friction
response signal observed in atomic systems. However, recently it has been observed in both experiments
and MD simulations that layered materials exhibit an initial period of friction strengthening, which is notfound within the Prandtl Tomlinson-model [1][2]. Furthermore, out-of-plane bending has been proposed
as a driving mechanism for this friction strengthening regime [2][3]. In this paper we propose an extension
to the Prandtl Tomlinson-model to account for friction strengthening. We show that this model
successfully exhibits friction strengthening for a wide range of parameters, and that it is consistent withthe out-of-plane bending hypothesis as well as previous experiments. Moreover, we make predictions as
to the importance of commensurability for friction strengthening in these systems.
[1] Frictional Characteristics of Atomically Thin Sheets, Lee at al, Science 2010, doi:
10.1126/science.1184167[2] The evolving quality of frictional contact with graphene, Li at al, Nature 2016, doi:
10.1038/nature20135[3] Flexible graphene strengthens friction, de Wijn, Nature Communication 2016, doi:10.1038/539502a

706

MMM 2018

(P1-78)
Analysis of Friction Characteristics of Steel Powders using Parallelized
Discrete Element Method*Naoki Yashiro1,2, Kouya Oohira2, Natsuko Sugimura1, Hitoshi Washizu1
1. Graduate School of Simulation Studies, University of Hyogo, 2. Advanced Technology R&D Center, NTN
Corporation

In order to increase the application of sintered metal parts, high dense and less pores parts have to be
produced with stable quality. Since sintered metal parts cannot be fully densified during sintering, it isessential to understand the steel powders' characteristics in detail and to densify them in the compaction
step. Sufficient powder characteristic data, however, cannot be evaluated by conventional versatilemethods, and there are few cases where actual powder characteristics are applied to large scale particle
simulation. In this study, we evaluated flowability and friction characteristics of steel powders
experimentally in detail, and applied them to parallelized discrete element method (DEM).
We prepared steel powders with different kinds of solid lubricant and addition process. Theircharacteristics were evaluated by a method using a powder yield locus (PYL). The evaluation indexes areflowability (uniaxial collapse stress), inter-particle friction (internal friction angle), and friction between
particles and mold (wall friction angle).
The PYL method, which can measure flowabilities under various vertical loads, enabled collection of
powder characteristics corresponding to the actual powder behavior in the mold. Furthermore,inter-particle and wall friction coefficients were calculated using the friction angles obtained from theseexperiments. These coefficients were higher than those which were applied in previous DEM simulation
reports.
We created a parallelized DEM code for powder behavior simulation, using FDPS (Framework forDeveloping Particle Simulator) developed by Iwasawa et al.[1] in order to develop into large scaleparallelization in the future. FDPS is designed to reduce the calculation load at the time of region divisionand interaction calculation by constructing a tree structure. In this code, we evaluated flowabilities andfriction characteristics by changing the model of the contact force between particles, especially thetangential component.
In the conference presentation, we will show the flowabilities and the friction characteristics of steel
powders, which are evaluated by this parallelized DEM code.

[1] Iwasawa et al., Publ. Astron. Soc. Japan, 68, 54 (2016)

MMM 2018

707

(P1-80)
Dynamics of Polymer Under Shear in Confinement Geometry*Taiki Kawate1, Soma Usui1, Hiroaki Yoshida2, Hitoshi Washizu1
1. Univ. of Hyogo, 2. Toyota Central R&D Labs

In lubricants, a small amount of polymer named viscosity index improver (VII) is used to control
temperature dependence of viscosity. When VII is confined in small space, such as space between piston
ring and cylinder bore in combustion engine, the dynamics of the solution changes due to theconfinement. Here we simulate the dynamics of polymer under shear flow in confinement geometry.
Polymer segment is modeled as sphere Brownian particle. The motion of the particle is tracked by theLangevin equation, whereas the Navier-Stokes equation governing the behavior of the base oil is analyzed
by using the lattice Boltzmann method. The two equations are coupled through the friction between theparticle and the fluid. The friction force is in proportion to the velocity of a particle relative to the host
fluid, which is evaluated locally based on the velocity difference at the position of the point particle. The
friction force acting on a particle is estimated by fitting the analytical solution or the flow around a
Stokes-let to the flow field obtained numerically. The reaction force acts on the position of the particle,
which realizes two-way coupling between the particle and fluid motions. The lattice Boltzmann method is
employed for the flow simulation, which is compatible with massive parallel computing, and is easy toapply various types of boundary conditions. In order to describe confined system, periodical boundary isadopted in x, y (flow and transverse) direction, and non-slip boundary is adopted in bottom plate in z
direction and moving wall is set in top plate of it. During the simulation, the polymer is pulled to upper
layer where the shear field is large. To show the orientation of the molecule, order parameter iscalculated. The orientation of the polymers changed to the direction of the external shear force.

