Hydrogen-enhanced vacancy embrittlement of grain boundaries in iron

Abstract

We have investigated atomistic and electronic mechanisms of hydrogen embrittlement at grain boundaries of iron in the presence and absence of vacancies by using first-principles calculations. Considering interactions between hydrogen, vacancies, and $\ensuremath{\Sigma}$3 grain boundaries in $\ensuremath{\alpha}$-Fe, we have searched for the most deleterious defect states by evaluating their influence on tensile strength under static tensile strain. The calculated results show that hydrogen and vacancies prefer to accumulate as defect complexes near grain boundaries, thereby decreasing the tensile strength of grain boundaries. Theoretical stress-strain curves show that a strength lowering at grain boundaries by the defect complexes is found to be much worse than the effect of each factor. Because of the low mobility of vacancies, this mechanism can account for the delayed brittle fractures induced by hydrogen in steel materials.