Plastic deformation mechanisms in nanocrystalline columnar grain structures
We present an atomistic study of the plastic deformation mechanisms occurring in columnar structures of nanocrystalline Ni, with particular emphasis on the process of dislocation emission form the grain boundaries. The samples are constructed with grain boundaries characterized by random tilt misorientations around a common (110) type axis. All samples contain the same 36 grains polycrystalline microstructure, with grain sizes ranging from 4 to 20 nm in order to isolate size effects on the deformation mechanisms. Tensile deformations up to 8% were simulated and the strain stress curves observed for these samples show grain size effects in both the elastic and plastic portions. An inverse Hall-Petch effect is observed for the nominal stress at a fixed strain, but disappears when the grain size-dependent elastic modulus is used to construct an 0.5% offset yield stress. Both dislocation emission from the grain boundaries and grain boundary accommodation of plasticity are observed. Dislocation emission comes largely from pre-existing dislocation-like structures in the grain boundaries, and increases rapidly for grain sizes textgreater 4 nm. However, the number of dislocations per unit length of grain boundary saturates to a constant value at large grain sizes, indicating a fixed density of pre-existing sources in the grain boundaries. A simple model wherein dislocation emission is prohibited within a small distance from grain triple junctions accounts for the overall density versus grain size. Grain boundary sliding was observed in the same regions of the microstructure in all grain sizes, and to approximately the same degree. A simple model accounting for both dislocations and sliding is consistent with the observed trends in plastic strain versus grain size. The implications of these observations for more realistic three-dimensional samples are briefly discussed. (c) 2005 Elsevier B.V. All rights reserved.