Diffusion induced grain boundary migration in the Ag–Zn system

Diffusion induced grain boundary migration (DIGM) has been studied in the Ag–Zn system by exposing polycrystalline Ag to Zn vapor with a Ag-25 wt.% Zn alloy as the source of Zn. The time and temperature dependence of the migration distance has been studied in the temperature range 660 to 810 K. The composition profile was obtained on the sheet cross-section along a line perpendicular to the edge to determine D_bδ at each temperature. Similarly, the Zn concentration profile was obtained from the region swept by the migrating grain boundary. The coherency strain energy, the total chemical free energy change and the effective free energy change were calculated. The regular solution model was used for calculating the free energy change. It has been observed that a fraction of the total free energy has been used for volume diffusion in front of the migrating grain boundary. The instantaneous rate of migration has been observed to be directly proportional to the chemical free energy change and the coherency strain energy. The instantaneous rate of migration versus the composition graph has indicated that the driving force for DIGM in the Ag–Zn system is the coherency strain energy.The fine-grained layer formed at the surface follows a parabolic growth behavior. The diffusion coefficients calculated from the composition profile as well as from the rate of growth of the fine-grained layer are of the same order of magnitude. The diffusivity values are four to six orders of magnitude higher than the volume diffusion coefficients. From the activation energy and the diffusivities it is clear that DIGM in the Ag–Zn system occurs by the diffusion of Zn along the grain boundaries of polycrystalline Ag.