Фазовая диаграмма системы C-Nb

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C-Nb (Carbon-Niobium) J.F. Smith, O.N. Carlson, and R.R. de Avillez The assessed phase diagram for the Nb-C system is a weighted composite of available information. Published phase diagrams concur with regard to the existence of two phases with stoichiometries near Nb2C and NbC, both with ranges of homogeneity. There is lack of definitive evidence concerning the polymorphism of Nb2C and NbC, and the details of the ordering of C in the NbC phase field at temperatures below ~1000 C. In view of the high temperatures associated with the invariant reactions in the assessed diagram, agreement among various investigators for the major features of the diagram is generally good, although some pronounced disparities may be noted. In systems of this type, contamination by other interstitials such as N, O, and H must be suspect. The Nb-rich solvus in the assessed diagram is drawn on the basis of resistometric measurements between 1500 and 2200 C by [74Hor]. Internal friction studies of low C alloys quenched and aged at 310 C for different times place the solubility limit at that temperature at less than 0.077 at.% C [57Pow]. There is general agreement that the Nb-rich eutectic temperature lies between 2330 and 2350 C. Nb2C exists as a disordered hexagonal phase, gNb2C, at high temperatures and transforms to an ordered hexagonal form, bNb2C, at about 2500 C. There is general agreement that the homogeneity range of the Nb2C phase field extends from ~28 to ~33 at.% C [66Geb, 67Rud, 67Sto] at the eutectic temperature of 2335 C. The Nb-rich boundary recedes sharply with decreasing temperature to form a narrow one-phase field at 2000 C and below; quantitative definition of the width of this narrow phase field at lower temperatures is in dispute. Several investigators have reported a peritectic decomposition temperature for Nb2C varying between 3035 and 3265 C with compositions varying from 33.3 to 34.5 at.% C. There is wide agreement with the values of [67Sto] of 3080 с 20 C and 33.3 at.% C, and these values have been incorporated into the assessed diagram. [59Bra] postulated the existence of a x phase intermediate between Nb2C and NbC1<310>x analogous to the x phase in the Ta-C [67Bri] and V-C [70Yvo] systems. [68Rud] concluded that this phase was metastable in the Nb-C system; however, [67Sto] and [70Yvo] have considered the phase to be a stable feature of the binary diagram. The most convincing evidence for the existence of the phase comes from [71Cra]. The possible existence of this phase as a stable member of the Nb-C equilibrium diagram is shown in the assessed diagram by dashed lines that show a peritectic reaction between Nb2C and NbC1<310>x to form the x phase, in analogy with the corresponding peritectic reaction in the V-C system. The Nb-rich boundary in the assessed diagram is from [59Bra], and the C-rich boundary between 3100 and 3500 C is based on the incipient melting data of [ 68Rud]. At lower temperatures, the C atoms tend to developing range order. The ordering temperature depends upon composition and has a maximum reported from 1025 C [72Ven] to 1082 C [84Rem]. The ideal stoichiometry for the ordered array is generally accepted as Nb6C5. Without evidence to the contrary and by analogy with the corresponding phase in the V-C system, the order-disorder transformation has been assumed to be first order. In the assessed diagram, the phase fields are drawn accordingly, with the general contour between the ordered and disordered regions being that of [72Bil]. 54Bra: G. Brauer, H. Renner, and J. Wernet, Z. Anorg. Allg. Chem., 277, 249- 257 (1954) in German. 57Pow: R. W. Powers and M.V. Doyle, J. Met., Trans. Sec. 9, 1285-1288 (1957). 59Bra: G. Brauer and R. Lesser, Z. Metallkd., 50, 8-10 (1959). 60Sto: E.K. Storms and N.H. Krikorian, J. Phys. Chem., 64 1471-1477 (1960). 64Ter: N. Terao, Jpn. J. Appl. Phys., 3, 104-111 (1964). 66Geb1: E. Gebhardt, E. Fromm, and U. Roy, Z. Metallkd., 57, 682-687 (1966) in German. 68Geb2: T.H. Geballe, B.T. Matthias, J.P. Reveika, A.M. Clogston, V.B. Compton, J.P. Maita, and H.J. Williams, Physics, 2, 293-310 (1966); quoted in [71Tot]. 67Bri: W.F. Brizes and J.M. Tobin, J. Am. Ceram. Soc., 50, 115-116 (1967). 67Rud: E. Rudy and C.E. Brukl, J. Am. Ceram. Soc., 50, 265-268 (1967). 67Sto: E.K. Storms, The Refractory Carbides, Academic Press, New York, 61-81 ( 1967). 68Rud: E. Rudy, St. Windisch, and C.E. Brukl, Planseeber. Pulvermetall., 16, 3- 33 (1968). 70Yvo: K. Yvon and E. Parth‚, Acta Crystallogr. B, 26, 153-163 (1970). 71Cra: R.L. Crane and F. Ostermann, Met. Trans., 2, 3487-3488 (1971). 71Tot: L.E. Toth, Transition Metal Carbides and Nitrides, Ch. 4, Academic Press, New York (1971). 72Bil: J. Billingham, P.S. Bell, and M.H. Lewis, Acta Crystallogr. A, 28, 602- 606 (1972). 72Ven: J.D. Venables and M.H. Meyerhoff, NBS Spec. Publ. No. 364, 583-590 ( 1972). 74Hor: G. H”rz, K. Lindemaier, and R.L. Klaiss, J. Less-Common Met., 35, 97- 105 (1974). 79Kha: B.V. Khaenko, Izv. Akad. Nauk SSSR, Neorg. Mater., 15, 1952-1960 (1979) in Russian; TR: Inorg. Mater., 15, 1535-1543 (1979). 84Rem: A.A. Rempel, S.Z. Nazarova, and A.I. Gusev, Phys. Status Solidi (a), 86, K11-K14 (1984). Submitted to the APD Program. Complete evaluation contains 8 figures, 7 tables, and 112 references. Special Points of the Nb-C System