Rationalizing the Grain Size Dependence of Strength and Strain-Rate Sensitivity of Nanocrystalline fcc Metals

Abstract

Low strain-rate sensitivity (SRS) of nanocrystalline metals measured by experiments often leads to the claim that grain boundary (GB)-mediated plasticity is insignificant, contrary to molecular dynamics simulation results. Here, we develop an crystal plasticity model to rationalize the important role of GB-mediated plasticity on the rate-controlling deformation of nano-grained (NG) and ultrafine-grained (UFG) face-centered-cubic (fcc) metals. Important phenomena such as the GB strengthening, the stress saturation, and the evolution of SRS are well captured. We show that the main reason for the low SRS measured experimentally in NG metals (several tens of nm) is the dominance of the localized dislocation activities over the GB process on the overall plasticity. Such localization of dislocation process may provide a reason for the formation of shear bands/zones in NG and UFG fcc metals.