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Dynamics of a Polymer in Bulk Solution under Shear Flow
*Soma Usui1, Taiki Kawate1, Hiroaki Yoshida2, Hitoshi Washizu1
1. Univ. of Hyogo, 2. Toyota Central R&D Labs.

This study describes dynamics of viscosity index improver (VII) in bulk lubricants under shear flow. Small
amount of polymer named VII is added in basis oil to control viscosity index (VI) which is a parameter toexpress temperature dependence of viscosity. Controlling VI is one of the commonly used methods toimprove the fuel consumption of motor vehicles. Here we simulate the dynamics of polymer under shear
flow in Bulk Solution. Polymer segment is modeled as sphere Brownian particle. The motion of the particle
is tracked by the Langevin equation, whereas the Navier-Stokes equation governing the behavior of thebase oil is analyzed by using the lattice Boltzmann method. The two equations are coupled through thefriction between the particle and the fluid. The friction force is in proportion to the velocity of a particle
relative to the host fluid, which is evaluated locally based on the velocity difference at the position of thepoint particle. The friction force acting on a particle is estimated by fitting the analytical solution or theflow around a Stokes-let to the flow field obtained numerically. The reaction force acts on the position of
the particle, which realizes two-way coupling between the particle and fluid motions. The lattice
Boltzmann method is employed for the flow simulation, which is compatible with massive parallel
computing, and is easy to apply various types of boundary conditions. In order to simulate the system, thecomputational domain has three-dimensions and Periodic boundary condition. External forces are
introduced to form simple shear flow. The dynamics of polymer in shear flow is examined by using two
computational methods.

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Mechanochemistry induced atomic wear in chemical mechanicalpolishing processes*Jialin Wen1, Tianbao Ma1, Xinchun Lu1
1. Dept. of Mechanical Engineering, Tsinghua Univ.

Chemical mechanical polishing (CMP) is widely applied in the semiconductor industry to achieveultra-high precision manufacturing of surfaces (such as Si and Cu), however, its mechanisms remain
elusive because of interaction between chemical effects (such as the chemical reactions with theexistence of H2O2 and glycine) and mechanical effects (such as the tribology process). By using molecular
dynamics simulations based on the ReaxFF reactive force field, we have explored the CMP mechanisms at
the atomic level and have revealed the importance of mechanochemistry induced atomic wear during the
CMP processes.

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(P1-83)
Adsorption property of a fatty acid on iron surface with grain boundary*Yuki Uchiyama1, Yoshinori Shiihara1, Ivan Lobzenko1
1. Toyota Technological Institute

Reducing the coefficient of boundary friction on steel surfaces is one of key technologies to improve theefficiency of machines such as automotive engines. It has been shown that the boundary friction on
nanostructured steel surfaces in the sliding test using particular lubricant is smaller than the friction of
normal steel surfaces [1]. This difference between the nanostructured and a normal surfaces lies in thedensity of the grain boundary as a surface defect. The surface defect can attract lubricant molecules andenhance lubricating film formation on metal surfaces. This can be one of the mechanism that induces thefriction reduction on the nanosructured steel surface. In this work, using first principles calculations, theadsorptivity of a lubricant molecule, a fatty acid, on iron surfaces has been studied.The chemical
adsorption of fatty acids is expected to be enhanced at the site near the crystal grain boundary which may
have high chemical activity. Adsorption energy calculations were preformed for acetic acids as thesimplest fatty acids. Adsorbability of a Fe (110) surface with symmetrical tilt Σ3 (111)grain boundarywas
compared to clean Fe(100), (110) and (111) surfaces. As a result, we found that the molecule can adsorb
on the particular site on grain boundary more strongly than that on Fe surfaces without grain boundary.
[1]
Yoshikazu Todaka, Kenichi Toda, Motohiro Horii, Minoru Umemoto, Tetsu-to-Hagané 101, 10 (2015)

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Adsorption of Volatile Organic Compounds (VOCs) on Silicene by
Density Functional Theory Calculations*Thi Viet Bac Phung1, Trong Lam Pham1, Van An Dinh1,2
1. Nanotechnology Program, Vietnam Japan University –Vietnam National University, 2. Center for Atomic andMolecular Technologies, Graduate School of Engineering, Osaka University

