Possibility of Subgrain Rotation during Recrystallization

The possibility of the rotation of a subgrain with respect to its neighbors as a natural process during recrystallization is analyzed thermodynamically and kinetically. It is found that it is energetically possible if the direction of rotation favors the elimination of low-angle boundaries over that of high-angle boundaries, the elimination of twist and asymmetric boundaries over that of the tilt and symmetric boundaries, and the elimination of the large-area boundaries over that of the small-area boundaries. Since the rotation direction has two degrees of freedom, there exists a relation between the two degrees of freedom such that directions whose two degrees of freedom fulfill such a relation provide no driving force for rotation. All other directions will supply free energy to rotate in one sense or the opposite. Kinetically one of the following two processes within one boundary can be rate-controlling: the cooperative movement of dislocations in the boundary, and the cooperative diffusion of vacancies in the lattice. The rotation of a subgrain favors the elimination of one of the boundaries which contributes the largest fraction of driving force and also the largest fraction of resistance. This causes the subgrain to coalesce with the other subgrain separated by this boundary. The estimated time required for one such coalescence compares favorably with the observed rate of subgrain growth in Al.