Competing regimes of rate dependent plastic flow in ultrafine grained metals

Low temperature (77 K, 243 K and 255 K) mechanical tests were carried out at strain rates between 7.10−4 s−1 and 5.10−2 s−1 on pure ultrafine grained (UFG) copper (grain size 100 nm) to mimick room temperature high strain rates. Variation of the activation volume measured as a function of the stress is consistent with the analytical model proposed in [C. Duhamel, Y. Bréchet, Y. Champion Int. J. Plast. 26 (2010) 747-757] to explain the rheology of UFG metals. This model is based on dislocations interactions at grain boundaries and grain boundaries sliding. The experiments confirm that the strain rate sensitivity (which should be a criterion for plasticity in absence of macroscopic work hardening) increases with the strain rate (or stress) at high strain rate, as also observed for nanotwinned copper. From the model, a strain rate sensitivity criterion is derived, function of a characteristic UFG strength parameter and grain boundary properties. The analytical description of deformation of UFG including microstructural properties should help for a quantitative control of the macroscopic properties of UFG.