Фазовая диаграмма системы Sn-Ti

К оглавлению: Другие диаграммы (Others phase diargams)

Sn-Ti (Tin-Titanium) J.L. Murray For reactions in the range 25 to 50 at.% Sn, the only experimental data are those of [57Pie], who made incipient melting studies using optical pyrometry. The reaction L = (bTi) + Ti3Sn was located at 1607 с 20 C. [57Pie] located the reaction (Ti3Sn + L = Ti2Sn) at 1552 C based on metallographic work. The reactions Ti2Sn + L = Ti5Sn3 and L = Ti5Sn3 + bTi6Sn5 were investigated by metallographic examination of as-cast and slowly cooled specimens. Using X-ray diffraction, [57Pie] found that alloys containing more than 45 at.% Sn were mixtures of bTi6Sn5 and (bSn). [62Ere] determined the Sn-rich liquidus and (bSn) eutectic temperature by differential thermal analysis. They also tentatively identified an exothermic reaction at 790 C with a polymorphic transformation in Ti6Sn5. That such a transformation exists was independently discovered in the X-ray diffraction study of [64Vuc]. The assignment of the hexagonal structure to the high-temperature form and the orthorhombic structure to the low-temperature form is based first on the lower symmetry of the hexagonal form and second on the predominance of bTi6Sn5 and the observation of aTi6Sn5 only in relatively Sn-rich alloys [57Pie, 64 Vuc]. (bTi) transforms congruently to (aTi) at 6.7 at.% Sn and 842 C [55Mcq]. Neither [54Fin] nor [57Pie] showed the congruent transformation, but both examined too coarse a temperature interval to have been able to distinguish the minimum. [55Mcq] and [60Gla] believed the single-phase (aTi) solution to be considerably more extensive than was reported elsewhere, attributing reports of two-phase microstructures to a failure to achieve equilibrium. [55Mcq] used extremely long annealing times, but did not work the alloys to hasten equilibration. Similarly, [60Gla] used levitation melting to make unusually homogeneous alloys. Neither [55Mcq] nor [60Gla] found evidence for a two-phase (aTi) + Ti3Sn field. [55Mcq] speculated that the ordering was continuous, and both [55Mcq] and [60Gla] showed the (aTi) and Ti3Sn phase fields joining continuously. It can be shown on the basis of symmetry, however, that the cph D019 ordering cannot occur by a higher order transition. One must therefore conclude that equilibrium was actually more closely approached by [54Fin] and [57Pie]. It is proposed that under the conditions studied by [55Mcq] and [60Gla] incoherent precipitation of Ti3Sn in (aTi) was suppressed and that they measured the metastable extension of the (aTi)/(bTi) boundary. The start temperature of the martensitic (bTi) <259> (aTi) transformation was measured by [60Sat]. In the range 2 to 9 at.% Sn, it is 740 to 760 C, apparently independent of composition. 54Fin: W.L. Finlay, R.I. Jaffee, R.W. Parcel, and R.C. Durstein, J. Met., 6, 25-29 (1954). 55Mcq: M.K. McQuillan, J. Inst. Met., 84, 307-312 (1955-1956). 57Pie: P. Pietrokowsky and E.P. Frink, Trans. ASM, 49, 339-358 (1957). 60Gla: V.V. Glazova and N.N. Kurnakov, Dokl. Akad. Nauk SSSR, 134, 1087-1090 ( 1960) in Russian; TR: Proc. Acad. Sci. USSR, Chem. Sect., 134, 1129-1133 (1961) . 60Sat: T. Sato, S. Hukai, and Y. Huang, J. Austr. Inst. Met., 5(2), 149-153 ( 1960). 62Ere: V.N. Eremenko and T.Ya. Velikanova, Zh. Neorg. Khim., 7, 1750-1752 ( 1962) in Russian; TR: Russ. J. Inorg. Chem., 7(7), 902-904 (1962). 64Vuc: J.H. Vucht, H.A. Brunning, H.C. Donkersloot, and A.H. Mesquita, Phillips Res. Repts., 19, 407-421 (1964). Published in Phase Diagrams of Binary Titanium Alloys, 1987. Complete evaluation contains 4 figures, 6 tables, and 22 references.