bluuebunny/crossref_metadata_2025 huggingface dataset:
A Study of the Mechanical Properties and Recrystallization of Two Types of Foil of Aluminium 1050 A
Authors: Pérez-Ilzarbe J. , Fernández Carrasquilla J. , Luis-Pérez C.J.
Published: December 2009

An experimental study of the variation of the mechanical properties of sheets of aluminium alloy 1050A due to heat treatment has been carried out. For this purpose, sheets and foils rolled from continuous casting strip (CCS) and from hot rolling strip (HRS) have been prepared in the laboratory. Initially, CCS and HRS had a thickness of 7,5 mm and 3,0 mm respectively. The final thickness achieved in both cases was 0,20 mm. In order to obtain hardening curves for CCS and HRS sheets and foils, specimens of different thickness were taken during the rolling process. Oven treatments were carried out for drawing softening curves of foils. Foil proceeding from CCS presents hardening due to the precipitation of Al3Fe particles at a temperature range of 160°C to 220 °C. HRS foil has a lower hardening in the same range of temperatures owing to its low supersaturation of Fe. Correlation between ultimate tensile strength Rm and Vickers hardness is studied. Micrographies of optical microscopy are offered at hard and recrystallized conditions.
Structural Effect on the Plastic Behavior in Highly Porous Glasses
Authors: Woignier Thierry , Hafidi Alaoui A. , Primera Juan , Phalippou J. , Scherer George W.
Published: December 2009

Silica aerogels have been studied with the objective of understanding the mechanical behavior of these extremely porous (pore volume higher than 85%) glassy materials. Elastic and plastic behaviors are investigated using Hg porosimetry. Because of the peculiar structure of these materials, Hg liquid cannot enter their porous network and consequently induces an isostatic pressure. Due to the high compliance of the solid network, under isostatic pressure aerogels display an irreversible shrinkage caused by plastic deformation. The magnitude of the plastic shrinkage and the increase of the associated mechanical properties depend on the different parameters (porosity, elastic properties and structural features). The structural features are followed by X Rays scattering. The irreversible compaction can be explained by siloxane bond formation between clusters constituting the porous materials, retaining the strained structure. The pore collapse mechanism is favored by the large pores structure and loose cluster structure (low fractal dimension). This densification process could offer a new way to synthesize porous glasses at room temperature.
Mechanical Properties of Organic-Inorganic Hybrid Black Coating of Silica and Polyurethane
Authors: Rebati Aziz , Piñero Manuel , de la Rosa-Fox Nicolás , Esquivias Fedriani Luis Maria
Published: December 2009

The mechanical properties of black thin film on glass substrates and metallic, obtained from the synthesis of organic-inorganic silica hybrids via sol-gel are described. Hybrid silica-based coatings with tetraethoxysilane (TEOS), methyltrietoxysilane (MTES), polyurethane (PU) and commercial silica colloidal solution (Ludox HS30) as precursors, were used. The procedure is divided in the following three steps: 1) obtaining homogenous and stable solutions of different organic silica-polymer composition, 2) aggregation of the black pigment with the formulate FeMnOx, 3) Coating substrates by both dip-coating and aerograph airbrush, at room temperature. The films obtained were characterized by SEM, AFM and nano-indentation. The coatings of the Ludox-MTES-PU/FeMnOx system showed the better characteristics concerning the morphology and the adhesion to the glass and metallic copper substrates
Creep and Stress Relaxation of Hybrid Organic-Inorganic Aerogels
Authors: de la Rosa-Fox Nicolás , Toledo-Fernández J.A. , Morales-Flórez Victor , Piñero Manuel , Esquivias Fedriani Luis Maria
Published: December 2009

Organic/inorganic hybrids silica aerogels were synthesized by the classical sol-gel method with application of high power ultrasounds to the liquid mixture. Precursors were tetraethoxysilane (TEOS), as inorganic phase, and polydimethyl siloxane (PDMS), as organic one. These hybrid organic-inorganic materials are known as ORMOSIL (ORganically MOdified SILicates). Monolithic aerogels were obtained by supercritical drying in ethanol. Failure tests by uniaxial compression shows an increase of the rupture modulus as well as a decrease of the Young’s modulus with the polymer content, tuning from a brittle solid to a rubbery-like one. These hybrid aerogels behave as elastomers showing a decrease in the relaxation viscoelastic modulus. Nanoindentation tests have been performed in these hybrid aerogels: load/unload cycles about 1.5 mN of maximum load have shown a decreasing value of the reduced modulus, as well as both plastic and elastic work with the organic content, while hardness remains almost constant. Elastic recovery parameter rised with the increasing organic content. Results from creep tests made with uniaxial compression configuration are discussed and compared with nanoindentation. Viscoelastic behavior of these hybrids materials can be described by a rheological model.
Grain Size Strengthening in Microcrystalline Copper: A Three-Dimensional Dislocation Dynamics Simulation
Authors: de Sansal Christophe , Devincre Benoit , Kubin Ladislas P.
Published: December 2009

