First-Principles Study of Hydrogen Impact on the Formation and Migration of Helium Interstitial Defects in hcp Titanium

We present a first-principles study of the effects of hydrogen on helium behavior in hcp titanium. The calculation indicates that the dissolved H atoms in hcp Ti change the formation energy of the interstitial He atom, but they do not change the energetically favorable occupying site of the He atom, i.e., the tetrahedral site is more favorable than the octahedral site. The impacts of H on the formation of interstitial He defects are directly related to the atomic environment around H atoms and their positions relative to interstitial He atoms as well. For He diffusion, a tetrahedral interstitial He atom can more easily migrate along the indirect tetrahedron-octahedron-tetrahedron path than the direct path of tetrahedron-tetrahedron. When a H atom exists in the first neighbor octahedral site from the He, the activation energy for He atom diffusion is 0.46 eV, which is higher than that of the He atom diffusion in perfect crystal, 0.41 eV. Increasing the number of H atoms to two, He diffusion needs much higher activation energy. This suggests that the H atoms around interstitial He may impede the migration of interstitial He atom in hcp Ti.