Фазовая диаграмма системы Au-Ti

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Au-Ti (Gold-Titanium) J.L. Murray The equilibrium solid phases of the Ti-Au system are (1) the bcc (bTi) solid solution, with a maximum Au solubility of about 15 at.%; (2) the cph (aTi) solid solution, with a maximum Au solubility of about 1.7 at.%; (3) the fcc ( Au) solid solution, with a maximum Ti solubility of 12 at.%; (4) the essentially stoichiometric compound Ti3Au, with the A15 structure and a congruent melting point; (5) the TiAu compounds, with a maximum homogeneity range of 38 to 52 at.% Au; (6) the essentially stoichiometric compound TiAu2, with the MoSi2 structure and a congruent melting point; and (7) the compound TiAu4, with a homogeneity range of 79 to 82 at.% Au and the Ni4Mo structure, which melts by a peritectic reaction with (Au). The main outlines of the Ti-Au system are based on the results of two studies [ 56Pie, 62Pie]. Thermodynamic calculations of the diagram agree very well with the experimental data, within the uncertainties expected for a Ti-based system for which there are no experimental thermodynamic data, i.e., within с35 C for three-phase equilibria. The assessed (Au) boundaries were drawn from the thermodynamic calculation because they agreed with the assessment within 2 C. The (bTi) liquidus and solidus were based on calculations as well as experimental data, because the experimental observations do not conform with thermodynamic constraints. The remainder of the diagram was drawn from the experimental data. The Ti-rich region of the diagram has been studied extensively, but the data of [78Pli] are preferred, because their results are least likely to be influenced by interstitial impurities. The (bTi) transus is difficult to determine metallographically due to the massive transformation. (bTi) that has transformed to (aTi) during quenching differs very little in appearance from equilibrium (aTi). The (bTi) structure cannot be retained metastably during quenching for alloys containing 0 to 6 at.% Au [54Mcq]. For cooling rates less than about 103 C/s, the mechanism of the transformation was shown to be of the massive type [78Pli]. There are at least two, perhaps three, equilibrium polymorphic forms of TiAu. The forms are designated gTiAu, bTiAu, and aTiAu, proceeding from high to low temperature and high to low Ti content. There are discrepancies concerning the structures of the allotropic forms. However, it is generally agreed that the room-temperature phase on the Ti-rich side of the phase field has the B19 structure. [70Don] splat quenched alloys between 40 and 52.6 at.% Au to prepare metastable single-phase TiAu alloys and to examine the martensitic transformation. The low-temperature phase was found to have the B19 structure for all compositions studied, and it transformed martensitically from the high- temperature CsCl structure. TiAu4 has been identified as TiAu3 [43Wal] and as TiAu6 [52Rau]. The latter identification was based on interpretation of a microstructure as single-phase at that composition; the former identification is clearly the result of large interstitial impurity content. The observation of a two-phase TiAu2 + TiAu4 assemblage at 75 at.% Au [62Pie, 69Sin] appears to preclude the occurrence of TiAu3 as an equilibrium binary phase. Thermal analysis (cooling) data were reported for Au-rich alloys by [52Rau], and incipient melting data were reported by [62Pie]. For alloys containing more than 67 at.% Au, reported liquidus and invariant temperatures are in agreement. 43Wal: H.J. Wallbaum, Naturwissenschaften, 31, 91-92 (1943) in German. 52Rau: E. Raub, P. Walter, and M. Engel, Z. Metallkd., 43, 112-118 (1952) in German. 54Mcq: M.K. McQuillan, J. Inst. Met., 82, 511-512 (1954). 56Pie: P. Pietrokowsky, E.P. Frink, and P. Duwez, Trans. AIME, 206, 930-935 ( 1956). 62Pie: P. Pietrokowsky, J. Inst. Met., 90, 434-438 (1962). 69Sin: A.K. Sinha, Trans. AIME, 245, 237-240 (1969). 70Don: H.C. Donkersloot and J.H.N. Van Vucht, J. Less-Common Met., 20, 83-91 ( 1970). 78Pli: M.R. Plichta, H.I. Aaronson, and J.H. Perepezko, Acta Metall., 26, 1293- 1305 (1978). Published in Phase Diagrams of Binary Gold Alloys, 1987, Phase Diagrams of Binary Titanium Alloys, 1987, and Bull. Alloy Phase Diagrams, 4(3), Nov 1983. Complete evaluation contains 3 figures, 6 tables, and 16 references. Special Points of the Ti-Au System