Mechanistic models for the activation volume and rate sensitivity in metals with nanocrystalline grains and nano-scale twins

This paper describes mechanistic models that seek to rationalize experimentally determined low values for the activation volume associated with the high strain rate sensitivity of nanocrystalline metals. We present models for the emission of partial or perfect dislocations from stress concentrations at a grain boundary or twin boundary. The emission of deformation twins is likewise examined as a competing mechanism to perfect dislocation emission. The approach illustrates the important roles of both the intrinsic stacking fault energy and the unstable stacking energy. We find that the models lead to estimates of activation volumes in the range 3 − 10b3 for truly nanocrystalline metals. Activation volumes are found to increase monotonically with increasing grain size. The findings are found to be in accord with available experimental evidence in both a quantitative and qualitative manner. Deficiencies in the available experimental evidence are noted, specifically in the context of explaining some of the difficulties in comparing theoretical predictions to experimental observation.