Kinetic Study of Vacancy and Interstitial Annealing in Metals

Abstract

The rate equations governing the annealing of vacancies and interstitials in irradiated and prequenched irradiated metals for stage-III recovery have been studied. Vacancies were considered to be the mobile defect for this recovery, and the effects of the formation and migration of small vacancy clusters were investigated. Interstitials, either in the form of single trapped interstitials or small interstitial clusters, were assumed to be immobile. This system gives rise to two annealing stages, one of which involves annihilation of vacancies and interstitials, and the other, at higher temperatures, arises from vacancy migration to permanent sinks such as dislocations or grain boundaries. The form of the interstitial, trapped or clustered, has little effect on the calculated results. Peak temperatures and calculated effective migration energies, however, are sensitive to the vacancy clustering parameters (migration and binding energies). The presence of small concentrations of mobile vacancy clusters can shift the low-temperature peak to lower temperatures without changing its magnitude, and give rise to low and variable measured activation energies. The recovery model suggested by these calculations ascribes stage III to interstitial-vacancy annihilation and stage IV to vacancy migration to sinks. Depending on the clustering parameters, the kinetics for stage III may be primarily related to single-vacancy migration or may be complicated by vacancy clusters. This latter case does not require any initial cluster concentration and does not require high cluster concentrations during the recovery process.