Фазовая диаграмма системы H-V

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H-V (Hydrogen-Vanadium) J.F. Smith and D.T. Peterson The assessed V-H phase diagram is based primarily on the work of [77Sch], [ 78Sch], [79Sch], and [82Pes], with review of the work of [73Asa], [76Asa], [ 79Asa], [81Asa], and [81Hir]. At elevated temperatures, the terminal solid solution, a phase or (V), extends to quite high hydrogen concentrations. Below 325 K, however, the terminal solubility of H in (V) is limited. The terminal solubilities in the low-temperature range are adequately described by [67Wes] as: -log10 C = 2.53 - 641/T, where C is the H concentration in at.% and T is in K. It has been established that the terminal solubilities of H in (V) are influenced by isotopic purity, the presence and concentration of other interstitial impurities, the temperature range that is considered, and the presence of crystallographic imperfections. The data of [79Fuj] at temperatures above 325 K and compositions below 33.3 at.% H have been used to shape the solvus in the assessed diagram. The b1 phase has a wide range of homogeneity extending on both sides of stoichiometric V2H. In addition, b1 (V2H) undergoes a second or higher order transformation near 445K to b2, which in turn undergoes a first-order transformation to the a terminal solution near 470 K. Below 400 K, a broad two-phase field exists between the a and b1 phases, with the V-rich boundary of the b1 phase being in the vicinity of 30 at.% H. With increasing temperature, the b1 phase disorders to form the b2 phase, with maximum heat absorption occurring near the temperature of 445 K at nearly ideal stoichiometry of V2H. The close relationship between the b1 and b2 structures is emphasized by the fact that the a <259> b2 <259> b1 sequence of transitions can be suppressed by the application of external stress to eliminate the b2 intermediate phase and yield a direct a <259> b1 transition. A low-temperature transformation of the b1 phase near V2H stoichiometry has been reported [77Sch, 78Sch], with the transformed structure being designated h phase, but subsequent studies have shown that this transformation is associated with oxygen contamination [79Sch] and is not characteristic of the pure binary V-H system. However, at the more hydrogen-rich stoichiometry of V3H2, a transformation has been confirmed near 223 K. The low-temperature structure resulting from this transformation (V3H2) is the d phase. This b1 <259> d transformation is a first-order transformation. Hydrogen additions to V in the a solid solution range lower the superconducting transition temperature of V, and none of the V-H intermediate phases have superconducting transition temperatures above 1.5 K. The g phase has a narrow range of homogeneity near a stoichiometry of VH2. All phases in the V-H system, except VH2, have crystal structures with V sublattices that are essentially the same as the bcc lattice of elemental V, but the H sublattices locate H atoms on what would be interstitial sites in the elemental structure with occupation distributions varying from phase to phase. 67Wes: D.G. Westlake, Trans. AIME, 239, 1341-1344 (1967). 73Asa: H. Asano and M. Hirabayashi, Phys. Status Solidi (a), 15, 267-279 (1973) . 76Asa: H. Asano, Y. Abe, and M. Hirabayashi, Acta Metall., 24, 95-99 (1976). 77Sch: T. Schober and A. Carl, Phys. Status Solidi (a), 43, 443-449 (1977). 78Sch: T. Schober and H. Wenzl, Topics in Applied Physics, Vol. 29, Hydrogen in Metals II, G. Alefeld and J. Volkl, Ed., Springer-Verlag, Berlin, 11-71 ( 1978). 79Asa: H. Asano and M. Hirabayashi, Z. Phys. Chem., 114, 1-19 (1979). 79Fuj: K. Fujita, Y.C. Huang, and M. Tada, Nippon Kinzoku Gakkaishi, 43, 601- 610 (1979). 79Sch: T. Schober and W. Pesch, Z. Phys. Chem., 114, 21-28 (1979). 81Asa: H. Asano and M. Hirabayashi, in Metal Hydrides, G. Bambakidis, Ed., Plenum Press, New York, 81-103 (1981). 81Hir: M. Hirabayashi and H. Asano, in Metal Hydrides, G. Bambakidis, Ed., Plenum Press, New York, 53-80 (1981). 82Pes: W. Pesch, T. Schober, and W. Wenzl, Scr. Metall., 16, 307-312 (1982). Published in Phase Diagrams of Binary Vanadium Alloys, 1989, and Bull. Alloy Phase Diagrams, 3(1), Jun 1982. Complete evaluation contains 1 figure, 4 tables, and 48 references. 1