We present an atomistic study of the behavior and interactions of hydrogen and vacancies in body centered cubic (bcc) iron, using both ab initio and classical molecular dynamics methods. Hydrogen causes damage to materials through embrittlement, hardening, and swelling; we investigate the role of vacancies in these processes. Hydrogen, which normally diffuses with a very small barrier, is strongly trapped at monovacancies and vacancy clusters, resulting in changes to its electronic structure. Following saturation of the surface of a vacancy cluster, the formation of H${}_2$ molecules is possible, at variance with the situation in the bulk. High local concentrations of hydrogen increase the likelihood of vacancy formation and stabilize vacancy clusters. Small hydrogen-vacancy clusters generally tend to diffuse by dissociation, but the trivacancy is shown to be capable of dragging hydrogen while migrating. We describe the structure of clusters of vacancies with varying hydrogen concentrations, finding that compact or spherical bubbles are generally lower energy than planar or linear configurations. Comparison with other bcc metals and with experiment is provided. For systems involving light elements such as hydrogen, corrections for zero-point energy are very important; we include these in our calculations and discuss their importance for different properties.

• Received 18 February 2013

DOI:https://doi.org/10.1103/PhysRevB.87.174103