Mechanical response of nanocrystalline platinum via molecular dynamics: size effects in bulk versus thin-film samples

Abstract

We use large-scale molecular dynamics simulations to characterize the mechanical responses of nanocrystalline bulk and thin-film samples with average grain size ranging from 5 to 40 nm and at two strain rates. Our simulations show Hall–Petch maxima for both yield and flow stresses and for both sets of specimens. We find that the presence of free surface decreases both the yield and flow stresses and, interestingly, the Hall–Petch maximum for slabs occur at a larger grain size than for the bulk samples. A quantitative analysis of plastic slip on grain interiors and boundaries reveals that the shift in the maximum results from a combination of higher intergranular slip and weaker size dependence of dislocation activity in the slabs as compared with the bulk. Finally, increasing strain rate increases both yield and flow stresses and this rate effect is dominated by the plasticity involving full dislocations; plastic slip by partial dislocations and grain boundary processes exhibit weaker size effects.