Фазовая диаграмма системы Ta-Ti

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Ta-Ti (Tantalum-Titanium) J.L. Murray The Ti-Ta phase diagram is of the simple isomorphous type, but data are lacking on the liquidus, and data on the solid phase boundaries are both mutually contradictory and inconsistent with thermodynamic properties of pure Ti. Thermodynamic calculations therefore played a major role in the construction of the assessed diagram. [52Sum] measured the temperature of the solid-liquid interface and suggested that these data represented points about midway between the solidus and liquidus. On the Ti-rich side, the data of [53May] were obtained by optical observation of melting; other data were obtained from microscopic examination of quenched alloys. [65Bru], [69Rud], and [67Bud] obtained solidus data by optical methods. Discrepancies in the melting temperatures are nearly 500 C for Ta contents exceeding 40 at.%. Alloy contamination at high temperature is thus a predominating effect. Because of the large discrepancies, the assessed solidus is based on the pure metal melting points and the approximate linearity of the solidus. The (aTi) solvus [52Sum, 53May, 67Bud] and (bTi) transus [53May, 67Bud, 69Nik] were examined by metallographic and X-ray techniques, supplemented by resistivity [53May] and physical and mechanical property [67Bud] measurements. There is agreement that the maximum solubility of Ta in (aTi) is about 3 с 0.2 at.% at 600 C. The absence of compounds strongly suggests that excess Gibbs energies are positive and that the (bTi) transus lies everywhere above a metastable (bTi) miscibility gap. The appearance of two bcc phases in tempered (bTi,Ta) supports the existence of a metastable miscibility gap with approximate tie line compositions of 20 and 70 at.% Ta at 400 C [72Byw2]. Gibbs energy functions were constructed that reproduce the (aTi) solvus and the approximate metastable bcc miscibility gap, and the calculation was used to draw the assessed diagram. The martensite transformation of (bTi) was reported to be partly suppressed in alloys containing more than 9 at.% Ta [52Sum] or 15 at.% Ta [53May] and fully suppressed in alloys containing more than 14 at.% Ta [52Sum], 15 at.% Ta [ 58Bag], or 21 at.% Ta [53May]. [72Byw1] showed that prior heat treatment can influence the product structures and also that samples which showed no optical evidence of martensite may nevertheless be fully transformed. In alloys containing up to 7 at.% Ta, the martensite has the cph structure; in alloys containing more than 7 at.% Ta, the martensite has an orthorhombic structure. The start temperature of the martensite transformation was measured by [53Duw] for alloys containing up to 5 at.% Ta; it varies approximately linearly with composition and reaches 750 C at 5 at.% Ta. In Ti-Ta alloys, w phase is not found in quenched specimens of any composition, but only forms during tempering of the bcc phase near 400 C [58Bag, 72Byw2]. 52Sum: D.J. Summers-Smith, J. Inst. Met., 81, 73 (1952). 53Duw: P. Duwez, Trans. ASM, 45, 934-940 (1952). 53May: D.J. Maykuth, H.R. Ogden, and R.I. Jaffee, Trans. AIME, 197, 231-237 ( 1953). 58Bag: Yu.A. Bagariatskii, G.I. Nosova, and T.V. Tagunova, Dokl. Akad. Nauk SSSR, 122, 593-596 (1958) in Russian; TR: Sov. Phys. Dokl., 3, 1014-1018 (1958) . 65Bru: C.E. Brukl et al., unpublished work (1965); cited in [69Rud]. 67Bud: P.B. Budberg and K.K. Shakova, Izv. Akad. Nauk SSSR Neorg. Mat., 3(4), 656-660 (1967) in Russian; TR: Russ. J. Inorg. Mater., 3(4), 577-580 (1967). 69Nik: P.N. Nikitin and V.S. Mikheyev, Fiz. Met. Metalloved., 28(6), 1127-1129 (1969) in Russian; TR: Phys. Met. Metallogr., 28(6), 190-192 (1969). 69Rud: E. Rudy, USAF Tech. Rep. AFML-TR-65-2, Part V (1969). 72Byw1: K.A. Bywater and J.W. Christian, Philos. Mag., 25, 1249-1273 (1972). 72Byw2: K.A. Bywater and J.W. Christian, Philos. Mag., 25, 1275-1289 (1972). Published in Phase Diagrams of Binary Titanium Alloys, 1987. Complete evaluation contains 5 figures, 3 tables, and 17 references.