Hydrogen in $\alpha$ iron: modification of elastic properties and solubility in strain fields

The effect of interstitial hydrogen on the elastic properties of $\alpha$-iron is investigated using \textit{ab initio} density functional theory. We find that while the overall strength properties are reduced by H, the effects are mainly due to the resulting local volume expansion. We use these concentration-dependent elastic moduli to model the effects of homogeneous strain fields on H solubility and make extrapolations to H-solubility in the spatially-varying strain fields of realistic dislocations. We find that H is strongly trapped by dislocations of both the edge and screw variety, leading to a remarkable increase in the local H concentration near the dislocation cores. The strain dependence of the solution energy leading to accumulation of H near dislocation cores, as well as the reduction of elastic moduli which our calculations predict, is consistent with numerous experimental studies which indicate that the trapping of H near dislocations plays a significant role in the embrittlement of iron and steels.