Vacancy loop formation by 'cascade collapse' in alpha-Fe: a molecular dynamics study of 50 keV cascades

Direct formation of a large vacancy loop by displacement cascade in alpha - Fe has been observed for the first time in a molecular dynamics (MD) comput er simulation study. This phenomenon occurred in one anomalous run out of 1 00 simulations of 50 keV primary knock-on atom energy cascades, in which on e large displacement cascade was produced instead of the formation of small er subcascades. Two large self-interstitial atom clusters were produced at the periphery of the cascade core, followed by the formation of;a very high concentration region of vacancies at the centre of the cascade during the quenching of the thermal spike phase. Finally, one large vacancy loop with Burgers vector b = a(0) (100) was formed by cascade collapse. A very-low-pr obability, one hundredth or probably less, for vacancy loop formation in th e present MD simulation is consistent with the experimental observation of a low defect yield in irradiated alpha -Fe.