A continuum model of nanocrystalline metals under shock loading

Nanocrystalline metals, i.e., polycrystalline metals with grain sizes in the nanometer range, have recently elicited significant interest due to their potential for achieving higher material strength, especially under shock loading, reaching strength as much twice the value under normal conditions. The main source of deformation, grain-boundary sliding, has been found to be coupled to friction-like mechanisms decreasing the proportion of sliding resistance by an amount proportional to the applied normal stress. In this work, we propose a continuum model describing the competing deformation mechanisms believed to determine the effective response of nanocrystalline materials. A phenomenological model considering grain boundary sliding and accommodation as uncoupled plastic dissipative deformation mechanisms is formulated to describe the constitutive behavior of grain boundaries. A Mohr-Coulomb friction model is then added by considering the normal stress as an inhibiter of sliding, in agreement with molecular dynamics findings. The model proposed aims at capturing the main feature of the effective behavior afforded by atomistic descriptions at a much lower cost, i.e., without the need of tracking the evolution of individual atoms. Copyright © 2006 by MIT