Driving Force for Pure Step Movement at Grain Boundaries Due to Vacancy Supersaturation and Anistropy

A relaxed pure step at the grain boundary is considered as a microfacet with a junction dislocation dipole. In the presence of non-equilibrium vacancies at the boundary, osmotic forces produce a stressed state and, due to anisotropy, unequal strains on two different sides of the riser plane. Strain compatibility is satisfied due to additional stresses which induce a gradient in the chemical potential of the atoms across the grain boundary. The driving force is determined by the amount of elastic energy dissipated during diffusion. Grain boundary migration is associated with movement of line defects on interfaces. The high migration rates observed in some cases during recrystallization and during diffusion induced grain boundary migration (DIGM) are in turn associated with movement of pure steps. Earlier, King proposed a model for pure step migration during DIGM. A relaxed pure step was considered as a microfacet with a junction dislocation dipole. In the presence of a vacancy concentration gradient along the boundary the net force exerted on the step, equals the difference of the two climb forces acting on the junction dislocations. This net force is associated with a thermodynamic driving force. The contribution of this force to the step movement may be significant only during diffusional transients which set up large vacancy gradients. In the present work an additional cause for a driving force acting on a pure step /dislocation dipole is considered. This force is due to two factors: non-equilibrium vacancy concentration at the boundary and crystal anisotropy.