This article reports on a study of the microstructure and mechanical response of copper polycrystals with grain sizes in the micrometer range. Three-dimensional dislocation dynamics simulations are used for the first time to investigate grain boundary strengthening and the Hall-Petch law. The methodology, which involves constructing a microcrystalline representative volume element with periodic boundary conditions, is briefly presented. Simulation results show that the initial density of dislocation sources and the cross-slip mechanism are two key factors controlling the heterogeneity of plastic deformation within the grains. At yield, the smaller the grains size, the more plastic deformation is heterogeneously distributed between grains and homogeneously distributed inside the grains. A size effect is reproduced and it is shown that the Hall-Petch exponent decreases from the very beginning of plastic flow and may reach a stable value at strains larger than the conventional proof stress.
Wear Resistance of Metal Oxide Sol-Gel Coatings Deposited over Ceramic Glazes
Authors: Galindo R. , Gargori C. , Cerro S. , Badenes J. , Monrós G.
Published: December 2009

This paper analyse the wear resistance of high reflective metal oxide films of TiO2 (anatase), Fe2O3 (hematite), ZrO2 (Zirconia) and SnO2 (Casiterite) deposited by screen printing over a conventional ceramic glaze (molar composition 0,2CaO.0,15ZnO.0,05K2O.0,1Al2O3 0,45SiO2.0,05ZrO2). All oxide coatings increase the gloss of the substrate producing a lustre effect. Anatase films stand out for their high gloss valour and low roughness Ra. Likewise, all lustres improve the wear resistance of the glaze substrate. Hematite and casiterite coatings stand out for their relatively high wear resistance maintaining relatively high reflectivity.
Spark Plasma Sintering and Microstructural Characterization of Additive-Free Polycrystalline β-SiC
Authors: Lara A. , Poyato R. , Muñoz A. , Ortiz A.L. , Domínguez-Rodríguez Arturo
Published: December 2009

Additive-free -SiC powders were sintered by means of Spark Plasma Sintering System. Experiments were performed in the temperature range from 1650°C to 2200°C, 3 to 10 min holding time and pressure from 50 until 150 MPa. In order to favour sinterization, the starting powder was mechanically activated: defect concentration was increased by centrifugal ball milling. Applied temperature, holding time and/or pressure were varied to analyze their effect on the densification and grain growth kinetics. The full sinterization of the material was obtained for temperatures as high as 1900°C and over. The relative density of the obtained material was high, up to 97.0  0.6 % the theoretical density for 2200°C sintering temperature. An intense grain growth took place while sintering. The final microstructure exhibited a grain size distribution range from 1.0 to 2.5 m, depending on the sintering conditions. Such grain growth strongly depends on the sintering time, not so much on the sintering temperature.
A Method for Elasticity Modulus Calculation in Porous Media with Known Geometry Using the Monte Carlo Technique
Authors: Añez Liz , Primera Juan , Hasmy Anwar , Franceschini Pedro , Sánchez Néstor , Woignier Thierry
Published: December 2009

This study introduces a method for a computational calculus of the Elasticity Modulus (E) of simulated porous media using the Monte Carlo technique. The porous media of known geometry is simulated as an elastic network of central forces, to which a known deformation is applied. The minimum strain energy is calculated applying the Monte Carlo technique. The Elasticity Modulus is obtained from the theoretical relations between the elastic energy of a system and its deformation. The computational method is validated by applying it in systems of known analytic solution and over porous media generated through aggregation algorithm in two dimensions i.e. Random Sequential Aggregation and Diffusion Limited Cluster-Cluster Aggregation (RSA and DLCA respectively). The latter used to simulate the structure of silica aerogels. As for the range of concentrations studied for the DLCA and RSA systems, it was found that the elasticity modulus E decreases as the porosity of the system increases, being the E value higher for the DLCA system with respect to RSA. The method used is able to differentiate the elastic properties for two different aggregation models. Being E values different for equal porosities, the coordination number (Z) was the geometric parameter that best explains the behavior of the Elasticity Modulus.
High Temperature Mechanical Properties of Ti(C,N)-Co-Mo2C Cermets
Authors: Gallardo-López Angela , Morales-Rodríguez A. , Domínguez-Rodríguez Arturo , Córdoba J.M. , Avilés M.A. , Gotor F.J.
Published: December 2009

The creep behavior of a TiCxN1-x-Co-Mo2C cermet has been investigated at temperatures between 1100-1200°C in an inert atmosphere to assess the one step mechanically induced self-sustaining reaction synthesis and pressureless sintering process, and the influence of the Mo2C additive in the high temperature mechanical properties of this cermet. The samples deform plastically at the chosen temperatures, and values of the stress exponent (n=1.70.6) and activation energy (Q= 4.30.5 eV) have been estimated from uniaxial compression tests. No significant grain growth has been detected after deformation. The reproducibility of the creep tests compared to other compositions indicates that the Mo2C addition contributes to increase notably the resistance to high temperature oxidation of the samples, so that the plastic behavior is not affected by oxidation when deformation experiments are performed in an inert atmosphere.
High-Temperature Mechanical Spectrometer for Internal Friction Measurements
Authors: Simas P. , San Juan José , Schaller Robert , Nó M.L.
Published: December 2009

