First-principles calculations of the effects of Mn, Cr, and Ni on hydrogen diffusion in BCC, FCC, and HCP Fe

Synopsis: The effects of Mn, Cr, and Ni addition on the hydrogen diffusion behavior in BCC, FCC, and HCP Fe was investigated by means of first-principles calculations. Diffusion coefficients were estimated quantitatively from the migration energy calculated by the nudged elastic band method and the vibrational energy at every stable and metastable site. The addition of Mn, Cr, and Ni to a BCC lattice has a blocking effect on hydrogen diffusion and decreases the diffusion coefficient of hydrogen. Crystal orbital Hamilton population (COHP) analysis revealed that the weakened bonding between the added element and hydrogen is the origin of the blocking effect. On the other hand, the addition of Mn, Cr, and Ni to FCC and HCP lattices resulted in the formation of hydrogen trap sites. In the FCC case, the diffusion coefficients of Fe ₃₁ MnH, Fe ₃₁ CrH, and Fe ₃₂ H, showed similar values, while that of Fe ₃₁ NiH was lower. In the HCP case, the diffusion coefficients of the three additional elements showed a decreasing trend. Based on the results of the COHP analysis, we conclude that the octahedral interstitial sites around the additional elements become trap sites in FCC and HCP Fe due to the strengthened bonding between Fe and H.