Temperature-dependent strain rate sensitivity and activation volume of nanocrystalline Ni

Repeated stress relaxation tests and strain rate jump tests have been carried out over a range of deformation temperatures (77-373 K) on electrodeposited nanocrystalline Ni with an average grain size of ∼30 nm. The strain rate sensitivity, the apparent and physical activation volume, and the activation energy have been determined. The magnitude observed for these characteristic deformation parameters, as well as their temperature-dependent behavior, is very different from those of coarse-grained Ni. This suggests that the thermally activated process in nanocrystalline Ni is different from the conventional forest dislocation cutting mechanism. It is concluded that grain boundary diffusion-controlled processes such as Coble creep and grain boundary sliding can be ruled out as dominant mechanisms for the grain sizes and temperature range studied. Instead, our experimental findings and analysis suggest that the deformation kinetics are controlled by the activities of dislocations. The dominant thermally activated mechanism is suggested to originate from three possible processes, all involving interactions of mobile dislocations with grain boundaries.