Фазовая диаграмма системы Cr-Ti

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Cr-Ti (Chromium-Titanium) J.L. Murray The equilibrium solid phases of the Ti-Cr system are (1) the cph (aTi) solid solution, in which Cr has a small solubility; (2) the bcc (bTi,Cr) solid solution-in a narrow temperature range below the congruent melting point, Ti and Cr are completely miscible in the bcc phase; and (3) aTiCr2, bTiCr2, and gTiCr2, Laves phases with the C15, C14, and C36 structures, respectively. aTiCr2 has the lowest Cr content, and bTiCr2 and gTiCr2 are low- and high- temperature forms of the hexagonal Laves phase. The present reassessment of the diagram incorporates recent work on the Laves phases and a new thermodynamic analysis of the system. There have been discrepancies in the reported experimental data about the number and homogeneity ranges of equilibrium intermetallic phases. The cubic C15 Laves phase (aTiCr2) was established by [52Cuf] and [52Duw]. The higher temperature, higher Cr content bTiCr2 phase was established by [53Lev]. gTiCr2 (with the hexagonal MgNi2 structure) may not be an equilibrium phase. It was reported by [62Sve] and [70Sve] to be the equilibrium form above about 1270 C. [71Min], however, attributed thermal arrests found at this temperature to the formation of a ternary Ti-Cr-O phase rather than a third binary Laves phase. However, they did not identify the structure of the high-temperature phase with any known ternary phase. A transition between bTiCr2 and gTiCr2 is indicated on the assessed diagram by a dotted line. Determination of equilibria among the Laves phases is hindered by sluggishness of the reactions and metastable persistence of phases. Moreover, these structures can be described as different stacking sequences of the same basic cell, small amounts of one phase may be interpreted as stacking fault variants of another, and a number of polytypes have been observed. The cph phase (a›Ti) can form martensitically during the quench from the (bTi) field. (bTi) cannot be fully retained during quenching unless the Cr content exceeds about 6 at.% [52Duw]. Start temperatures for the martensitic b <259> a› transformation and for the reverse transformation have been determined. w appears in metastable (bTi) alloys either during cooling or during low- temperature aging. As-quenched w forms in alloys with compositions between approximately 3 and 9 at.% Cr. [69Hic] determined the compositions of w and ( bTi) after aging for various times and at temperatures between 300 and 400 C. The composition of w approaches 6.5 at.% Cr, and the composition of (bTi) approaches approximately 20 at.% Cr. 52Cuf: F.B. Cuff, N.J. Grant, and C.F. Floe, Trans. AIME, 194, 848-853 (1952). 52Duw: P. Duwez and J.L. Taylor, Trans. ASM, 44, 495-513 (1952). 53Lev: B.W. Levinger, Trans. AIME, 197, 196 (1953). 62Sve: V.N. Svechnikov, Yu.A. Kocherzhinskii, and V.I. Latysheva, Problems in the Physics of Metals and Metallurgy, (16), 132-135 (1962) in Russian. 69Hic: B.S. Hickman, Trans. AIME, 245, 1329-1336 (1969). 70Sve: V.N. Svechnikov, M.Yu. Teslyuk, Yu.A. Kockerzhinsky, V.V. Petkov, and E. V. Dabizha, Dop. Akad. Nauk Ukr. RSR A, 32(9), 837-841 (1970) in Russian. 71Min: S.A. Minayeva, P.B. Budberg, and A.L. Grave, Izv. Akad. Nauk SSSR, Met., (4), 205-209 (1971) in Russian; TR: Russ. Metall., (4), 144-147 (1971). Published in Phase Diagrams of Binary Titanium Alloys, 1987. Complete evaluation contains 8 figures, 8 tables, and 64 references. Special Points of the Ti-Cr System