First-Principles Study on Segregation Energy and Embrittling Potency of Hydrogen in NiΣ5(012) Tilt Grain Boundary

Abstract

The electronic structures of Σ5(012) symmetrical tilt grain boundary (GB) and (012) free surface (FS) systems for nickel including hydrogen are calculated by the full-potential linearized augmented plane wave method with the generalized gradient approximation. The GB/FS energy and the binding energy of hydrogen at an inner bulk site and GB/FS site are calculated. The calculated binding energy of hydrogen on Ni(111) surface is in good agreement with experiment. The difference of the binding energies between the Σ5(012) GB/FS site and the inner bulk site is obtained as the GB/FS segregation energy, and the difference between GB and FS segregation energies as the embrittling potency energy. The segregation position of hydrogen atom is determined by the force minimization. We find that hydrogen atom prefers Σ5(012) GB to inner bulk energetically by about 0.3 eV/H, while it prefers (012) FS to the GB by about 0.3–0.4 eV/H. The open space at GB may be occupied by an additional interstitial Ni atom since it increases the GB energy by only about 10%. In this case, the GB segregation energy of hydrogen reduces, which makes the segregation difficult. The calculated zero-point energies of hydrogen at inner bulk, GB, and FS sites are within 0.12–0.16 eV/H. The difference of the zero-point energies is within 0.04 eV/H, which is small comparing with the segregation energy and embrittling potency.