Mechanical Properties Depending on Grain Sizes of Fcc Nanocrystalline Metals by Using Molecular Dynamics Simulation. Investigation of Stacking Fault Energy's Influence.

Abstract

Mechanical properties and deformation mechanisms of nanopolycrystalline (NPC) materials under tensile test are studied by using molecular dynamics. An embedded atom mehod (EAM by Mishin, et al. 1999) and an effective medium theory (EMT by Jacobsen, et al. 1987) are adopted as the interatomic potentials of Al to investigate the influence of stacking fault energy (SFE) on the phenomena, for the typical difference of them is that the latter underestimates the SFE of Al. Simulations using 3 different models are carried out to study the dependence of grain size under different strain rate conditions. For all cases, the dependency of maximum stress on grain size can be expressed as an inverse Hall-Petch relation. This tendency is considered and may be directly explained by volume effect of grain boundary (GB). Both crystal slips and GB slidings are observed, but the GB sliding is predominant in small grain model. The most crystal slip is motion of perfect dislocation in the EAM potential cases, but in the EMT potential cases the most crystal slip is caused by motion of Shockley's partial dislocation. Because the EMT potential underestimates the SFE, the core length of the extended dislocation is comparable to the grain size and there remains a lot of stacking fault in grains. During the unique deformation process by partial dislocation movement, a strange rotation of grain is also observed. It is to say that the deformation mechanism of NPC materials is strongly influenced by the SFE. As another interesting deformation mechanism, the grain switching process is observed. In the macroscopic viewpoint, this is quite similar to the switching mechanism proposed by Ashby and Verrall (1973). But, there is a significant difference; in the present study both the GB migration and sliding are predominant but Ashby and Verrall's model is based on diffusion contrarily.