Фазовая диаграмма системы Co-Ti

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Co-Ti (Cobalt-Titanium) J.L. Murray The equilibrium solid phases of the Ti-Co system are (1) the cph solid solutions, (aTi) and (eCo), where (aTi) is stable below 882 C and (eCo) is stable below approximately 422 C; (2) the bcc solid solution, (bTi), stable in pure Ti above 882 C, with a maximum solubility of Co in (bTi) of 14.5 at.% at 1020 C; (3) the fcc solid solution, (aCo), stable in pure Co above 422 C, with a maximum solubility of Ti in (aCo) of 14.1 at.% at 1190 C; (4) Ti2Co, an ordered fcc structure with a homogeneity range of not more than about 0.3 at.% near stoichiometry; (5) TiCo, with the CsCl structure, which melts congruently at 1325 C and has a homogeneity range of 49 с 1 to 55 с 0.5 at.% Co at 1200 C; (6) cubic (C15) and hexagonal (C36) Laves phases of approximate stoichiometry TiCo2, designated in the present evaluation as TiCo2(c) and TiCo2(h), respectively, where TiCo2(h) is slightly richer in Co, because the homogeneity ranges of TiCo2(c) and TiCo2(h) are approximately 66.5 to 67.0 at.% Co and 68.75 to 72 at.% Co, respectively; and (7) TiCo3, with the ordered fcc AuCu3 structure and a maximum homogeneity range of 75.7 to 80.7 at.% Co. The assessed phase diagram contains two "deep eutectics," in which amorphous alloys can be formed, and the higher order ferromagnetic transition has a pronounced effect on the solvus. However, some features of the diagram have received only cursory examination or are the subject of controversy. Notably, the liquidus has not been determined from 0 to 20 at.% Co, and there are discrepancies between 20 and 80 at.% Co. There are conflicting reports about which of the TiCo2 Laves phases are stable phases, and it has been suggested ( based on observation of polytypism) that this part of the diagram may be more complex than had been imagined. Conflicting reports suggest that only a hexagonal Laves phase TiCo2(h) [50Duw] or only a cubic Laves phase TiCo2(c) [83Uhr] is stable. By X-ray diffraction, [ 50Duw] found only TiCo2(h) in alloys containing 63 to 68 at.% Co. [83Uhr] found only TiCo2(c) formed from the liquid at 71 at.% Co. [59Fou] found only TiCo2(h) in an as-cast 66.1 at.% Co sample. Using electron microscopy, [72All] found that Ti-Co exhibits polytypism: the stable hexagonal form can be described as a 4H stacking and 6H and 12-layer stackings were also found. These contradictory observations are at present unresolvable. The minimum Co content for which (bTi) can be retained metastably during quenching ranges from 4.5 [59Yak] to 10 at.% Co [58Swa]. The composition reported depends on the investigator's judgment of whether (bTi) was fully retained, as well as on the quenching rate. [77Nik] found that whereas cph Co martensite is formed from 100 to 98 at.% Co, a faulted structure appeared between 98 and 96 at.% Co, and that from 96 to 93 at.% Co the martensite has a 126-layer rhombohedral structure. [80Ino] produced mixed crystalline and amorphous phases near 23 and 88 at.% Co by rapid solidification. Between 87 and 89 at.% Co, only the amorphous phase was present without any trace of the crystalline phase. 50Duw: P. Duwez and J.L. Taylor, Trans. AIME, 188, 1173-1176 (1950). 58Swa: P.R. Swann and J.G. Parr, Trans. AIME, 212, 276-279 (1958). 59Fou: R.W. Fountain and W.D. Forgeng, Trans. AIME, 215, 998-1008 (1959). 59Yak: F.W. Yakymyshyn, G.R. Prudy, R. Taggart, and J.G. Parr, Trans. ASM, 53, 283-294 (1959). 72All: C.W. Allen, P. Delavignette, and Amelinckx, Phys. Status Solidi (a), 9, 237-246 (1972). 77Nik: B.I. Nikolin, Dokl. Akad. Nauk SSSR, 223, 587-590 (1977) in Russian; TR: Sov. Phys. Dokl., 22(4), 226-228 (1977). 80Ino: A. Inoue, K. Kobayashi, C. Suryanarayana, and T. Masumoto, Scr. Metall., 14, 119-123 (1980). 83Uhr: B. Uhrenius and K. Forsen, Z. Metallkd., 74(9), 610-615 (1983). Published in Phase Diagrams of Binary Titanium Alloys, 1987. Complete evaluation contains 4 figures, 8 tables, and 60 references. Special Points of the Ti-Co System