Incipient plasticity of twin and stable/unstable grain boundaries during nanoindentation in copper

An incipient plastic deformation of several types of grain boundaries subjected to nanoindentation was investigated by atomistic simulations. Our previous study showed that the dislocation nucleation in the inner region of the defect-free metals occurs when the resolved shear stress exceeds a particular stress level slightly higher than the ideal shear strength. However, crystal defects such as grain boundaries undermine the nucleation resistance. In this paper, we examined the dislocation nucleation mechanism at the twin and several coincidence site lattice grain boundaries and the resulting weakening of the dislocation nucleation resistance. We found that for the twin and the relatively stable Sigma 11((1) over bar 13 [110] grain boundary, the primary slip deformation is activated on the grain-boundary plane prior to the defect-free region because of the low fault energy of the grain boundaries during slip deformation. Subsequently, the secondary slip is activated from the grain boundary. On the other hand, the dislocation is initially generated from the heterogeneous grain-boundary plane for the unstable high-energy grain boundaries.