Effect of Mo addition on hydrogen segregation at α-Fe grain boundaries: A first-principles investigation of the mechanism by which Mo addition improves hydrogen embrittlement resistance in high-strength steels

•Reason why Mo suppresses hydrogen embrittlement of martensitic steels was studied.•Thermodynamic model was developed to evaluate the effect of Mo on GB H concentration.•H segregation energies at α-Fe GB with Mo were obtained and inserted into the model.•Segregation energies at Σ3(111) GB were obtained by first-principles calculations.•We found that a small amount of Mo markedly decreased the GB H concentration in α-Fe.