Stochastic annealing simulation of differential defect production in high energy cascades

Recent molecular dynamics (MD) studies have confirmed that significant clustering of both vacancies and self-interstitial atoms (SIAs) takes place by the end of the quenching stage of a cascade, and that small interstitial clusters are glissile, with migration energies on the order of 0.1 eV. The spatial segregation and clustering of the vacancies and SIAs give rise to a differential production of mobile vacancies and SIAs that has a strong temperature dependence.

At temperatures above recovery Stage V, vacancies can evaporate from clusters, while large SIA clusters produced in the cascade remain stable, leading to a differential increase of mobile vacancies that represents a ‘production bias’ that may be responsible for void swelling. The stochastic annealing simulation code ALSOME is used to investigate quantitatively the differential production of mobile vacancy and SIA defects as a function of temperature for isolated 25 keV cascades in copper generated by MD simulations.

The ALSOME code and cascade annealing simulations are described. The annealing simulations indicate that above Stage V, where the cascade vacancy clusters are unstable, nearly 80% of the post-quench vacancies escape the cascade volume, while about half of the post-quench SIAs remain in clusters. The results are sensitive to the relative fractions of SIAs that reside in small, highly mobile clusters and in large, sessile clusters, respectively, which may depend on the cascade energy.