Stacking fault emission from grain boundaries: Material dependencies and grain size effects

When load is applied to fcc nanograins, leading partial dislocations nucleate at grain boundary steps and propagate into the grain, leaving stacking faults behind. The extent to which these faults expand before a trailing partial is emitted generally does not equal the equilibrium separation distance of the corresponding full dislocation. Here we use a density functional theory - phase field dislocation dynamics model to study the effect of applied stress, 3D grain size, material stacking fault energies, and grain boundary ledge size on the stress-driven emission of leading and trailing partial dislocations from a grain boundary. The calculation accounts for the nucleation and glide of leading and trailing partial dislocations by incorporating the entire material γ-surface into the formulation. We show that the nucleation stress for a Shockley partial from a grain boundary is controlled by the size of the grain boundary ledge, scales with the unstable stacking fault energy γU, and is insensitive to grain size. We also reveal a gigantic γ-surface effect where small changes in γI/μb can lead to large changes in the extent of the stacking fault region. Last, we find that the stacking fault region increases with grain size and eventually saturates at larger grain sizes, which our analyses suggest can be attributed to local grain boundary stresses. These findings can provide insight into transitions in the mechanical behavior of nanostructured metals.