Фазовая диаграмма системы H-Ta

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H-Ta (Hydrogen-Tantalum) A. San-Martin and F.D. Manchester In H-in-metal (metal hydride) systems, the equilibrium pressure of the H surrounding the metal is always a significant thermodynamic variable, in contrast to most situations involving metallic alloys. Thus, sections of the P- X-T surface in a T-X plane and a P-X plane are always necessary. In this evaluation, P is the pressure (in Pascals), T is the temperature (plotted both in K and C), and X is the H concentration (expressed either as at.% H or H/Ta, the atomic ratio). The assessed T-X phase diagram for the Ta-H system at atmospheric pressure consists of the following phases: a random interstitial solid solution of H in bcc Ta, which is designated by (aTa) up to H concentration X = 0.5 (33.33 at.% H) and by (a›Ta) at X >> 0.5; two phases, one of which, b, is fully ordered and the other, e, is partially ordered; the d phase, an ordered, monoclinic, TaHx alloy, in which the minimum composition value is 0.667 (40 at.% H) at 45. 3 C; the z phase, a fully ordered, monoclinic TaHx alloy formed first at - 38 C in the d phase; and the h phase, a pseudo-orthorhombic phase, which exists from -165 C to near absolute zero, where X ~0.64 (39 at.% H). Two other ordered phases, g and q of unknown structure, have also been included in the assessed diagram. The q phase exists between -78 and -215 C, while the g phase extends from -65 C to near absolute zero. The dashed line in the assessed diagram is the isobar at atmospheric pressure; it demarcates the limit of existence for this system at that hydrogen pressure. Both the b and e phases have the same orthorhombic structure, but have different hydrogen arrangements in the nominal stoichiometric TaH0.5 structure. b exists from temperatures close to absolute zero up to 46.3 C. The minimum and maximum values in the composition range of this phase, 0.468 < X < 0.632 ( 31.9 to 38.9 at.% H), are located at temperatures of 10 and 45.3 C, respectively. e forms at 10 C and exists up to 61 C, where it transforms to a. The minimum (0.395 or 28.3 at.% H) and maximum (0.580 or 36.7 at.% H) composition values for this phase occur at 10 and 46.3 C, respectively. The d phase exists from -79 to about 50 C, where it decomposes into the disordered (a›Ta) phase. z exists down to temperatures close to absolute zero, where X ~0.72 (41.9 at.% H). Below about 60 C, the a phase decomposes into a, b, or a and e, or a and d depending on the H concentration. At 61 C, TaH0.5 (33.33 at.% H) transforms into e [77Sch]. At 10 C, the a phase extends up to X = 0.131 (11.6 at.% H), and the two-phase field of (a + b) exists from 0.131 to 0.468 (31.9 at.% H). Above 10 C, a coexists with e up to the critical temperature. The a/(a + b) boundary in the assessed diagram is drawn following the revised version of the T-X diagram reported by [82Kob, 83Sch]. In the assessed diagram, the a›/(a› + d) and (a› + d)/d boundaries follow the limits of the two-phase (a› + d) region reported by [82Kob, 83Sch]. Equilibrium measurements established that the solubility of H decreases with increasing temperature, and that conformity with Sieverts' law (X • P{1/2} ) is approached at temperatures of around 500 C or higher for X < 0.2 [62Mal, 69Vel, 77Zuz]. According to [76Hos], the maximum solubility of H in (Ta) is X = 0.79 at 20 C. At hydrogen pressures higher than 0.1 MPa, the H solubility increases considerably. Thus, [84Sza] obtained compositions of X = 0.86 (46.2 at.% H) after charging Ta sheets at 120 C with hydrogen gas under pressures of 16 kbar (1.6 GPa) and found that a new phase was formed below temperatures of -80 C. 62Mal: M.W. Mallett and B.G. Koehl, J. Electrochem. Soc., 109, 611-616 (1962). 69Vel: E. Veleckis and R.K. Edwards, J. Phys. Chem., 73(3) (1969). 76Hos: S.M. Hosseini, Z. Metallkd., 67, 123-127 (1976) in German. 77Sch: T. Schober and A. Carl, Scr. Metall., 11, 397-400 (1977). 77Zuz: E. Zuzek, Hydrogen in Metals, 2nd Int. Congr. Paris, Vol. 3, Pergamon Press, Oxford, England, U.K., 11 pp (1977) in French. 82Kob: U. Kobler and J.M. Welter, J. Less-Common Met., 84, 225-235 (1982). 83Sch: T. Schober, Electronic Structure and Properties of Hydrogen in Metals, P. Jena and C.B. Satterthwaite, Ed., 1-10 (1983). 84Sza: A.W. Szafranski, M. Tkacz, and S. Majchrzak, J. Less-Common Met., 101, 523-527 (1984). Submitted to the APD Program. Complete evaluation contains 7 figures, 6 tables, and 90 references. Special Points of the Ta-H System