A new high temperature mechanical spectrometer, based on an inverted torsion pendulum, has been constructed for the measurement of the internal friction and the dynamic shear elastic modulus in two different working modes: (a) as a function of temperature (300 – 1800 K) at imposed frequency, during heating or cooling; and (b) as a function of frequency (10-3 – 10 Hz) in isothermal conditions. The whole installation is computer controlled by a dedicated software specifically developed. We describe the different parts of this new installation, as well as its performances in both temperature and frequency through an original example study on a high temperature structural intermetallic of Fe-Al.
Influence of the Martensitic Transformation on the Fatigue Life of Austenitic Stainless Steels
Authors: Fargas Gemma , Anglada Marc , Mateo Antonio
Published: December 2009

The martensitic transformation in austenitic stainless steels can be induced by plastic deformation at room temperature. The benefit of this transformation is commonly used to strengthen stainless steels grades, i.e. their yield and tensile resistance can be adjusted according to the requirement by cold rolling. In this paper, the martensitic transformation was induced by means of torsion deformation. Several torsion angles were selected to achieve different percentages of martensite at the surface of the specimens and then the effect on the fatigue life of the steel was studied. Fatigue testing results showed dissimilar behavior depending on the stress ratio (R) applied during the test. As a conclusion, the presence of martensite in the surface increases the fatigue life for high stress ratios (R=0.8), while at low R values martensitic transformation has no positive effect.
Numerical Analysis of Aluminum Alloys Extrusion through Porthole Dies
Authors: Zasadziński Józef , Rękas Artur , Libura W. , Richert Jan , Leśniak D.
Published: December 2009

In the work the distribution of stresses and strains as well as temperature field within the welding chamber of the porthole die were determined. The analysis was performed by the use of the computer program DEFORM based on a finite element method. The direct hot extrusion of 2024 aluminum alloy was investigated with the use of the porthole dies of different geometry. Particularly, a different height of the welding chamber was adopted in calculations. The calculations allowed determining both pressure and temperature levels and their distributions within the welding chamber leading to the best welding conditions for the alloy tested.
Effect of Tube Wall Thickness in Joining of Aluminum Tube and Holed Rib by Extrusion
Authors: Moroi T. , Kuboki T. , Murata Makoto
Published: December 2009

Existence of some holes at internal ribs enhances the function and value of the tubes. A new extrusion method is proposed here for the forming of this shape by extrusion with joining. The method involves the use of a unique mandrel that has a slit along its axis and two guides at the slit exit. A holed sheet is fed through the slit and joined with the inner surface of extrude tube. Effect of two parameters, that are tube-wall thickness and guide position h which is distance from guide top to die surface were clarified by FEA. Two kinds of three-dimensional analysis models were prepared. One of the analysis models treats the rib as rigid body to examine a gap between rib and tube. Another model treats the rib as the elasto-plasticity body as well as the billet to examine the effect of the guide position and the tube wall thickness on the rib deformation. The series of analyses was carried out with emphasis on the metal flow. The gap between tube and rib is able to be suppressed small and joining condition becomes satisfactory when guide position rose or tube wall was thin. When the guide position rose further, or the tube wall thickness was excessively thinner, the amount of the deformation of the rib increases, and it causes defects.
The Use of Extruded Profiles as Filling Material in Friction Stir Welding (FSW)
Authors: Donati Lorenzo , Tomesani Luca
Published: December 2009

In this paper, an innovative approach is presented for joining two sheets with an extruded profile all made by AA6082-T6 aluminum alloy. The tested configuration is the T-joint and the innovation presented in this paper is the use of a specially design appendix of the extruded profile as filler material during the friction welding. In particular three configurations were analyzed: without appendix, with I appendix and with T appendix. In the experiments, several process parameters and PIN shapes were investigated in order to determine optimal processing conditions able to produce an effective sound weld. Specimens were extracted from the joint and tensile tests were performed along the sheet direction thus allowing a comparison of the welded sections respect to the base material. It was found that the appendixes of the extrude profile are able to effectively fill the distance between the sheets and, in particular with the T shape, a gap up to 1,7 mm on the retreating side was successfully welded.
A New Cone-Friction Test for Evaluating Friction Phenomena in Extrusion Processes
Authors: Karadogan C. , Grueebler R. , Hora Pavel
Published: December 2009

Friction is one of the most influential parameters in extrusion processes. The flow distribution is controlled using the effect of friction lengths on the extrusion dies. A comparison between multi-hole die aluminum extrusion experiments and simulations [1] necessitated studies on numerical and physical modeling of friction during aluminum extrusion. In order to measure the friction under conditions similar to extrusion processes a new experimental setup is proposed.
Numerical Analysis of Four-Hole Extrusion of Aluminum Alloys
Authors: Libura W. , Rękas Artur , Leśniak D.
Published: December 2009

