Nano-mechanical Characterization for Multi-scale Modeling in
Mechanical Behavior of Metallic Materials
NIMS, RCSM, Takahito OHMURA
Background & Purpose

Experimental procedure

Muti-scale modeling of mechanical behavior especially in nano scale iscrucial for understanding and controlling mechanical properties of metallic
materials. This paper is aiming to present plastic deformation behavior
focusing on plasticity initiation characterized through nano-mechanicaltesting in a small scale associated with microstructures including interphase and grain boundary. A statistical analysis is also shown to discuss
physical models in an indentation-induced deformation behavior.

Every specimen surface was mechanically polished carefully andsubsequently electropolished to remove the mechanically damaged layer.
Nanoindentation tests were performed using a Hysitron Triboindenter(Bruker Nano Surface division, MN) with 60°or 115°(Berkovich) threesided pyramidal indenter. The position of the indent on the specimen
surface was confirmed using scanning probe microscopy (SPM) before andafter the indentation measurements.

Plasticity initiation associated with a coherency of - interface (1)Dislocation-grain boundary interaction through TEM in- situ straining(2)Message: Continuous/discontinues yielding on stress-strain behavior in
macro scale could be dominated by lower/higher critical stressin plasticity initiation at ferrite-cementite interface.

Message: Grain interior dislocation could be absorbed at grain boundary
resulting in a reduction of dislocation density leading toincreasing the yield stress in ultra-fine grained IF steel.

(a)

(c)

(b)

Bright field mode(a)Load-displacement curve

(b)
(d)

Fig. 1 Load-displacement curves obtained by nanoindentation in thevicinity of ferrite-cementite interface. (a)semi-coherent in thespheroidized sample, (b)incoherent in the recrystallized sample. The
three curves with Arabic numbers correspond to the indent markson the SPM image. The critical load (Pc) for plasticity initiation isindicated by the arrow on the loading segment. The #1 in the vicinityof the interface of the semi-coherent sample only shows significantly
lower Pc than the others, suggesting an easy micro-yielding leading
to the continuous macro-yielding on stress-strain curve.

Fig. 2 TEM in-situ straining for UFG IF steel. (a)BF image before straining
including six grains. (b)Stress-strain curve obtained during straining.
(c)Snap shots of dislocation motion in grain 1 in DF mode showing
absorption of dislocation at GB. Image (a) to (e) corresponds to thesymbols on the S-S curve in (b). (d)Dislocation density reduction ingrain 1 with applied compressive stress. Since the series of thedislocation-GB reaction is the stress range significantly lower thanmacro yielding (solid arrow in (b)), the reduction in dislocation
density could lead to the high yield stress in UFG materials.

TEM in-situ observation of strain-induced grain boundary formation(3)Physical modeling in the first and subsequent pop-ins(4)Message: Inhomogeneous plastic strain evolves dislocation interaction in
grain interior generating a reacted dislocations to form a new
grain boundary as an elementally step of grain refinement.

Message: Plasticity initiation and subsequent plastic deformation in
crystalline materials could include different physical origins of
stochastic and avalanche models.
(a)Fig. 3 TEM in-situ straining demonstrating grain boundary formationinduced by plastic strain in bcc Iron. Left: (a)-(f) snap shots of
dislocation motion and interaction in TEM and schematics showing
formation of three grain boundaries. Right: analysis in the activatedslip system based on the TEM snap shots. Assuming the activateddislocation as <111> screw type, Burgers vector in each region of A
to D can be specified. The reaction of the <111> dislocations
generates <001> or <110> dislocations, and they are arrayed tominimize the interaction elastic energy, forming a new grainboundary at the positions of 1,2 and 3. This is a direct observation ofgrain refinement induced by plastic strain.

(b)

Fig. 4 (a)Typical Load-displacement curve showing pop-ins including firstone at lower load in high magnitude and subsequent ones at higher
load in much smaller event size. (b)probability distribution density of
the first (in blue) and the subsequent (in red) pop-in events. The
blue plot shows a Gaussian-like distribution suggesting a thermallyactivated dominancy while red marks represent power-law functions
indicating a catastrophic model presuming dislocation avalanche.
This result messages that an elementally step of the plasticity
initiation can be modeled in single dislocation motion but thesubsequent behavior should be treated by a different model likecollective motion of dislocations.

Summary
Applications of nano-mechanical characterization to various metallic materials are shown with the effectivity, and given results are discussed for the multi-scale
physical modeling of a crystal plasticity. The characterization technique can probe a local mechanical behavior at a specific position in a microstructure, therefore astructure-property relation can be revealed more clearly. The probed behavior can be modeled based on dislocation theory on a closer scale with dislocation structures.
References
1)
2)
3)
4)

Y. Wang, Y. Tomota, T. Ohmura, W. Gong, S. Harjo, M. Tanaka, Acta Mater., 196, (2020) 565-575.
H. Li, S. Gao, Y. Tomota, S. Ii, N. Tsuji, T. Ohmura, Acta Mater., 206, (2021) 116621.
H. Li, S. Ii, N. Tsuji, T. Ohmura, Scripta Mater., 207, (2022) 114275.
Y. Sato, S. Shinzato, T. Ohmura, T. Hatano, S. Ogata, Nature Comm., 11, (2020) 4177.