Hydrogen behaviors in molybdenum and tungsten and a generic vacancy trapping mechanism for H bubble formation

We investigate the trapping behaviors of hydrogen (H) in molybdenum (Mo) and tungsten (W) using a first-principles method with a zero point energy correction as well as the molecular dynamics (MD) method. The H trapping is found to generally satisfy the “optimal charge density” rule at the vacancy, and a monovacancy is shown to simultaneously trap 14 H with a H2 molecule formed at the vacancy center in both Mo and W. On the other hand, the MD simulation shows the temperature decreases the number of trapped H at the vacancy. We further propose a generic vacancy trapping mechanism for H bubble formation in metals. The H atoms will first saturate the internal surface of the vacancy (or other vacancy-type defects) to form a “screening layer”, which can screen the interaction between the further trapped H and the surrounding metal atoms. This leads to the formation of H2 molecule at the vacancy center, which can be considered as the preliminary stage of H bubble nucleation.