Electronic structure calculations were carried out for bcc iron grain boundaries (GB) with or without hydrogen, using the self-consistent Discrete Variational embedded cluster method within the first-principles local density formalism. Bonding characteristics were mainly investigated. Simple rigid body translations perpendicular to the GB plane were used for estimation of relaxed GB geometry. Analysis of bond order summation over the GB shows considerable volume expansion normal to the GB plane of a dense 23(111) twist/tilt GB and some compression for the rather open 23(110) twist configuration. These results are discussed in the context of atomistic simulations which suggest that higher energy GB's generally have larger volume expansion normal to the GB plane. H in a 23(111) GB reduces Fe-Fe bonding strength by —3% within a 0.25 nm spherical volume around the H site, associated with reduction of the 4s and 4p occupancy of the nearest neighbor Fe. Since these orbitals contribute mainly to metallic bonding, the action of H atoms as an embrittlement inducer can be understood.

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Electronic structure calculations were carried out for bcc iron (Fe) clusters with or without hydrogen (H), and also involving a vacancy, using the self-consistent Discrete Variational method (DV-Xa) within the local density functional formalism. Bonding characteristics investigated show the following: (i) Interstitial H notably decreases interatomic Fe-Fe bond strengths, but acts over a small distance (within 0.3 nm).(ii) In the perfect Fe lattice field, interstitial H feels a repulsive force at any site. As a result of lattice relaxation, volume expansion may be expected. (iii) H in combination with a vacancy prefers a position shifted from the octahedral site toward the vacancy. This is fairly consistent with an experimental result.

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In this study, the microstructure, tensile strength, elongation, and reduction of area of near-¢ Ti alloys (Ti-17) were investigated after being subjected to solution and aging treatments. Ti-17 was forged at temperatures between 700 and 850°C followed by air cooling. Then, the forged Ti-17 was subjected to solution treatment at 800°C for 4 h followed by water quenching and aging treatment at 620°C for 8 h followed by air cooling. Tensile tests were performed at room temperature, 450°C, and 600°C. The change in microstructure at different forging temperatures was exhibited by only the volume fraction and morphology of the grain boundary (GB) ¡ phase. That is, a granular GB ¡ phase was formed in the samples forged at 700 and 750°C. Moreover, a film-like GB ¡ phase was formed in the samples forged at 800 and 850°C. The tensile strength was the same for all the tested samples, indicating that the microstructure has little effect on the tensile strength. The elongation and reduction of area increased with decreasing volume fraction in the GB ¡ phase. It is considered that the film-like morphology slightly improves ductility.