First-principles study of strain effects on hydrogen trapping at vacancy clusters in BCC iron

Vacancies, as typical hydrogen traps, play a critically important but poorly understood role in the hydrogen-induced failure of body-centered cubic (BCC) iron. This study employs first-principles calculations to systematically investigate hydrogen-vacancy interactions. It is found that in the absence of hydrogen, uniaxial tensile strain and hydrostatic tensile strain lead to a trend in vacancy formation energy that first increases and then decreases, whereas hydrostatic compression promotes vacancy formation; shear strain has a marginal effect. Crucially, hydrogen introduction significantly reduces formation energies across all strain levels. This corroborates the Hydrogen-Enhanced Strain-Induced Vacancy (HESIV) mechanism. Without strain, mono-vacancy and 2NN di-vacancy spontaneously trap up to five and ten hydrogen atoms, respectively, achieving peak incremental binding energies at the second and third trapped atoms. Uniaxial tension decreases binding energies, hindering hydrogen trapping. Conversely, hydrostatic compression enhances trapping, increasing capacities to six and eleven atoms, respectively, while hydrostatic tension suppresses it, reducing the di-vacancy capacity to below ten. While pristine vacancies and V 1 H 1 -based clusters predominantly aggregate spherically, V 1 H 2 -based clusters primarily form {100} and {110} tabular configurations. This preferential accumulation on the typical crystallographic planes fundamentally exacerbates hydrogen-induced quasi-cleavage fracture and failure in BCC iron.