Energetics of formation and migration of self-interstitials and self-interstitial clusters in α-iron

Energetic primary recoil atoms from fast neutron irradiation generate both isolated point defects and clusters of vacancies and interstitials. Self-interstitial mobility as well as defect cluster stability and mobility play key roles in the subsequent fate of defects and, hence, in the overall microstructural evolution under irradiation. Self-interstitials and two, three and four-member self-interstitial clusters are highly mobile at low temperatures as observed in molecular-dynamics simulations and high mobility probably also extends to larger clusters. In this study, the morphology, energetics and mobility of self-interstitials and small self-interstitial clusters in α-iron are studied by molecular-statics and molecular-dynamics simulations using a Finnis-Sinclair many-body interatomic potential. Self-interstitial migration is found to be a two-step process consisting of a rotation out of the 〈110〉 split-dumbbell configuration into the 〈111〉 split-dumbbell configuration and 〈111〉 translational jumps through the crowdion configuration before returning to the 〈110〉 dumbbell configuration. Self-interstitial clusters of 〈111〉 type split-interstitials assembled on adjacent {110} planes migrate along 〈111〉 directions in an amoeba-like fashion by sequential local dissociation and re-association processes.