Segregation of hydrogen and vacancies at the Σ5(210)[001] symmetric tilt grain boundary in Ni and influence on cohesion

The segregation of hydrogen and vacancies at the Σ5(210)[001] symmetric tilt grain boundary (GB) was studied by atomic scale simulations in Ni. First, the hydrogen segregation energies and hydrogen–hydrogen pair interaction energies were calculated on every interstitial site of the GB. The vacancy–hydrogen clusters' formation energies were also determined on the most favorable site. All these calculations were done using the density functional theory. Second, based on these elementary energies, a free energy functional was built to determine the concentration of segregated hydrogen and of vacancy-hydrogen clusters, as a function of the bulk hydrogen concentration and the temperature. It was found that two configurations exits in typical conditions where embrittlement is observed experimentally: H segregation only, with up to 3 hydrogen atom per structural unit or 50% occupancy by VH₅ clusters (1 cluster every two structural unit). The cohesive stress and ideal work of fracture were evaluated by fracturing the GB with different degrees of hydrogen and vacancy segregation. H segregation alone (no vacancy) decreased the work of fracture by 25%. A significantly larger decrease of cohesion was obtained when considering vacancy-hydrogen clusters. A maximum drop of the cohesive stress, of a magnitude of 40%, was obtained when every structural unit was hosting a VH₄ cluster. Finally, these data were transformed into cohesive stress models. They were used to evaluate the degree of localization of the shear displacement at the crack tip. The conclusion is that, even if cohesion is very significantly decreased, shear localization is still effective, meaning that dislocation emission should occur at the expense of crack propagation. The comparison with other grain boundaries in the literature shows that the GB studied is almost an ideal sink and therefore is very favorable for the formation of equilibrium VH_n. It represents more an upper bound of the effect. Therefore, extra ingredients should be considered to explain the embrittlement observed experimentally. (paper)