Comparative study on plastic deformation of nanocrystalline Al and Ni

The tensile plastic deformation of nanocrystalline (nc) Al and Ni, with grain sizes of 13.5 and 15.5 nm, respectively, is studied by molecular dynamics simulations. The results are analyzed using common neighbor analysis to characterize grain boundaries, dislocations and twins, and atomic strains to characterize plastic deformation. It is revealed that grain boundary (GB)-mediated process occurs before dislocation activities are initiated. The plasticity inside grains is carried by full dislocations in nc Al and by partial dislocations in nc Ni. For the first time, we observe zigzag dislocation motion initiating from grain boundaries where partial dislocations nucleate on parallel planes in nc Al without cross-slip. We also observe partial dislocation cross-slip via Fleischer mechanism in nc Ni. No twin is found in the deformation process of nc Al while plenty of twins are observed in nc Ni. In general, both dislocation-mediated processes and GB sliding contribute to the global plastic deformation significantly.