Density Functional Theory Study of the Role of Molybdenum in Weakening Hydrogen Effect on bcc Iron: A Comparison with Carbon and Nitrogen Effects

In body-centered cubic (bcc) iron (Fe), highly diffusible solute hydrogen (H) atoms are easily trapped at lattice defects, i.e., grain boundaries (GBs) and vacancies, where concentrations reach thermal equilibrium. The presence of H atoms can increase vacancy concentration under plastic deformation. Although molybdenum (Mo) additions mitigate hydrogen embrittlement (reducing elongation loss by H), the underlying mechanisms remain unclear. To address this gap, herein, the interactions of H atoms with additive atoms (carbon (C), nitrogen (N), and Mo) and vacancies in the bcc Fe lattice and at the Σ3 and Σ5 symmetrical tilt GBs were analyzed via density functional theory calculations. In the bcc Fe lattice, C and N atoms exhibited a stronger repulsion toward H atom than Mo atom. However, the higher solubility of Mo atoms is expected to reduce the overall H diffusion coefficient by reducing the H diffusion paths. C and N atoms promote vacancy formation, while Mo atom exhibit negligible interactions with a vacancy and do not reduce the vacancy formation energy. C and N atoms at the GB plane showed strong H repulsion, whereas Mo atom showed weak H repulsion. Thus, C and N atoms more effectively suppressed H segregation at the GBs. The effects of these additive atoms (C, N, and Mo) on the vacancy-trapping energy at the GBs were generally negligible. Finally, vacancy trapping at the GBs with an additive atom recovered the H-trapping energy to the value for the GB with only a vacancy.