Mechanical Properties Depending on Grain Sizes of Face-Centered-Cubic Nanocrystalline Metals Using Molecular Dynamics Simulation

Abstract

The mechanical properties and deformation mechanisms of nano-polycrystalline (NPC) materials under tension are studied using molecular dynamics. The embedded atom method (EAM by Mishin et al. 1999) and the 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. The main difference between EAM and EMT potentials is that the latter underestimates the SFE of Al. Simulations using three different models are carried out to study the grain size dependence of the mechanical properties 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 the volume effect of the grain boundary (GB). Both crystal slips and GB sliding are observed, but GB sliding is predominant in the small-grain model. Most of the crystal slips are caused by motion of perfect dislocation in EAM potential cases, but in EMT potential cases most of the crystal slips are 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 hence many stacking faults remain in grains. During the unique deformation process by partial dislocation movement, a strange rotation of grains is also observed. That is, the deformation mechanism of NPC materials is strongly influenced by the SFE. Another interesting deformation mechanism observed is the grain switching process. From the macroscopic viewpoint, this is quite similar to the switching mechanism proposed by Ashby and Verrall (1973). However, there is a significant difference: in the present study both the GB migration and sliding are predominant but in Ashby and Verrall's study, the mechanism of switching is based on diffusion.