Hydrogen accommodation in α-iron and nickel

Highlights: • The effect of H on the vacancy content in Ni and α-Fe is studied by ab intio method. • Vacancy formation energies were predicted and compared well with previous results. • In α-Fe the vacancy formation energy is lowered by the presence of single H atoms. • In contrast to α-Fe, only clustered H solutes reduce vacancy formation energy in Ni. • The increase in vacancy concentration has been compared to experimental observations. -- Abstract: Ab initio calculations have been used to study the effects of hydrogen on vacancy concentrations in α-Fe and Ni. The presence of H interstitials aided vacancy formation in both metals but via two different mechanisms. In α-Fe, trapping of H by a vacancy is favourable. However binding of further hydrogen atoms was not predicted to proceed. The thermal equilibrium concentration of H interstitials in comparison to vacancies in α-Fe is many orders of magnitude higher over a wide temperature range. Excessive H interstitials in solid solution facilitate vacancy formation, lowering the required energy by 0.79 eV (down to 1.41 eV). In Ni, a single H interstitial is not expected to have an impact on the vacancy population, increasing the vacancy formation energy by 0.32 eV. Two bound H interstitials however are predicted to decrease the vacancy formation energy by 0.52 eV with expected arrangement along a 〈1 1 1〉 direction around the vacancy. The calculations show reasonable agreement with experimental data when comparing crystal lattice contractions of the pure metals and predicted melting temperature of the Me–H alloys