Фазовая диаграмма системы C-V

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C-V (Carbon-Vanadium) O.N. Carlson, A.H. Ghaneya, and J.F. Smith The assessed phase diagram for the V-C system is based primarily on the work of [68Rud], with modifications based on the work of [62Sto], [68Ade], and [ 73Sto]. The liquidus is taken from [68Ade], who equilibrated V or VC with C at different temperatures and chemically analyzed the liquid phase in equilibrium with the solid at each temperature. At elevated temperatures, only two intermediate phases (V2C and VC) exist; both exhibit wide ranges of homogeneity. Decreasing temperature results in a peritectoidal reaction between V2C and VC near 1320 C to form the V4C3-x phase, which in contrast to V2C and VC appears to be nearly invariant in stoichiometry. This invariance exists even though the phase is C deficient, with the crystal structure indicating an ideal stoichiometry of V4C3, but with an actual stoichiometry near V3C2. On this basis, the composition of the phase is designated V4C3-x. At still lower temperatures, transformations occur within the phase fields of both V2C and VC to produce more complicated phase relationships. The phase relationships for the region between 27 and 34 at.% C must be considered tentative. The relationships as drawn in the assessed diagram agree with the majority of the available experimental data, but some uncertainties exist. Crystallographic data indicate that the V-rich portion of the high- temperature form of V2C is identifiably different, with space group symmetry distinguishable from the C-rich high-temperature form. On this basis, the C- rich portion of the high-temperature region is designated bV2C, and the V-rich portion is designated b›V2C. The b and b› crystal structures are hexagonal, and their V sublattices are identical. The crystallographic considerations, in combination with the lack of any experimental evidence for a two-phase separation between the b and b› forms, are the bases for suggesting that the b›V2C = bV2C transformation is higher order, as indicated by the dash-dot line. The temperature-composition contour is speculative. 62Sto: E.K. Storms and R.J. McNeal, J. Phys. Chem., 66, 1401-1408 (1962). 68Ade: L.M. Adelsberg and L.H. Caddof, J. Am. Ceram. Soc., 51(4), 213-220 ( 1968). 68Rud: E. Rudy, S. Windisch, and C.E. Brukl, Planseeber. Pulvermetall., 16, 3- 33 (1968). 73Sto: E. Storms, The Refractory Carbides, Chapt. 4, Academic Press, New York ( 1973). Published in Phase Diagrams of Binary Vanadium Alloys, 1989, and Bull. Alloy Phase Diagrams, 6(2), Apr 1985. Complete evaluation contains 2 figures, 3 tables, and 77 references. 1