Cancer can be regarded as a rising threat towards modern societies. Detecting cancer at an early stage
significantly improves the curability of the disease [1]; unfortunately, currently available methods for early
diagnosis of cancer are scarce and inefficient. In fact, the concentration of VOCs in cancer patients in thebreath is different from that in normal people [2]. Therefore, development of new sensors that can detect
VOCs at low concentrations, corresponding to the early stage of cancer, is desirable. 2D materials are
expected as attractive materials for these sensors due to their large surface area to volume ratio. In thiswork, we investigated the adsorption mechanism of some small-to-medium VOCs on the surface ofsilicene by the quantum simulation method. The images of the potential energy surfaces for different
positions of the adsorbate on the silicene surface were explored by Computational DFT-based Nanoscope[3] for determination of the most stable configurations and diffusion possibilities. The adsorption energy
profiles were calculated by three approximations of van der Waals interation: revPBE-vdW, optPBE-vdW,
and DFT-D2. It is found that the adsorption energies of the VOCs in question vary in the range of 0.6-1.0
eV, which indicates that silicene is considerably sensitive with these VOCs. The charge transfer between
the substrate and VOCs and the effect of an electric field on the adsorption configurations, energies, andband structures were also addressed.

References[1] M. Hakim, Y. Y. Broza, O. Barash, N. Peled, M. Phillips, A. Amann, H. Haick Chem. Rev. 2012, 112, 11,5949–5966.
[2] E. M. Gaspar, A. F. Lucena, J. D. da Costa, and H. C. das Neves. Journal of Chromatography A 2009,
1216, 14, 2749–2756.
[3] Developed by V. A. Dinh, Vietnam Japan University.

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Atomistic insights into defect formation mechanism in single crystal
SiC based on molecular dynamics simulation*Kenji Nishimura1, Koji Miyake1, Ken-ichi Saitoh2
1. AIST, 2. Kansai Univ.

Silicon carbide (SiC) is a promising next-generation semiconductor for high-power, high-temperature, andhigh-voltage devices because of its wide bandgap, high breakdown field, and high thermal conductivity.
However, the efficient and smooth machining of SiC is technically difficult due to its intrinsic nature ofhigh hardness and brittleness as well as strong anisotropy. It is still a great challenge to understandmachining processes in which damage layers caused by stress-induced phenomena such as plastic
deformation and fracture are introduced beneath grinding surface. In this study, we perform one
million-atom molecular dynamics simulations of nanoindentation tests on cubic SiC single crystal using anano-sized spherical indenter to clarify the plastic deformation mechanism and defect formation criteria
in SiC. An analytical bond-order Tersoff-type interatomic potential for SiC developed by Erhart et al.,which reproduces the elastic, defect, and thermal properties, is adopted. The load-displacement curves of
the nanoindentation tests obtained by our simulations demonstrate transition from elastic deformation toplastic deformation so-called “pop-in” event. Our results also predict the decrease of the CRSS (criticalresolved shear stress) of single crystal SiC with increasing temperature from 300 K to 2000 K for both(001) and (111) indent, which means less energy is required to activate slip systems at higher
temperature. These results are similar to the feature of ductile materials such as metals, although SiC is
known as brittle materials. In addition, we identify crystalline slips and defects generated beneath theindenter after the pop-in event by means of a novel type of structural analysis method using sub-lattice ofSi or C which is based on common neighbor analysis. The structural analysis we propose reveals that
dislocation loops in {111} planes which correspond to the slip plane of SiC are developed with increasing
indenter depth. Furthermore, we find that for lower temperature perfect dislocations are dominantly
formed, while for higher temperature partial dislocations together with stacking faults are superior to theperfect dislocations, resulting in the dramatic increase of the partial dislocations.

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Potential cathode material NaxVOPO4 for rechargeable Sodium –ionbatteries: DFT investigation*Duc Huu Luong1, An Van Dinh1,2, Yoshitada Morikawa3, Yoji Shibutani2,1
1. Nano Technology Program, Vietnam Japan University, 2. Center for Atomic and Molecular Technology, Graduate
School of Engineering, Osaka University, 3. Department of Precision Science and Technology, Graduate School ofEngineering, Osaka University

