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 similar to 90 nm to similar to 30 nm 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. (C) 2014 Elsevier B.V. All rights reserved.