A Three Dimensional Model for Nanocrystalline Materials based on Grain Interior and Grain Boundary Deformation Mechanisms
Ercan Gurses (METU)
Plasticity at Different Length Scales
Mon 2:40 - 4:00
CIT 219
Nanocrystalline metals, because of their distinct features, e.g., high strength, low ductility, pronounced rate dependence, tension-compression asymmetry and susceptibility to plastic instability, has become the subject of intense research over the past two decades. Owing to a large volume fraction of grain boundary (GB) atoms, the deformation mechanisms in nanocrystalline metals are different from traditional coarse-grained polycrystals. Indeed, there are several inelastic deformation mechanisms, e.g., GB-diffusion, GB-sliding, dislocations, grain rotation, grain growth and GB- migration, which can be simultaneously operative in nanocrystalline metals. In this work, a three dimensional viscoplastic constitutive model for nanocrystalline metals is presented. Different from most constitutive models where nanocrystalline metals are considered as multi-phase composites with corresponding volume fractions, the proposed model does not assume a parameter for a GB-thickness or GB affected zone. The proposed model is based on several competing grain boundary and grain interior deformation mechanisms. GB-diffusion (Coble creep), GB-sliding, and the grain interior dislocation-mediated-plasticity are the three different types of deformation mechanisms that are assumed to be operative in this work. The first two micro-mechanisms are GB-based, whereas the last one is related with the grain interior. Effects of pressure on the grain boundary diffusion and sliding mechanisms are taken into account. Furthermore, the influence of grain size distribution on macroscopic response is studied. Finally, the model is shown to capture the fundamental mechanical characteristics of nanocrystalline metals. These include grain size dependence of the strength, i.e., both the traditional and the inverse Hall–Petch effects, the tension–compression asymmetry and the enhanced rate sensitivity.