Competing plastic deformation mechanisms in nanophase metals

The mechanisms of plastic deformation in nanocrystalline Ni are studied using three-dimensional molecular-dynamics computer simulations for samples with mean grain sizes ranging from 3 to 12 nm under uniaxial load at finite temperatures. At the lower limit of this size range, we observe a plastic regime controlled by intergrain sliding; at the upper limit, however, we observe a regime with two competing mechanisms: intergrain sliding and dislocation emission from the grain boundaries (GB's). The latter mechanism constitutes a transition behavior, precursor to the dislocation-dominated regime typical of large grain polycrystals. In samples with mainly low-angle GB's, the transition occurs at a smaller grain size.