ON THE ORIGINS OF GHOST BOUNDARIES DURING INSITU STRAIN-INDUCED GRAIN-BOUNDARY MIGRATION

Grain boundaries in high-purity (99.999%) aluminum with misorientations of 8-degrees-15-degrees have been studied in situ under conditions of applied stress. At both high and low homologous temperatures (T almost-equal-to 0.8T(mp) and T = 0.32T(mp), respectively), substantial grain-boundary-dislocation motion and grain-boundary migration was observed. In conjunction with grainboundary migration was the formation of "ghost" boundaries (indicating prior positions of the boundary) during both highand 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 cxperimentally.