Direct observations of grain boundary dislocation motion due to thermal stresses

Grain boundaries in high-purity (99.999%) aluminum with misorientations of 8°–15° have been studied in situ under conditions of applied stress. At both high and low homologous temperatures (T ≈ 0.8T_mp and T ≈ 0.32T_mp, respectively), substantial grain-boundary-dislocation motion and grain-boundary migration was observed. In conjunction with grain-boundary migration was the formation of “ghost” boundaries (indicating prior positions of the boundary) during both high- and low-temperature experiments. Experimental observations suggest that one cause of “ghost” boundary formation was a discontinuous change in the thickness of the region swept out by the migrating boundary, which was apparently due to grain-boundary diffusion from the grain boundary to the grain surface during strain-induced grain-boundary migration. Evidence that grain boundary diffusion was involved received further support from the formation of epitaxial hillocks at the grain boundary during migration. Finally, a simple model was derived which showed that grain-boundary diffusion during migration was capable of producing a discontinuous thickness change such as that observed experimentally.