Dissolution and diffusion of hydrogen in a molybdenum grain boundary: A first-principles investigation

We have performed a first-principles investigation with a zero point energy correction on dissolution and diffusion of hydrogen (H) in a Σ5 (310)/[001] molybdenum (Mo) symmetrical tilt grain boundary (STGB). H prefers to stay at the interstitial site in the vacant space of the Mo GB with a negative solution energy (-0.42 eV) and segregation energy (-1.16 eV), which are decreased in the presence of vacancy in the GB. Furthermore, the H solution energy at the vacancy in the Mo GB is lower than that in the Mo bulk, suggesting a larger trapping capability for H of the vacancy in the Mo GB. The dissolving stability of H in the Mo GB can be explained by the low charge density that the GB provides for H. Kinetically, H prefers to easily diffuse within the vacant space along the Mo GB with a small diffusion barrier (0.04 eV), and to migrate to the GB from the bulk. These results provide a systematically exploration and analysis of H behaviors in the Mo GB at an atomic scale, which suggest that the Mo GB can serve as a trapping center for H, similar to the vacancy. In particular, a detailed comparison of H dissolution and diffusion behaviors in a Mo GB with those in a tungsten GB is performed. This can be helpful in understanding the experimentally observed H bubble formation in the Mo GB from the microscopic view.