Hydrogen inα-iron: Stress and diffusion

Abstract

First-principles density-functional theory has been used to investigate equilibrium geometries, total energies, and diffusion barriers for H as an interstitial impurity absorbed in $\ensuremath{\alpha}\text{-Fe}$. Internal strains/stresses upon hydrogen absorption are a crucial factor to understand preferred absorption sites and diffusion. For high concentrations, H absorbs near the octahedral site favoring a large tetragonal distortion of the bcc lattice. For low concentration, H absorbs near the tetrahedral site minimizing the elastic energy stored on nearby cells. Diffusion paths depend on the concentration regime too; hydrogen diffuses about ten times faster in the distorted body-centered-tetragonal (bct) lattice. External stresses of several GPa modify barriers by $\ensuremath{\approx}10%$, and diffusion rates by $\ensuremath{\approx}30%$.