Solubility and Anisotropic Migration Behaviors of Helium in bcc Iron Under Strain

The different solution and migration behaviors of tetrahedral and octahedral interstitial helium in bcc iron have been investigated by using first principles calculations. We showed that the tetrahedral site has less charge transfer and less redistribution of the density of states but stronger bonding and a lower solution energy. This is due to the coupling between the symmetrical facts of the two interstitial atoms and the 3 d orbitals of Fe atoms. The solution energies of both sites are not significantly influenced by applied normal strains of 2% and 4%. In contrast, the migration barriers have the reverse trends for different migration directions under strain, which can be explained by an anisotropic elastic energy change and charge transfer. The lower migration energy along certain directions under strain can facilitate the segregation of helium and the formation of helium bubbles.