The work presents the results of FEM simulations of extrusion process with the use of four – hole dies of various geometry. The traditional flat dies and pocket dies were adopted for comparison. The calculations were conducted for indirect extrusion of rods of 14 mm in diameter from the 6061 aluminium alloy. The distributions of strains, stresses, metal velocities and temperature within the deformation zone were the basis for metal flow analysis. Based on the conducted calculations the die configuration, leading to the best product quality was recommended. The product quality was evaluated by the flow direction of rods from the die orifices.
Measuring the Deformation of a Flat Die by Applying a Laser Beam on a Reflecting Surface
Authors: Assaad W. , Geijselaers H.J.M. , Nilsen K.E.
Published: December 2009

The design of extrusion dies depends on the experience of the designer. After the die has been manufactured, it is tested during an extrusion process and machined several times until it works properly. The die is designed by a trial and error method which is expensive interms of time consumption and the amount of scrap. Research is going on to replace the trial pressing with finite element simulations that concentrate on material and tool analysis. In order to validate the tool simulations, an experiment is required for measuring the deformation of the die. Measuring the deformation of the die is faced with two main obstacles: high temperature and little free space. To overcome these obstacles a method is tried, which works by applying a laser beam on a reflecting surface. This cheap method is simple, robust and gives good results. This paper describes measuring the deformation of a flat die used to extrude a single U shape profile. In addition, finite element calculation of the die is performed. Finally, a comparison is performed between experimental and numerical results.
Localization of the Shear Zone in Extrusion Processes by Means of Finite Element Analysis
Authors: Kammler Matthias
Published: December 2009

A characteristic feature of extrusion processes is the formation of a shear zone, which separates the deformation zone and the dead metal zone [1, 2]. The deformations occurring in the shear zone cause inner separation and welding effects, which are of great importance for the material flow and the microstructure development of the extruded profiles. The material in the dead metal zone is not participating directly in the forming process but the shape of this zone influences eminently the forming zone and thus the forming of the extruded profile. Furthermore the extreme shear deformation causes according to the Hall-Petch relationship a significant grain refinement in these regions of extruded profiles [3, 4]. So the knowledge on the effects in the shear zone during extrusion processes is fundamental for subsequent numerical investigations on the microstructure development for example regarding quenching techniques. The aim of this study is to localize the formation of the shear zone during extrusion processes by means of the finite element analysis. On the basis of the assumption that separation and welding effects take place in the shear zone, numerical investigations were carried out to indicate these microscopic effects on the macroscopic scale. The considered process was the extrusion of a solid round profile of the alloy EN AW 6082 at 450°C with a punch velocity of 10.5 mm/s. For the localization of the shear zone mechanical parameters were chosen for a shear criterion, which are taken from numerical simulations. A user subroutine was implemented into the FE-models in order to evaluate the shear criterion for the localization of the shear zone. According to [5, 6] the friction model used for the numerical simulations has a strong influence on the formation of the shear zone. In this study a combined friction model according to [7] was used.
Study of Flow Balance and Temperature Evolution over Multiple Aluminum Extrusion Press Cycles with HyperXtrude 9.0
Authors: Farjad Bastani Amin , Aukrust Trond , Skauvik Inge
Published: December 2009

In this research, transient finite element simulations of the aluminum extrusion process have been performed in order to study how process parameters influence flow balance and exit temperature. This has been achieved by investigating the influence of billet taper, front billet temperature and ram speed on the run-out velocity and temperature of two separate outlets. Analysis of variance (ANOVA) has been employed to study the effect of each parameter on the velocity and temperature variation of the extruded section. Results show that increasing each of these three parameters results in an undesired increase in exit velocity and temperature. The front billet temperature is found to be the most significant factor affecting the variation. The finite elements software used was Altair HyperXtrude 9.0.
Modeling and Simulation of Microstructure Evolution in Extruded Aluminum Profiles
Authors: Parvizian Farhad , Kayser T. , Svendsen Bob
Published: December 2009

The purpose of this work is to predict the microstructure evolution of aluminum alloys during hot metal forming processes using the Finite Element Method (FEM). Here, the focus will be on the extrusion process of aluminum alloys. Several micromechanical mechanisms such as diffusion, recovery, recrystallization and grain growth are involved in various subsequent stages of the extrusion and the cooling process afterward. The evolution of microstructure parameters is motivated by plastic deformation and temperature. A number of thermomechanical aspects such as plastic deformation, heat transfer between the material and the container, heat generated by friction, and cooling process after the extrusion are involved in the extrusion process and result in changes in temperature and microstructure parameters subsequently. Therefore a thermomechanically coupled modeling and simulation which includes all of these aspects is required for an accurate prediction of the microstructure evolution. A brief explanation of the isotropic thermoelastic viscoplastic material model including some of the simulation results of this model, which is implemented as a user material (UMAT) in the FEM software ABAQUS, will be given. The microstructure variables are thereby modeled as internal state variables. The simulation results are finally compared with some experimental results.
Simulation of the Quench Sensitivity of the Aluminum Alloy 6082
Authors: Güzel Ahmet , Jäger Andreas , Ben Khalifa Nooman , Tekkaya A. Erman
Published: December 2009

