Фазовая диаграмма системы Os-V

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Os-V (Osmium-Vanadium) J.F. Smith Two rather different phase diagrams have been proposed for the Os-V system by [ 66Rau] and [79Sus]. Many of the differences between the two proposed phase diagrams can be rationalized. The diagram of [79Sus] is based on more extensive data and is preferred as the assessed diagram. The two diagrams both show a cph Os-rich phase representing the terminal solid solution, a Cr3Si-type intermediate phase, a CsCl-type structure, and an elemental bcc structure corresponding to the V-rich terminal solution at high V compositions. Thus, basic details of the two diagrams are in rough accord, but phase boundaries agree only with regard to the Os-rich solvus, which in the investigation of [66Rau] was determined from the lattice parameters of quenched samples. This boundary increases from ~28 at.% V at 900 C to ~38 at.% V at 1570 C. There is also qualitative agreement between the two diagrams with regard to the liquidus-solidus contours, with both showing a eutectic reaction near the central portion of the system and a melting maximum in the V- rich portion of the system. There are significant differences between the two diagrams with regard to the temperatures and compositions associated with the invariant reactions and with the remaining phase boundaries. [66Rau] determined liquidus temperatures by thermal analysis, whereas [79Sus] determined solidus temperatures by direct observation of melting. Because data from thermal analysis are more likely to suffer error from supercooling, the results of [79Sus] are preferred. The two investigations interpreted the melting maximum in quite different ways, with [66Rau] taking it to be the congruent melting point of a CsCl-type phase and with [79Sus] taking it to be an azeotropic melting of the V-rich solid solution. The difference in interpretations hinges on whether the formation of the CsCl-type structure is first order. [66Rau] assumed the CsCl-type structure to represent a separate and distinct phase with a median stoichiometry of V3Os2 and with the requirement of a two-phase field to separate it from the V-rich terminal solution. The diagram of [66Rau] was drawn accordingly and used dashed lines to show a two-phase field whose existence was unsupported by experimental evidence. [79Sus] made a careful search for such a two-phase field and was unable to detect it. Their interpretation of the CsCl-type structure was that it was associated with an order-disorder reaction whose formation was thermodynamically second rather than first order. In this sense, the Os-V system is analogous to the Ru-V system in the V-rich region with the ordering becoming detectable by the appearance of superlattice lines below some threshold V composition, which in Os-V is near 78 at.% V. Though [79Sus] admitted the possibility of a two-phase field between the bcc and CsCl-type phase above 1500 C, the probability seems small. The preponderant weight of their data supports the view that the terminal solution grades into the CsCl-type structure by an ordering reaction without an intervening two-phase field. In the assessed diagram, the temperature- composition contour separating their observation of the presence or absence of superlattice lines is shown by a dashed line. In the investigations of [66Rau] and [79Sus], the CsCl-type structure was reported not to extend to equiatomic stoichiometry; rather, the Os-rich limit is indicated to be near 55 at.% V. [66Rau] and [79Sus] agree that a Cr3Si-type phase occurs nonstoichoimetrically near equiatomic composition; however, there is a significant difference between the two diagrams with regard to the temperature and composition ranges of stability of this phase. The reason may be associated with the fact that the Cr3Si-type phase tends to metastability and can easily be retained at lower temperatures with only moderate rates of quenching [79Sus]. Within the limits of 50 to 55 at.% V, the superconducting transition temperature was reported to decrease from 5.7 to 3 K [79Sus]. Earlier measurements of the superconducting transition temperature of this phase [ 69Bla, 70Spi] fall within this range. In the V-rich region, additions of Os to V cause the superconducting transition temperature of elemental V to fall below 1 K at 90 at.% V and to reappear below 60 at.% V in the ordered CsCl- type region to reach 1.7 K at the Os-rich boundary of 55 at.% V. Low to vanishing superconductivity between 90 and 60 at.% V is corroborated by the observation of [63Mat] that a 71 at.% V alloy was a normal conductor to the lowest temperature of measurement of 0.37 K. The decrease in the superconducting transition temperature of V with Os additions followed by a subsequent rise with further Os enrichment is qualitatively similar to the behavior of Ru additions to V in the Ru-V system [79Sus]. 58Kna: A.G. Knapton, J. Inst. Met., 87, 28-32 (1958-59). 63Mat: B.T. Matthias, T.H. Geballe, and V.B. Compton, Rev. Mod. Phys., 35, 1- 22 (1963). 66Rau: E. Raub and E. R”schel, Z. Metallkd., 57, 470-474 (1966). 69Bla: R.D. Blaugher, R.A. Hein, J.E. Cox, and R.M. Waterstrat, J. Low Temp. Phys., 1, 539-547 (1969). 70Spi: P. Spitzli, R. FlЃkiger, F. Heiniger, A. Junod, J. Muller, and J.L. Staudenmann, J. Phys. Chem. Solids, 31, 1531-1537 (1970) in French. 79Sus: C. Susz, R. FlЃkiger, J.L. Jorda, and J. MЃller, J. Less-Common Met., 63, 45-52 (1979). Published in Phase Diagrams of Binary Vanadium Alloys, 1989. Complete evaluation contains 1 figure, 2 tables, and 9 references. Special Points of the Os-V System