ON THE KINETIC-BEHAVIOR AND DRIVING-FORCE OF DIFFUSION-INDUCED GRAIN-BOUNDARY MIGRATION

The diffusion induced grain boundary migration (DIGM) has been studied in the Ni(Cu) system over the temperature range 723-1023 K using light microscopy, scanning electron microscopy and electron probe microanalysis (EPMA). Four different stages of the grain boundary (GB) migration during DIGM were found, involving nucleation, initial, stationary and mixed stages. During the mixed stage the DIGM process occurs simultaneously with the diffusion induced recrystallization, and it is impossible to separate the two phenomena. The EPMA measurements reveal that the Cu concentration in the DIGM zone does not remain constant, but increases with increasing annealing time. The highest Cu concentration which does not depend on annealing conditions is situated in the middle of the DIGM zones. Based on the measured Cu distribution in the DIGM zones, the coherency strain driving force for GB migration is calculated. In some cases the calculated values are lower than the energy difference across the GB due to its curvature. It is concluded that some other driving force should be involved. The nucleation and initial stages of the DIGM process can be explained by a model based on the diffusion induced GB stresses. The Arrhenius parameters of the diffusion along and across the GB in Ni-rich Ni-Cu alloys have been determined.