Using the density functional calculations, we investigated the crystal and electronic structures,electrochemical properties and ion diffusion mechanism of Sodium –polaron complex in Sodium Vanadyl
phosphate NaVPO5. A calculated voltage of 3.77 V (GGA+U) and 3.58 V (HSE06) corresponding to aredox reaction of potential couple V4+/V5+ were in good agreement with experimental result [1]. The

diffusion mechanism of charge carriers was explored using GGA+U. In the charge process, a Sodium ion isremoved from the crystal structure so that the Sodium vacancy appears and a positive small polaron forms
at one the two first nearest VO6 octahedron. The diffusion of Na+ ion which is accompanied by a positive
small polaron is described by three elementary diffusion processes, including single, crossing and parallel
diffusions [2]. With the smallest activation energy of 395 meV, the pathway of Sodium diffusion along the[010] direction is the most favorable diffusion pathway and it is significantly higher than previous
calculation which did not mention the small polaron formation. In the discharge process, Sodium ion isintercalated to structure of β-VOPO4, then the negative small polaron forms at one of the nearest
neighbour VO6 octahedron. In addition, the elementary diffusion process of sodium ion is more favorable
in the [010] direction. However, because of about 10% smaller volume, the diffusion activation energy(627 meV) is significantly higher than those required in the charge process. Compared with the othermaterials, it is obviously that this material would perform as well as some common materials for cathodesuch as Olivine.
[1] G. He, A. Huq, W. H. Kan, and A. Manthiramm, Chem. Mater. 2016, 28, 1503−1512[2] V. A. Dinh et al. App. Phys. Express, 2012, 5, 045801

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(P2-81)Two-dimensional NaxSiS as a promising anode material forrechargeable Sodium-based batteries: Ab initio material design.
Thi Dung Pham1, *Van An Dinh1,2, Kazunori Sato3, Yoji Shibutani1,2
1. Nanotechnology Program, Vietnam Japan University, 2. Center for Atomic and Molecular Technology, Graduate
School of Engineering, Osaka University, 3. Division of Materials and Manufacturing Science, Graduate School ofEngineering, Osaka University

The rapidly rising demand of energy storage system for electronic devices presents an imperative need todevelop sodium-ion batteries with high energy density, high conductivity, and low barrier energy. In thiswork, we present the density functional study on properties of the two-dimensional (2D) NaxSiS as apromising anode material for rechargeable sodium ion batteries (SIBs). Energetically stable structures ofNa-adsorbed Silicene sulfide NaxSiS were explored. It is found that silicene sulfide has an adsorptionenergy to sodium atom about of -0.4 eV, which is large enough to ensure a good stability for sodium
inserting into SiS during sodiate process. The electronic structure and capacity of NaxSiS were calculated.
The electronic structure of pristine SiS monolayer and Na adsorbed layer shows the distinction of asemiconductor material. The fully sodiated phase of SiS is Na0.5SiS corresponding to a highest theoretical
capacity of 187.2 mAh/g per one side layer. The diffusion mechanism of Na ions was also investigated byusing NEB method. Two possible elementary processes are explored: one is along a-and the other is alongb-direction. Most importantly, Silicene sulfide shows a good sodium mobility with an energy barrier along
two dimension is only 183 meV, which is much smaller than that in LixSiS (430meV), 2D TiS2 (220meV),and 2D MoS2 (280-680 meV). Our investigations also reveal that SiS exhibits the better electrochemical
performance as an anode in the SIBs than in the LIBs. All these characteristics suggest that 2D SiS can
expected to be a promising anode material for sodium batteries.

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Modelling and analysis of SiO2 interfaces of non-firing solids*Tomohiro Sato1, Ken-ichi Saitoh1, Masayoshi Fuji2, Chika Yamashita Takai2, Hadi Razavi2,
Masanori Takuma1, Yoshimasa Takahashi1
1. Dept. of Mechanical Engineering, Kansai Univ., 2. Advanced Ceramics Reserch Center, Nagoya Institute ofTechnology

Generally, ceramics are manufactured by using sintering process. However, this process needs hightemperature and loss a lot of fuels. CO2 emission in the process is also need to improve. So, non-fireprocess is focused to make ceramics products.
For non-fire process, surfaces of SiO2 particles are polished and put hydrogen on the activate surfaces.
These SiO2 particles are compressed and put into water that is non-fire process of ceramics. In this study,
SiO2 interface models were constructed for molecular dynamics simulation. Interactions between SiO2
and H2O were presented by using ReaxFF potential. At first, SiO2 interfaces model without OH as end
groups were conducted. By put water molecules between SiO2 interfaces, some atoms were changed
their combinations. Some of them achieved lower potential energy through the simulation. It is seemedthat a part of non-fire process was reproduced. For example, hydrogen atom connected the oxygen atomof SiO2. However, connection of SiO2 did not observed over SiO2 interfaces.
Then, SiO2 interfaces model with OH as end groups were conducted. At relaxation of the models, SiOH
exists in the model. After relaxation, water molecules were put into the surfaces. However, changes ofconnection between SiO2 and H2O or SiO2 interfaces did not observed. Energetic or structural stability of
SiOH surfaces were seemed to effect the result.
Distance of interfaces, conducting compressed SiO2 including OH as end groups models might be key toimprove the ability of reaction between atoms.

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