He-induced vacancy formation in bcc Fe solid from first-principles simulation

Using first-principles calculations, we investigated the He and H effects on vacancy formation in bcc Fe. From energetic point of view, the presence of interstitial He (H) atom reduces vacancy formation energy and single He prefers to occupy vacancy center. One monovacancy can accommodate at least 20 He (or 5 H) atoms, with He-He (H-H) distances of 1.5-1.7 Å (1.8-2.38 Å) in the Hem-vacancy (Hm-vacancy) complexes. The shorter He-He distance and larger expansion of vacancy space after multiple He insertion may account for more number of He atoms trapped in vacancy with regard to H. Moreover, second vacancy formation energy around the He-vacancy complex decreases remarkably with increasing amount of He atoms. Thus, trapping of multiple He in a Fe monovacancy can induce formation of new vacancy at nearby lattice site, and several vacancy-SIA pairs would form concurrently.