First-principles study of helium effect in a ferromagnetic iron grain boundary: Energetics, site preference and segregation

Using a first-principles method based on density functional theory, we have investigated energetics and site preference of helium (He) in a ferromagnetic bcc-iron (Fe) ∑ 5 (3 1 0) / [0 0 1] grain boundary (GB). We calculate the binding energies of He atom in the GB, which show that the substitutional He is energetically favored in comparison with the interstitial He with a small energy difference of 0.06 eV. The segregation energy is calculated to be ∼1.4 eV for the energetically favorable GB substitutional and interstitial sites, which is large enough for the He atoms to segregate to these sites, independent of the temperature and the bulk He concentration. This leads to the conclusion that all the He atoms will segregate into the GB at a typical temperature range of 573-1173 K.