A method for the numerical estimation of the final hardness distribution of heat treated aluminum alloys was developed and implemented into a commercial finite element (FE) tool. Jominy end-quench tests were carried out in order to determine the quench sensitivity of the aluminum alloy EN AW-6082. The hardness distribution of the alloy after end-quenching was related to the corresponding cooling rates. The derived relation was tested for an industrial application by investigating the local heat treatment of a prototype crash absorbing structure. Numerical estimations were validated with experimental measurements. Effectiveness of the derived method and possible improvements were discussed.
Analysis of Polypropylene Deformation in a 135° ECAE Die: Experiments and Three-Dimensional Finite Element Simulations
Authors: Aour Ben , Zaïri Fahmi , Naït-Abdelaziz M. , Gloaguen J.M. , Lefebvre J.M.
Published: December 2009

Plastic deformation of polypropylene (PP) resulting from equal channel angular extrusion (ECAE) process was investigated in a 135° die. A phenomenological elastic-viscoplastic constitutive model was identified and coupled with the three-dimensional finite element (FE) method in order to predict the different processing parameters governing the deformation behaviour of PP during the extrusion. An optimal agreement between FE results and experimental data was obtained for a friction coefficient of 0.2. A detailed three-dimensional FE analysis of stress-strain field distribution was then carried out. The effects of both the number of extrusion passes and the processing routes were experimentally highlighted. The results show that the pressing force decreases with the increase of the number of extrusion passes and reaches its saturation state rapidly for routes A and C while, for routes BA and BC, it requires a high number of passes.
Accurate Welding Line Prediction in Extrusion Processes
Authors: Kloppenborg Thomas , Ben Khalifa Nooman , Tekkaya A. Erman
Published: December 2009

In contrast to conventional extrusion processes, where a lot of research is done on in the welding quality, in composite extrusion, research is investigated into the welding line positioning. As a result of the process principle, the reinforcing elements are embedded into the longitudinal welding line. Hence, an undefined material flow inside the welding chamber induces reinforcement deflection, which can lead to reduced mechanical properties, as momentum of inertia. Therefore and to reduce costly experimental investigations, a new method of an automated numerical welding line prediction was developed. The results form HyperXtrude finite element calculations are used for special particle tracing simulations to predict the welding line in the profile cross section accurately. The procedures of segmentation and characteristic extraction are presented to approximate the welding line by cubic spline functions. The method was fully programmed in the Java program language, and works well for all HyperXtrude process models consisting of tetrahedral elements.
Simulation of Porthole Die Extrusion Process Comparing NEM and FEM Modelling
Authors: Alfaro I. , Gagliardi Francesco , Cueto E. , Filice Luigino , Chinesta Francisco
Published: December 2009

Porthole die extrusion is a process typology that can give great advantages in the forming processes. Due to the complexity of the die assembly, experimental analyses are often carried out in order to investigate the parameter influence on the quality of the final parts. Finite Element Analyses, however, have been often used for the cost reducing and for a better local investigation of variables like pressure and effective stress inside the welding chamber. In spite of that, up to now, commercial FE codes present a “structural” limit during the welding phase due to the impossibility to simulate element joining when material reaches the required process conditions. From this point of view, the Natural Element Method (NEM) provides significant advantages; in fact, the meshless characteristic of NEM is “natively” able to simulate joining of free surfaces, as it occurs during porthole die extrusion, simulating the welding line formation inside the welding chamber. In this paper, using experimental tests recognizable in literature, the authors tried to validate the effectiveness of this technique; moreover, even a comparison between NEM and FEM results was carried out. More in detail, different geometries of the welding chamber were analyzed; in some cases, the process conditions were suitable to guarantee material welding while, in other cases, the material came out from the porthole die without joint formation. The variable that was used to verify the process goodness is the maximum pressure inside the welding chamber. Furthermore, to evaluate the effectiveness of 2D analyses, even in a complex shape, a significant section was extrapolated for each die, performing a NEM vs. FEM assessment of the results. A good comparison was obtained between the two different methods that, moreover, were in agreement with the experimental tests.
Deformation Behavior and Plastic Strain Localization of Nanostructured Materials Produced by Severe Plastic Deformation
Authors: Naydenkin Evgeny V. , Grabovetskaya Galina P.
Published: December 2009

The literature on the deformation behavior and plastic strain localization inherent to nanostructured metallic polycrystals produced by severe plastic deformation techniques is reviewed. The effects of the texture, structure heterogeneity and state of grain boundaries on the special features and evolution of mesoscopic and macroscopic localized deformation bands are investigated. The role of grain-boundary sliding in the development of mesoscopic plastic deformation bands is discussed.
Processing, Strength and Ductility of Bulk Nanostructured Metals Produced by Sever Plastic Deformation: An Overview
Authors: Mashreghi A. , Ghalandari L. , Reihanian M. , Moshksar M.M.
Published: December 2009

