The occurrence of diffusion-induced grain-boundary migration was studied by means of electron probe micro-analysis and analytical electron microscopy of the Al substrates of Al/Zn diffusion couples which were annealed at temperatures ranging from 395 to 535K. The results indicated the operation of 2 basic types of diffusion-induced grain-boundary migration: laminar and turbulent. The former occurred over the entire temperature range and was characterized by a small migration distance and large migration depth. The enrichment of Zn at the sample surface was between 4.0 and 5.0wt%, and gradually decreased with increasing depth. The turbulent type of migration was limited to annealing temperatures which were greater than 450K. In this case, the width of the alloy zone was much greater close to the surface of the sample and then markedly decreased; exhibiting a behavior which was similar to that of the laminar morphology. The Zn content at the sample surface was between 8.0 and 9.0wt%. The diffusivities for diffusion-induced grain-boundary migration, which were calculated on the basis of Cahn’s equation, agreed well with the values of stationary grain boundaries in dilute Al-Zn alloys. The evidence for the existence of diffusion-induced grain-boundary migration was the asymmetry of the Zn profile with regard to the final position of the boundary. Micro-analytical scans across the alloyed zone revealed a sudden change in Zn concentration at the moving boundary. It was concluded that the role of volume diffusion during diffusion-induced grain-boundary migration was not so important, and that a considerable chemical contribution to the total driving force should exist.

P.Zieba, A.Pawlowski: Journal of Materials Science, 1994, 29[23], 6231-40