On the annealing of prismatic dislocation loops in aluminum

Abstract

A recently proposed mechanism for coalescence of dislocation loops on annealing by a pipe diffusion mechanism la examined. The driving force for pipe diffusion arises from two sources. In the first, differences in concentrations of vacancies round the perimeter of a loop due to differences in the forces exerted by a neighbouring loop cause a net flow of vacancies tending to make the loops coalesce. In the second, vacancies, formed at a given loop, interact with the stress field of a neighbouring loop. The driving forces for both these processes seem to be of the same order of magnitude, and the dependence on separation of the loops is similar. Estimates of the driving force are made from the stress field of a prismatic loop. The driving force is very small owing to the rapid decrease of the stress field with distance from a prismatic loop. It is shown that the times of coalescence of typical neighbouring loops are much longer than those recently proposed. It is concluded that pipe diffusion cannot be the rate-controlling mechanism for annealing of loops in quenched aluminium.