Nanostructured metals which have nano-scale microstructure are classified into ultrafine grained metals and nanocrystalline metals. In recent years, many processing techniques have been developed for producing nanostructured metals. Nanostructured metals possess ultrahigh strength but the low ductility is an important limitation on development of these materials for structural applications. This paper overviews various methods of producing nanostructured metals and recent investigations of strength and ductility of nanostructured metals processed by sever plastic deformation.
Ductility of Nanocrystalline Metals: Intrinsic or Extrinsic
Authors: Guo Y.Z. , Wei Q. , Li Yu Long
Published: December 2009

While the super-high strength of various nanocrystalline metals (NC metals, grain size d<100 nm) has drawn great attention in the past few decades, the poor ductility has become a seemingly insurmountable obstacle for the wide application of such metals. Other limiting factors include, in particular, the high cost associated with delivering such materials. Recent efforts from various groups have brought forth strong hope that the ductility of NC metals can be greatly improved by the application of “microstructure engineering”. However, the testing methods used to evaluate the mechanical properties, especially at small scales, have invited concerns. A natural question arises as to whether the reported allegedly good ductility of many NC metals is intrinsic or extrinsic. This article attempts to critically evaluate such issues. We will examine the experimental results in conjunction with some finite element modeling on the stress state and strain measurement of specimens during mechanical testing. Factors such as sample geometry and loading mode will be critically assessed. We point out that while intrinsic ductility may be possessed by some NC metals, careful attention should be paid to how strain measurement is made.
A Short Summary of Present Knowledge and some Experimental Observations on the Ductility of Sub-Microcrystalline Aluminium Alloys
Authors: Ferrante Maurizio
Published: December 2009

It is well known that the low ductility of nanostructured materials seriously impairs their commercial development. In its turn that mechanical property is associated to the work-hardening behaviour and the vast literature on this relationship is a measure of its importance. This paper presents a short review of the basic models of work-hardening, dealing initially with conventional “coarse” grain metals and alloys, then moving to the behaviour of sub-microcrystalline materials within the bounds of Al alloys and Equal Channel Angular Pressing. Finally, the interrelations of tensile properties, work-hardening behaviour and microstructure are illustrated by data obtained on a precipitation and a non-precipitation hardening Al alloys, namely Al-4%Cu and AA3004. Results show that low temperature aging results in higher strength and high work hardening rate, besides high ductility. The effects of precipitation and of annealing heat treatments are discussed.
The “Ductilities” in Single Phase Steels from Usual to Nano-Scale Microstructures
Authors: Bouaziz Olivier
Published: December 2009

The control and the improvement of the ductility of nano-structured structural steels is one of the key challenges in assessing the technological viability of this metallurgical strategy. In the present paper, it is shown that more rigorous definitions of the ductility are required in order to avoid possible confusions. After this preliminary work, a more transparent analysis is done concerning the effect of the microstructural scale showing clearly the weaknesses and the strengths of ultra-fine single phase steels. Finally, possible ways to overcome the main limitations are presented.
Effects of Microalloying on the Mobility and Mechanical Response of Interfaces in Nanocrystalline Cu
Authors: Caro A. , Farkas Diana , Bringa E. M. , Gilmer G. H. , Zepeda-Ruiz L. A.
Published: December 2009

We utilize a novel computational approach to model the problem of impurity segregation at grain boundaries in nanophase materials. It is based on a parallel MonteCarlo algorithm that places the impurities according to the local chemical potential for the species, following the thermodynamic driving force for segregation. This technique is combined with molecular dynamics techniques to study the role played by Fe impurities in the properties of nanocrystalline Cu grain boundary properties. The impurities were found to improve microstructural stability as studied by high temperature annealing simulations, and grain boundary cohesion as studied via spall resistance high stresses produced by simulated laser irradiation. Virtual tensile tests of samples with and without impurities revealed that the impurities did not affect the high flow stress typical of nanostructured material. We interpret these results in terms of impurity dragging and grain boundary sliding.
Measures of Ductility for UFG Materials Obtained by SPD
Authors: Beygelzimer Yan , Prokof'eva O. , Kulagin R. , Varyukhin Viktor , Synkov Sergey
Published: December 2009

It is shown that for ultrafine grained materials obtained with severe plastic deformation methods, the value of elongation up to fracture does not determine ductility, while the reduction of area up to fracture does determine it. The latter characteristic gives information about how an alloy structure resists the formation of discontinuity flaws under deformation in a hard stress state. We show that for a commercial grade titanium that underwent Twist Extrusion (TE), the value of , and thus ductility, is higher in the UFG state than in the coarse-grained state.
The Analysis of SPD Paradox by Computer Modeling Technique
Authors: Alexandrov Igor V. , Chembarisova Roza G.
Published: December 2009

