Driving Force for Grain Boundary Migration During Alloying by DIGM and DIR in Binary Systems

Abstract

A large number of experimental studies have been carried out on diffusion induced grain boundary migration (DIGM) and diffusion induced recrystallization (DIR) in binary alloy systems. A region with different composition is left behind a moving grain boundary (GB) owing to GB migration combined with the diffusion of solute atoms along the moving boundary in DIGM. On the other hand, during DIR, new grains with different solute concentrations are produced behind moving GBs due to recrystallization combined with diffusion of solute atoms along the moving GBs. The compositional discontinuity between the regions ahead and behind the moving GB should be the most important driving force for DIGM and DIR. The driving force due to this compositional discontinuity is usually called chemical driving force. Hillert and Purdy proposed the chemical driving force model for DIGM during alloying in binary systems. However, there are several sinks for the chemical driving force. For instance, if the migration of a GB is slow enough, solute atoms will penetrate into the untransformed matrix ahead of the moving GB and such a process may consume all the driving force. This can be avoided by coherency stresses building up in the penetration zone due to the compositionmore » dependence of the lattice parameter of the matrix. A new model has been proposed to evaluate the driving force for DIGM and DIR during alloying in binary systems as a function of the migration rate on the basis of the idea in a previous paper. Although this model is applicable to both DIGM and DIR, attention will be focused on DIGM to simplify the description in this paper.« less