Atomistic mechanism of hydrogen trapping in bcc Fe–Y solid solution: A first principles study

For many years trapping of hydrogen by solutes has been proposed as one of the mechanisms to mitigate hydrogen embrittlement in steels. In the 1980s, Myers et al. reported strong hydrogen trapping around yttrium centers in α-Fe with trapping energies of around −0.7 to −0.9 eV. To date, this is one of the strongest reported solute traps for hydrogen. Here, we use Fe–Y alloy as a model system to study mechanistic aspects of hydrogen trapping from first principles. We examine hydrogen binding at various defect complexes in Fe–Y. We show that binding at simple point defects such as Y atom, Y-monovacancy, and Y-divacancy complexes is weaker than what is required to explain experimental results, but binding at more complex defects involving multiple Y atoms and multiple vacancies is well in the range of the experimental observations. This leads us to infer that these complex defects are the strong hydrogen traps underlying experimental results. The effect of solutes in trapping hydrogen, however, is found to be very local, limited largely to within second nearest neighbor shells.