The paper has viewed the manifestation of the paradox of severe plastic deformation (SPD), caused by the occurrence of preexisting deformation twins in ultrafine-grained Cu, which has been obtained by the combination of the SPD method, accomplished by an equal-channel angular pressing with the conventional methods of deformation-thermal treatment. The high strength of the obtained samples has proved to be conditioned by the occurrence of the high density of the coherent twin boundaries, serving as effective obstacles on the way of slipping dislocations. Moreover, the occurrence of the twins creates favorable conditions for the dislocation density increase both in the grains with the twins and in the grains without them. As a result the sample hardens, contributing additionally into its strength. Simultaneously it manifests high ductility. By doing so the deformation behavior of the sample is mainly conditioned by the grain boundaries of grains free from the twins. The results were obtained on the basis of the dislocation-based model which develops models of Y. Estrin and L. Tóth, M. Zehetbauer, and L. Remy.
High-Rate Plastic Deformation of Nanocrystalline Tantalum to Large Strains: Molecular Dynamics Simulation
Authors: Rudd Robert E.
Published: December 2009

Recent advances in the ability to generate extremes of pressure and temperature in dynamic experiments and to probe the response of materials has motivated the need for special materials optimized for those conditions as well as a need for a much deeper understanding of the behavior of materials subjected to high pressure and/or temperature. Of particular importance is the understanding of rate effects at the extremely high rates encountered in those experiments, especially with the next generation of laser drives such as at the National Ignition Facility. Here we use large-scale molecular dynamics (MD) simulations of the high-rate deformation of nanocrystalline tantalum to investigate the processes associated with plastic deformation for strains up to 100%. We use initial atomic configurations that were produced through simulations of solidification in the work of Streitz et al [Phys. Rev. Lett. 96, (2006) 225701]. These 3D polycrystalline systems have typical grain sizes of 10-20 nm. We also study a rapidly quenched liquid (amorphous solid) tantalum. We apply a constant volume (isochoric), constant temperature (isothermal) shear deformation over a range of strain rates, and compute the resulting stress-strain curves to large strains for both uniaxial and biaxial compression. We study the rate dependence and identify plastic deformation mechanisms. The identification of the mechanisms is facilitated through a novel technique that computes the local grain orientation, returning it as a quaternion for each atom. This analysis technique is robust and fast, and has been used to compute the orientations on the fly during our parallel MD simulations on supercomputers. We find both dislocation and twinning processes are important, and they interact in the weak strain hardening in these extremely fine-grained microstructures.
Effect of Ageing on Strength and Ductility of Ultrafine Grained Al 6061 Alloy
Authors: Panigrahi Sushanta Kumar , Devanand D. , Jayaganthan R.
Published: December 2009

An effect of ageing on mechanical properties of ultrafine grained Al 6061 alloys has been investigated in the present work. The solution treated bulk Al 6061 alloy was subjected to cryorolling to produce ultrafine grain structures and subsequently ageing treatment to improve its both strength and ductility. The hardness and tensile properties of solution treated, cryorolled, cryorolled and aged Al alloys were measured and explained by using their corresponding microstructural morphologies. The pre-cryorolled solid solution treatment combined with post-CR ageing treatment (1300C-30h) has been found to be the optimum processing condition to obtain the ultrafine grained microstructure with improved tensile strength (362MPa) and good tensile ductility (10.7%) in the Al 6061 alloy. The combined effect of precipitation hardening and recovery are responsible for the simultaneous improvement of both strength and ductility observed in the present work.
Molecular Dynamics Simulations of Nanocrystalline Nickel and Copper Revealing Different Failure Model of FCC Metals
Authors: Cao Ajing
Published: December 2009

We have previously reported that the fracture behavior of nanocrystalline (NC) Ni is via the nucleation and coalescence of nano-voids at grain boundaries and triple junctions, resulting in intergranular failure mode. Here we show in large-scale molecular dynamics simulations that partial-dislocation-mediated plasticity is dominant in NC Cu with grain size as small as ~ 10 nanometers. The simulated results show that NC Cu can accommodate large plastic strains without cracking or creating damage in the grain interior or grain boundaries, revealing their intrinsic ductile properties compared with NC Ni. These results point out different failure mechanisms of the two face-centered-cubic (FCC) metals subject to uniaxial tensile loading. The insight gained in the computational experiments could explain the good plasticity found in NC Cu not seen in Ni so far.
An Investigation of Hardness and Microstructure Evolution of Heat Treatable Aluminum Alloys during and after Equal-Channel Angular Pressing
Authors: Cerri Emanuela
Published: December 2009

The influence of severe plastic deformation induced by ECAP on microstructure modification and aging effect was studied in two modified Al-Mg-Si aluminium alloys. The microstructure of both alloys in different heat treated and deformed state was characterised by X-Rays diffraction and polarised light microscopy. The effect of artificial aging was investigated after ECAP performed on samples in the as extruded condition. The aging effect was followed by hardness and electrical conductivity measurements. At higher aging temperature (170°C) the alloys showed an increasing softening with time due to recovery or/and grain coarsening effect. At the lower aging temperature, the hardness remains almost constant due to enhanced precipitation hardening effect.
Microstructure Development and Ductility of Ultra-Fine Grained Mg-Gd Alloy Prepared by High Pressure Torsion
Authors: Čížek Jakub , Procházka Ivan , Smola Bohumil , Stulíková Ivana , Očenášek Vladivoj , Islamgaliev Rinat K. , Kulyasova Olya B.
Published: December 2009

