Diffusion Induced Grain Boundary Migration in the Ni-Zn System

Abstract

Diffusion induced grain boundary migration (DIGM) has been studied in the Ni-Zn system by exposing polycrystalline Ni to Zn vapour with the Ni-30 wt% Zn alloy acting as a source. The time and temperature dependence of the migration distance and hence the rate of migration was studied in the temperature range 773 to 933 K. A parabolic growth behaviour was observed. The diffusivity, Dbδ, was calculated from the rate of migration and the value of v/Dbδ obtained through the concentration profile and Cahn's equation at each temperature. The diffusion coefficient was observed to be 5 to 6 orders of magnitude higher than the corresponding volume diffusion of Zn in Ni. The activation energy for solute transport corresponds to that required for grain boundary diffusion of Zn in Ni. The microstructural features during DIGM have been studied in detail and analyzed. These will also be presented.The diffusion induced growth of fine grains and growth of the fine grained layer were also studied. From the rate of growth of fine grains and the value of Dbδ calculated for the growth of fine grained layer, it has been observed that these two processes also occur by solute transport through the grain boundaries.