Ab initio study of helium in α-Fe: Dissolution, migration, and clustering with vacancies

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