Diffusional penetration during diffusion-induced grain-boundary migration process in an Al-Zn couple

Abstract

The existence of diffusion-induced grain-boundary migration (DIGM) has been re-examined by electron probe micro analysis and analytical electron microscopy in the aluminium substrate of Al-Zn diffusion couple annealed in the temperature range 395–535 K. The investigation revealed two basic kinds of DIGM: laminar and turbulent. The laminar kind occurs over the whole temperature range and is characterized by a small migration distance and large migration depth. The zinc enrichment at a sample surface is 4.0–5.0 wt% and gradually decreases with increasing depth. The turbulent kind is limited to annealing temperatures above 450 K. In this case, the width of the alloyed zone is much greater, close to the surface of sample and then dramatically decreases, showing a behaviour similar to the laminar morphology. The zinc content at the surface of sample is 8.0–9.0 wt%. The diffusivities of DIGM calculated based on Cahn's equation agree well with the values of stationary grain boundary in diluted AlZn alloys. Evidence for the existence of DIGM was the asymmetry of the zinc profile with regard to the final position of the boundary. Microanalytical scan across the alloyed zone showed an abrupt change of the zinc concentration at the moving boundary. This suggests that the role of volume diffusion during DIGM is not so important and a considerable chemical contribution to the total driving force should exist.