Orientation dependence of the nucleation and growth of partial dislocations and possible twinning mechanisms in aluminum

Plastic deformation in certain nanocrystalline metals (grain size less than 100 nm) or under certain conditions is associated with profuse activation of partial dislocations with grain boundaries acting as both the source and the sink. We use molecular dynamics (MD) simulations of pure aluminum to investigate the orientation dependence of deformation mechanisms, and show that two types of twins can be generated. Theoretical calculations are used to understand the effect of loading orientation on the activation of twins. The theoretical framework incorporates (a) a tensorial stress state, (b) dislocation loops and (c) non-singular core energies for accurate descriptions of criteria for partial dislocation activation. Both the theoretical predictions and the MD simulation demonstrate that twins should be possible (but rare) in nanocrystalline aluminum.