The direct formation of a large vacancy loop, in the α-phase, by displacement cascades was observed for the first time in a molecular dynamics simulation study. This phenomenon occurred in a single anomalous run among 100 simulations of 50keV primary knock-on atom energy cascades, in which a single large displacement cascade was produced instead of smaller sub-cascades. Two large self-interstitial atom clusters were produced at the periphery of the cascade core, followed by the formation of a region with a very high vacancy concentration at the centre of the cascade during quenching of the thermal spike phase. One large vacancy loop, with a Burgers vector of a<100>, was formed by cascade collapse. The very low-probability (less than 0.01) of vacancy-loop formation in the present simulation was consistent with experimental observations of a low defect yield in irradiated α-Fe.

Vacancy Loop Formation by Cascade Collapse in α-Fe - a Molecular Dynamics Study of 50keV Cascades. N.Soneda, S.Ishino, T.Diaz de la Rubia: Philosophical Magazine Letters, 2001, 81[9], 649-59