Density functional theory calculations were performed to study the dissolution and migration of He in α-Fe, and the stability of small He-vacancy clusters HenVm (n, m = 0 to 4). Substitutional and interstitial configurations of He were found to have similar stabilities. The tetrahedral configuration was more stable than the octahedral by 0.2eV. Interstitial He atoms were predicted to have attractive interactions and a very low migration energy (0.06eV), suggesting that He bubbles could form at low temperatures in initially vacancy-free lattices. The migration of substitutional He by the vacancy mechanism was governed by the migration of the HeV2 complex, with an energy barrier of 1.1eV. The activation energies for He diffusion by the dissociation and vacancy mechanisms were estimated for the limiting cases of thermal-vacancy regime and of high supersaturation of vacancies. The trends of the binding energies of vacancy and He to He-vacancy clusters were discussed in terms of providing additional knowledge on the behavior of He in irradiated Fe, necessary for the interpretation of complex experimental data such as thermal He desorption spectra.

Ab initio Study of Helium in α-Fe - Dissolution, Migration and Clustering with Vacancies. C.C.Fu, F.Willaime: Physical Review B, 2005, 72[6], 064117 (6pp)