Quantitative analysis of observations on diffusion induced grain boundary migration for random boundaries in the Cu(Zn) system using a driving force model

Diffusion induced grain boundary migration (DIGM) in the Cu(Zn) system was experimentally studied by Li and Hillert using polycrystalline Cu specimens zincified with binary Cu-Zn alloys containing 3.9 30.5 wt% of Zn at temperatures between 573 and 773 K. Their experimental results have been quantitatively analyzed using the energy balance model proposed by Kajihara and Gust. The effective driving force Δ^efG for DIGM has been evaluated from the migration rate v of the moving boundary and the composition in the region alloyed with Zn behind the moving boundary, and then the mobility M of the moving boundary has been calculated using the relationship M = v/Δ^ef G. According to the analysis, the grain boundary migration obeys the chemical driving force model proposed by Hillert and Purdy for the largest experimental values of v at 573 and 623 K. However, the chemical driving force is partially consumed by the volume diffusion of Zn in the Cu matrix ahead of the moving boundary for the other experimental values of v at 623-773 K. In such a case, the migration rate dependence of the effective driving force should be taken into consideration. The temperature dependence of the mobility gives a value of Q_M = 177 kJ/mol as the activation enthalpy for the grain boundary migration. This value is close to the activation enthalpy for volume diffusion of Zn in Cu, 191 kJ/mol. Consequently, the grain boundary migration is considered to be controlled by the solute drag effect due to the volume diffusion of Zn in the Cu matrix in the neighborhood of the moving boundary.