Effect of Hydrogen on The Electronic Structure of a Grain Boundary In Iron
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EFFECT OF HYDROGEN ON THE ELECTRONIC STRUCTURE OFA GRAIN BOUNDARY INIRON GENRICH L.KRASKO*, RALPH J. HARRISON* and G. B.OLSON" *U. S. Army Materials Technology Laboratory, Watertown, MA 02172-0001, **Department of Materials Science and Engineering, Northwestern University, Evanston, IL, 60208 ABSTRACT LMTO-ASA calculations were performed on a 26-atom supercell model of a Z3 (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 a-type, connecting H with the Fe atoms in the GB plane; the 8-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. INTRODUCTION The reduced cohesion of grain boundaries (GB's) is often the controlling factor limiting the ductility of high strength metallic alloys [1). This is particularly so when there is an environmental interaction , as in the case of intergranular hydrogen stress corrosion cracking [2]. Typically, hydrogen drastically degrades mechanical properties of metal alloys, making them unreliable and poses a significant technological problem. Intergranular embrittlement in metals is usually associated with prior segregation of impurities toward the GB's[3-6]. Impurities present in bulk concentrations of 10-3 -10-4 atomic percent can result in a dramatic decrease in plasticity. The solid solubility of hydrogen in bcc Fe below room temperature is only 1 ppm (10-4 atomic percent )[7]. Previous works attempted to explain GB decohesion due to hydrogen on the electronatom level by using small atomic clusters as a simple model of GB's[8]. Decohesive effects of hydrogen were also successfully investigated using semiempirical calculations and modelling[9] based on the Embedded Atom Method (EAM)[10]. It was shown that a single hydrogen atom in a unit cell at a GB of fcc Ni can weaken the metallic bonds across the GB, lowering the fracture stress by 15%. Progress in developing new efficient first-principles methods in the recent decade has allowed more detailed treatment of a GB with an impurity. Calculations making use of both two-dimensional[11,12] and supercell[13] models have provided a deeper insight into the mechanisms of im
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