Variation of strain rate sensitivity with grain size in Cr and other body-centred cubic metals

The strain rate sensitivity (i.e. m -value) of body-centred cubic Cr was measured over a wide grain size range (10 −9 –10 −2 m) using an instrumented nanoindentation technique. The m -value was observed to decrease initially as the grain size was reduced from mm to µ m, it reached a minimum, and then increased drastically when the grain size was below 20–30 nm. There was a corresponding variation in activation volume, particularly when the grain size was in the nm range and the grain-boundary-mediated process became dominant. The variation of m -value and activation volume over the entire grain size range could be rationalized by a model based on a double-kink mechanism coupled with the dislocation emission processes from the grain boundaries.