Microstructure of ultra fine grained (UFG) Mg-Gd alloy prepared by high-pressure torsion (HPT) was investigated in the present work. Lattice defects introduced by HPT were characterized at first. Subsequently thermal stability of UFG structure and its development with annealing temperature were studied and correlated with changes of hardness and ductility. Precipitation effects in the alloy with UFG structure were compared with those in a conventional coarse-grained alloy. Defect studies were performed by positron annihilation spectroscopy (PAS), which represents well established non-destructive technique with a high sensitivity to open volume lattice defects like vacancies, dislocations, misfit defects etc. PAS investigations were combined with transmission electron microscopy (TEM) and X-ray diffraction (XRD). Changes of mechanical properties were monitored by Vicker’s microhardness (HV) and deformation tensile tests. It was found that HPT deformed Mg-Gd alloy exhibits UFG structure with mean grain size of 100 nm and a dense network of dislocations distributed uniformly throughout the whole sample. Although recovery of dislocations takes place at relatively low temperatures, it is not accompanied by grain growth and the mean grain size remains around 100 nm up to 300oC. Tensile tests performed at elevated temperatures to examine ductility showed that HPT-deformed alloy exhibits a superplastic behavior at 400oC. Moreover, it was found that the precipitation sequence in HPT-deformed alloy differs from that in conventional coarse-grained material.
Mechanical Properties of Nanocrystalline Cu and Cu-Zn Using Tensile and Shear Punch Tests
Authors: Tao J.M. , Zhu Xin Kun , Wong P.Z. , Scattergood Ron O. , Koch Carl C.
Published: December 2009

Shear punch test (SPT) has been used to study the mechanical properties of Cu, Cu–10 wt.% Zn, Cu–20 wt.% Zn and Cu–30 wt.% Zn after ball milling with an average grain size in the range of 33-12nm. The strain rate sensitivity (SRS) and physical activation volume have been determined. The magnitude observed for these characteristic deformation parameters is very different from their course-grained (cg) counterpart. This suggests that the thermally activated process in nanocrystalline (nc) metal/alloys is different from the conventional forest dislocation cutting mechanism. The stacking fault energy (SFE) of Cu-Zn alloys decreased with the adding of Zn, and deformation twins are anticipated to introduce into the nc Cu-Zn alloys during process of ball milling. Dislocations could accumulate along the TBs and carry the plastic strain, so the ductility of nc Cu-Zn alloys could be improved.
A Computational Model for Intergranular Fracture in Nanocrystalline and Ultra-Fine Polycrystalline Metals
Authors: Wu Bo , Wei Yue Guang
Published: December 2009

By means of finite element method which is based on the conventional theory of mechanism-based strain gradient plasticity, cohesive interface model is used to study the intergranular fracture in polycrystalline metals with nanoscale and ultra-fine grains. A systematical study on the overall strength and ductility of polycrystalline aggregates which depend on both grain interiors and grain boundaries for different grain sizes is performed. The results show that the overall strength and ductility of polycrystalline aggregates with nanoscale and ultra-fine grains are strongly related to the competition of grain boundaries deformation with that in grain interiors. Finally, the deformation and failure behavior of nanocrystalline nickel are described by using the computational model.
Effect of Ppm Level Dopant on Ductility of Ultrafine Grained Gold Wires
Authors: Chew E. , Kim H.H. , Ferraris C. , Zhao Yong Hao , Lavernia Enrique J. , Wong C.C.
Published: December 2009

The addition of calcium (Ca) simultaneously improves the ductility and strength of UFG Au wires. Based on the observation on stacking faults, microstructures, simulation results and significant effect of Ca on grain boundary related properties, it is inferred that segregation of Ca to stacking faults and grain boundaries has occurred to induce effective stacking fault energy (SFE) reduction and properties improvement. Considering the known greater impact of SFE in UFG/ NC metals, segregating dopants are proposed to be an effective strategy for achieving dual improvement in this class of materials. Also, dopant selection criteria for this purpose is also suggested and verified.
Strategies for Improving Ductility of Cryomilled Nanostructured Titanium
Authors: Ertörer Osman , Topping Troy D. , Li Ying , Zhao Yong Hao , Moss Wes , Lavernia Enrique J.
Published: December 2009

The room temperature tensile behavior of commercially pure titanium (CP-Ti), cryomilled under different conditions and forged quasi-isostatically into bulk form, was studied in detail. The results demonstrate that the ductility of cryomilled titanium can be improved, and that the mechanical properties can be tailored using three specific strategies: the use of liquid argon as cryomilling media, introduction of coarse grained regions, and low temperature heat treatment. Cryomilling in a liquid argon environment, which differs from the widely used nitrogen cryogenic environment, was found to have a particularly strong influence on ductility, as it prevents nitrogen embrittlement. The contribution of coarse grains and heat treatment to ductility are also introduced and discussed using a comparative approach.