Atomistic study of hydrogen embrittlement of grain boundaries in nickel: II. Decohesion

Atomistic simulations of bicrystal samples containing a grain boundary are used to examine the effect of hydrogen atoms on the nucleation of intergranular cracks in Ni. Specifically, the theoretical strength is obtained by rigid separation of the two crystals above and below the GB and the yield strength (point of dislocation emission) is obtained by standard tension testing normal to the GB. These strengths are computed in pure Ni and Ni with H segregated to the grain boundaries under conditions typical of H embrittlement in Ni, and in artificially highly-H-saturated states. In all GBs studied here, the theoretical strength sigma(y) is not significantly reduced by the presence of the hydrogen atoms. Similarly, with the exception of the Ni Sigma 27(115) < 110 > boundary, the yield strength sigma(y) is not significantly altered by the presence of segregated H atoms. In all cases, the theoretical strengths are similar to 25 GPa and the yield strengths are similar to 10 GPa, so that (i) the theoretical strength is always well above the yield strength, with or without H, and (ii) both strengths are far above the bulk plastic flow stress, sigma(B)(y) of Ni and Ni alloys. Significant reductions in fracture energy (25%-45%) are only achieved for some of the artificially high-H-segregation cases and then only when all the H around the GB is allow to diffuse locally to the fracture surface, which corresponds to unlikely out-of-equilibrium segregation plus local kinetics. Complementing recent work showing that H does not change the ability of GB cracks to emit dislocations and blunt, the present work indicates that equilibrium segregation of hydrogen atoms to GBs has little effect on lowering the GB strength and energy, and so does not significantly facilitate nucleation of intergranular cracks.