Atomistic simulations of plasticity heterogeneity in gradient nano-grained FCC metals

Abstract

Gradient nano-grained (GNG) metals are an emerging class of heterogeneous structured materials with extraordinary mechanistic performances and unnatural plasticity mechanisms, which are unachievable in traditional homogeneous nanocrystalline metals. Here, the metal-type dependence of plasticity heterogeneity in three different GNG structured face-centered cubic metals (Cu, Al, and Ni) was studied by large-scale molecular dynamic simulation, from the plasticity perspectives of dislocation, deformation twinning, and grain boundary. The dislocation and deformation twinning-based plasticity show clear metal-type dependence in GNG structure of different selected metals. For grain boundary activities, GNG Cu, Al, and Ni are equivalent in the overall grain boundary sliding and micro-cracking process, but GNG Ni shows markedly higher grain boundary-free volume and stress due to the grain size gradient-induced twinning process. We find that GNG Al shows the weakest plasticity-induced strengthening due to its weak strain hardening and ununiform plasticity distribution, while GNG Ni exhibits prominent plasticity-induced strengthening because of the obvious extra strain hardening process. Besides, we further discuss the reasonable gradient grain size design of GNG structure, considering the critical grain size of the Hall-Petch and inverse Hall-Petch relationship, which will demonstrate a more rational plasticity heterogeneity and internal atomic stress distribution.