Grain size dependent strain rate sensitivity in nanocrystalline body-centered cubic metal thin films

The strain rate sensitivity (m) and activation volume (v^*) of three nanocrystalline (NC) body-centered cubic (bcc) metals, i.e., W, Mo and Ta, with various grain sizes were evaluated by nanoindentation testing. Opposite to the conventional trend that NC bcc metals exhibit reduced m as the grain size was decreased, elevated m was observed as the grain size was reduced from ~90nm to ~30nm for all the samples concerned. It was proposed that the unusual variation trends of m for NC bcc metals were dominated by GB-related mechanisms when the grain size drops below a critical value.