Molecular dynamics calculations were performed to evaluate the thermal stability of He–vacancy clusters (HenVm) in Fe using the Ackland Finnis–Sinclair potential, the Wilson–Johnson potential and the Ziegler–Biersack–Littmark–Beck potential for describing the interactions of Fe–Fe, Fe–He and He–He, respectively. Both the calculated numbers of He atoms, n, and vacancies, m, in clusters ranged from 0 to 20. The binding energies of an interstitial He atom, an isolated vacancy and a self-interstitial Fe atom to a He–vacancy cluster were obtained from the calculated formation energies of clusters. All the binding energies did not depend much on cluster size, but they primarily depend on the He-to-vacancy ratio (n/m) of clusters. The binding energy of a vacancy to a He–vacancy cluster increased with the ratio, showing that He increased cluster lifetime by dramatically reducing thermal vacancy emission. On the other hand, both the binding energies of a He atom and an Fe atom to a He–vacancy cluster decreased with increasing the ratio, indicating that thermal emission of self-interstitial atoms (Frenkel-pair production), as well as thermal He emission, may take place from the cluster of higher He-to-vacancy ratios. The thermal stability of clusters was decided by the competitive processes among thermal emission of vacancies, self-interstitial atoms and He, depending on the He/vacancy ratio of clusters. The calculated thermal stability of clusters was consistent with the experimental observations of thermal He desorption from α-Fe during post-He-implantation annealing.

Thermal Stability of Helium–Vacancy Clusters in Iron. K.Morishita, R.Sugano, B.D.Wirth, T.Diaz de la Rubia: Nuclear Instruments and Methods in Physics Research B, 2003, 202, 76-81