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

Abstract

Direct formation of a large vacancy loop by displacement cascade in α-Fe has been observed for the first time in a molecular dynamics (MD) computer simulation study. This phenomenon occurred in one anomalous run out of 100 simulations of 50 keV primary knock-on atom energy cascades, in which one large displacement cascade was produced instead of the formation of smaller 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 was formed by cascade collapse. A very-low-probability, one hundredth or probably less, for vacancy loop formation in the present MD simulation is consistent with the experimental observation of a low defect yield in irradiated α-Fe.