Direct evidence of a deformation mechanism crossover in nanocrystalline nickel

The deformation mechanism crossover in nanocrystalline nickel was investigated using dark-field TEM (DFTEM). The grain boundary mediated plasticity become prominent when the average grain size of a material decreases below some critical value. The deformation mechanism crossover is resulted from the competition between the deformation controlled by nucleation and motion of dislocations and the deformation controlled by grain boundary related deformation accommodated mainly by grain boundary diffusion with decreasing grain size. The results suggest that the nucleation of partial dislocations is only a necessary but not sufficient requirement for the formation of deformation twining.