Thermally activated glide of small dislocation loops in metals

Recent computer simulation studies have shown that small clusters of self¬interstitial atoms (SIAs) formed in displacement cascades consist of crowdions 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 a-Fe and Cu are also mobile. Loops containing more than about 30 vacancies in Fe are found to produce atomic displacements during annealing, due to thermally activated movement in the direction of their Burgers vector that is qualitatively similar to the mechanism of SIA cluster motion. Although vacancy clusters are slightly less mobile than SIA clusters under the same conditions, their mobility is significantly higher than that of the monovacancy. The motion of vacancy loops in Cu does not occur because they transform into sessile configurations similar to stacking-fault tetrahedra. These results point to the possibly important contribution of vacancy loop mobility to the difference in radiation damage between bcc and fcc metals, and between fcc metals with different stacking-fault energies.