Growth behavior of fine grains formed by diffusion induced recrystallization in the Cu(Zn) system Original Research Article

The growth behavior of fine grains produced due to diffusion induced recrystallization (DIR) was experimentally studied for the Cu(Zn) system using single crystal Cu specimens. The Cu specimens were annealed at 693 K for various times together with a Zn source alloy by a capsule zincification technique and then DIR was observed to take place on the surfaces of the specimens. The DIR region consists of an almost single layer of fine grains. Although the average height l and the average diameter s of the fine grains increase with increasing annealing time, the aspect ratio s/l is kept to be nearly unity during annealing. The Zn concentration on the specimen surface is smaller than that in the Zn source alloy at early stages of the reaction. Such lower Zn concentrations could be explained using the maximum driving force model proposed by Kajihara and Gust (Acta metall. mater., 1991, 39, 2565). The composition on the specimen surface gradually approaches the equilibrium value with increasing annealing time. This means that the amount of the Zn source alloy is large enough to maintain the supply of Zn atoms to the specimen in each zincification capsule. The kinetic model by Li and Hillert (Acta metall., 1981, 29, 1949) and the extended model by Kawanami et al. (ISIJ Int., 1997, 37, 921) could rather well reproduce the annealing time dependence of the growth rate of the DIR region. The contribution of volume diffusion to the transfer of Zn atoms from the surface to the moving boundaries in the DIR region was estimated to be negligible. Thus, the chemical driving force is considered to be the most important driving force for the grain boundary migration and the boundary diffusion along the grain boundaries in the DIR region is recognized to be the rate controlling process of DIR.