Thermally activated glide of small dislocation loops in metals

Recent computer simulation studies have shown that small clusters of self-i nterstitial atoms (SIAs) formed in displacement cascades consist of crowdio ns and are highly mobile in the crowdion direction. In the present work, we use molecular dynamics to investigate whether small perfect vacancy loops formed in cascades in alpha-Fe and Cu are also mobile. Loops containing mor e than about 30 vacancies in Fe are found to produce atomic displacements d uring annealing, due to thermally activated movement in the direction of th eir Burgers vector that is qualitatively similar to the mechanism of SIA cl uster motion. Although vacancy clusters are slightly less mobile than SIA c lusters under the same conditions, their mobility is significantly higher t han that of the monovacancy. The motion of vacancy loops in Cu does not occ ur because they transform into sessile configurations similar to stacking-f ault tetrahedra. These results point to the possibly important contribution of vacancy loop mobility to the difference in radiation damage between bce and fee metals, and between fee metals with different stacking-fault energ ies.