Stability and clusterization of hydrogen-vacancy complexes inα−Fe:Anab initiostudy

Abstract

By means of ab initio supercell calculations based on the density-functional theory, we have investigated stability of hydrogen-monovacancy complexes $({\mathrm{VH}}_{n})$ and their binding preferences in $\ensuremath{\alpha}\ensuremath{-}\mathrm{Fe}.$ We have found that ${\mathrm{VH}}_{2}$ is the major complex at ambient condition of hydrogen pressure, which corrects the conventional model implying the ${\mathrm{VH}}_{6}$ predominance. It is also demonstrated that monovacancies are not hindered from binding by the hydrogen trapping in the case of ${\mathrm{VH}}_{2}$ predominance. Besides, the presence of hydrogen is found to facilitate formations of line-shaped and tabular vacancy clusters without the improbable accumulation. These anisotropic clusters can be closely associated with the fracture planes observed in experiments on hydrogen embrittlement in Fe-rich structural materials such as steel. The present results should suggest implications of hydrogen-enhanced vacancy activities to microscopic mechanism of hydrogen embrittlement in those materials.