Effect of Hydrogen on The Electronic Structure of a Grain Boundary In Iron

Abstract

ABSTRACTLMTO-ASA calculations were performed on a 26-atom supercell model of a Σ3(111) grain boundary (GB) in bcc Fe. The supercell emulated two GB's with 11 (111)planes of Fe atoms between the GB planes. One of the GB's was clean, with a structural vacancy at the GB core in the center of a trigonal prism of Fe atoms, while on the other GB this site was occupied by a H atom. The interplanar spacings of the supercell were relaxed using a modified embedded atom method. As in the case of P and S in a similar GB environment in Fe there is only a weak interaction between H and nearest Fe atoms. Almost all the Fe d-states are nonbonding. A very weak covalent bond exists between H and Fe due to s-d hybridization, the hybrid bonding part located far below the Fermi energy. This bond is mostly of σ-type, connecting H with the Fe atoms in the GB plane; the δ-component of this bond across the GB is weaker. A weak electrostatic interaction attracts Fe-atoms across the clean GB, but results in repulsion if a H atom is present. The magnetic contribution to intergranular cohesion is decreased when H is present due the suppression of the magnetic moments of the nearest Fe atoms both in the GB plane and directly across the GB.