Hisakazu Ezaki

Electronic structure and hydriding property of titanium compounds with CsCl-type structure

The electronic structures of titanium compounds, TiFe, TiCo and TiNi with CsCl-type structure are investigated by the DV-Xα molecular orbital method. From the results of the calculation, it is found that hydrogen interacts more strongly with constituent elements, X (=Fe, Co, Ni), rather than Ti atoms. The bond strength between Ti and X atoms changes in the sequence, Fe>Co>Ni. This is the same order as the capacity of the hydrogen absorption in these TiX compounds. The characteristics of the hydrogen absorption in these compounds are discussed in view of the nature of the chemical bond between atoms.

Electronic and atomic-size effects on the omega phase formation in transition-metal based B.C.C. alloys

Abstract Two criteria for the omega phase formation in transition-metal based b.c.c. alloys have been proposed based on the electronic structure calculations and experiments on the omega phase. One is the electronic effect and the other is the atomic-size effect. The former effect is enhanced when the alloying transition element has more valence electrons than the mother metal. The latter effect is remarkable when the alloying transition element has a smaller atomic size than the mother metal. The importance of such a size effect has been confirmed from a series of experiments on the omega phase formation in ZrTi alloys. Following these criteria the compositional range for the omega phase to exist in various Ti and Zr alloys can be understood consistently. Further, the second-nearest-neighbour atomic interactions were shown to be large in metals and alloys where the omega phase is formed.

Prediction of the Occurence of the [sgrave] phase in Fe[sbnd]Cr[sbnd]Ni alloys

Abstract A method for evaluating solid solubilities of alloys proposed by Morinaga, Yukawa, Ezaki and Adachi (1985 a, b) has been applied to Fe-based f.c.c. alloys. The method predicts the occurrence of the [sgrave] phase and the other phases in commercial Fe[sbnd]Cr[sbnd]Ni-based alloys. Using this method, the systematic design and development of Fe-based alloys should be possible by controlling their composition so that they are free from the precipitation of undesired phases.

Correlation between electronic structure and phase stability of metal hydrides

Abstract Local electronic structures around hydrogen in a small octahedral cluster are simulated by the DV-Xα molecular orbital method. It is shown that the ionicity of hydrogen in the cluster is a good indicator of the stability of hydrides, as long as the charge transfer from metal to hydrogen atoms is responsible for the hydride stability. For example, the heat of formation of pure metal dihydrides, ΔH, correlates well with the calculated ionicity of hydrogen. A similar correlation is observed in binary vanadium alloys and ternary V–Ti–Ni alloys.

Observation of omega and h.c.p. phases in a quenched Cr-30at.% Ni alloy

Abstract The existence of two new phases has been established in a quenched Cr-30 at. % Ni alloy. By the electron diffraction method, they are identified to be the omega phase and the h.c.p. phase. These phase transformation behaviours are very similar to those observed in Ti and Zr alloys.

Solid solubilities of molybdenum and tungsten in F.C.C. transition metal alloys

Abstract A method for evaluating solid solubilities of alloys proposed by Morinaga, Yukawa, Ezaki and Adachi has been applied to the transition-metal based f.c.c. alloys containing molybdenum and tungsten. It has been shown that the method is useful in fitting the phase boundaries in ternary phase diagrams. This method has been shown to be applicable to the quantitative estimation of the phase boundaries relevant not only to the intermetallic compounds (for example, the μ phase and the Laves phase) but also to the terminal solid-solutions such as the α-tungsten phase.