Recent experiments on face-centered cubic and hexagonal close-packed nanocrystalline metals had reported a more than tenfold increase in strain-rate sensitivity, in contrast to conventional coarse-grained counterparts. In order to improve the understanding of this behaviour, a mesoscopic continuum model of a two-dimensional polycrystal was considered with deformation mechanisms which included grain-interior plasticity, grain-boundary diffusion and grain-boundary sliding. The model captured the transition from sliding- and diffusion-dominated creep, in nanocrystals having relatively small grain sizes, at low strain rates to plasticity-dominated flow in nanocrystals having larger grain sizes at higher strain rates.

The strain-rate sensitivity deduced from the calculations agreed well with experimental data on nanocrystalline Cu. Based upon this analysis, an analytical model which incorporated the competition between grain-interior plasticity and grain-boundary deformation was proposed in order to provide an intuitive understanding of the transition in strain-rate sensitivity in nanostructured metals.

Enhanced Strain-Rate Sensitivity in FCC Nanocrystals due to Grain-Boundary Diffusion and Sliding. Y.Wei, A.F.Bower, H.Gao: Acta Materialia, 2008, 56[8], 1741-52