Grain Size Effect on Deformation Mechanisms of Nanocrystalline bcc Metals

Nanocrystalline (NC) body-centered cubic (bcc) metals behave very differently from how NC metals with other crystal structures behave. Their strain rate sensitivity decreases with decreasing grain size, which is an observation that has not been well understood. Here, we report a significant effect of grain size on the deformation mechanism of NC bcc Mo. With decreasing grain size, the density of mixed and edge dislocations increases, while the density of screw dislocations decreases. When the grains become very small, the overall dislocation density decreases with decreasing grain size. These observations provide a logical explanation for the observed effect of grain size on strain rate sensitivity.