Фазовая диаграмма системы Hf-Nb

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Hf-Nb (Hafnium-Niobium) H. Okamoto The equilibrium phases of the Hf-Nb system are: (1) the liquid, L; (2) the bcc W-type continuous solid solution, (bHf,Nb); and (3) the cph Mg-type terminal solid solution, (aHf). Earlier reviews on this system are available in [71Age] and [81Spe]. The liquidus and solidus boundaries reported by [64Sie], [64Tay], [64Tyl], and [69Rud] are in considerable disagreement. The minimum melting point was found to be at 25 at.% Nb and 2100 C by [64Sie], at 32.5 to 55 at.% Nb and 2080 с 25 C by [64Tyl], and at 42 at.% Nb and 2065 с 20 C by [69Rud]; [64Tay] found no minimum melting point. The result of [64Tay] is not accepted here because their observed melting point of Nb was very high (2585 C) in comparison with the assessed value. The solidus data of [69Rud], which show a similar trend to that of [64Tyl], are accepted as the basis of the present thermodynamic modeling. The data of [ 64Sie] were not used as the basis because the reported melting point of Hf ( 2190 C) is substantially lower than the presently accepted value, presumably due to inferior purity of Hf. Considerably different forms of liquidus were speculated by [64Tyl] and [69Rud] based on very little data. For thermodynamic self-consistency, the assessed liquidus and solidus are drawn according to the present thermodynamic model. The (bHf,Nb)/[(aHf) + (bHf,Nb)] boundary was reported variously by [64Sie], [ 64Tay], [64Tyl], [69Ale], and [71Car]. The solid miscibility gap proposed by [ 64Tyl] does not exist according to cautious XRD or TEM investigations by [ 69Ale] and [71Car]. [69Ale] proposed two (bHf,Nb)/[(aHf) + (bHf,Nb)] boundaries, corresponding to two different amounts of oxygen and nitrogen contained in the alloys as impurity. The boundary observed in the purer alloys, however, includes an abrupt change of slope, which is thermodynamic-ally improbable [91Oka]. In addition, the angle between the initial slopes of ( bHf,Nb)/[(bHf,Nb) + (aHf)] and [(bHf,Nb) + (aHf)]/(aHf) appears to be very much wider than that expected from the van't Hoff relationship. Thus, the influence of the impurity level on the form of the phase diagram has not been reliably established. Therefore, the assessed solvus boundary also follows the present thermodynamic model, which was derived to represent X-ray analysis data of [71Car]. The solid solubility of Nb in (aHf) is 4 at.% [64Sie, 64Tyl] or ~2 at.% [64Tay] at 1000 C. Because of the lack of low-temperature solubility data, these investigators assumed that the solubility limit is nearly temperature independent at low temperatures. The present thermodynamic model indicates possible retrograde solubility. The crystal structure data are from [King 1]; lattice parameter data are available in [51Duw], [58Beg], [61Dwi], [64Tay], [70Jac], and [71Car]. 51Duw: P. Duwez, J. Appl. Phys., 22(9), 1174-1175 (1951). 58Beg: R.T. Begley, A.F. Contract 33(616)5754, Rep. No. A-2525Z (1958). 61Dwi: A.E. Dwight, Columbium Metallurgy, Interscience Publishers, New York, 383-406 (1961); quoted in [Shunk]. 64Sie: R.E. Siemend, H.R. Babitzke, and H. Kato, U.S. Bur. Mines, Rep. Invest. 6492, 11 p (1964); quoted in [Shunk]. 64Tay: A. Taylor and N.J. Doyle, J. Less-Common Met., 7(1), 37-53 (1964). 64Tyl: M.A. Tylkina, I.A. Tysganova, and E.M. Savitskii, Z. Neorg. Khim., 9, 1650-1652 (1964) in Russian; TR: Russ. J. Inorg. Chem., 9, 893-895 (1964). 69Ale: G.K. Alekseyenko and L.N. Aleksandrova, Izv. Akad. Nauk SSSR, Met., (3), 170-172 (1969) in Russian; TR: Russ. Metall., (3), 131-132 (1969). 69Rud: E. Rudy, Technical Report AFML-TR-65-2, Air Force Materials Laboratory, Wright-Patterson Air Force Base, OH, 95-96 (1969). 70Jac: W.A. Jackson, A.J. Perkins, and R.F. Hehemann, Metall. Trans., 1(7), 2014-2016 (1970). 71Age: N.V. Ageev, Phase Diagrams of Metallic Systems, Akad. Nauk SSSR, Moscow, 67 (1971). 71Car: R.W. Carpenter, C.T. Liu, and P.G. Mardon, Metall. Trans., 2(1), 125- 131 (1971). 81Spe: P.J. Spencer, O. von Goldbeck, R. Ferro, R. Marazza, K. Girgis, and O. Kubaschewski, Hafnium: Physico-Chemical Properties of Its Compounds and Alloys, K.I. Komarek, Ed., Atomic Energy Review Special Issue No. 8, International Atomic Energy Agency, Vienna (1981). 91Oka: H. Okamoto and T.B. Massalski, J. Phase Equilibria, 12(2), 148-168 ( 1991). 1