Фазовая диаграмма системы Au-Ta

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Au-Ta (Gold-Tantalum) H. Okamoto and T.B. Massalski The possible equilibrium phases of the Au-Ta system are the liquid, L; the fcc terminal solid solution, (Au); the bcc terminal solid solution, (Ta); the bct intermetallic phase, Au2Ta3; the s phase; and the Cr3Si-type cubic intermetallic phase, AuTa5. The homogeneity ranges of these phases are shown in the assessed phase diagram and should be regarded as tentative. The possible occurrence of additional phases, although with less certainty, is as follows: a low-temperature phase (below ~700 C) with an unknown composition range between (Au) and Au2Ta3; and a polymorphic, or disordered, form of Au2Ta3 above ~1600 C. The assessed phase diagram is based mainly on the detailed work of [73Cam]. However, the (Au) phase boundaries are drawn as a compromise between the results of [61Rau] and [73Cam]. Gold in Au-Ta alloys behaves as a highly volatile element as a consequence of the large difference in the melting points of Au and Ta (1064.43 C and 3020 C, respectively). Thus, the composition of Au-Ta specimens is difficult to maintain during heat treatment, especially in the Au-rich range. This seems to be the cause of the differences reported for composition ranges of the three intermediate phases existing in this system [61Rau, 73Cam]. Thus, the "AuTa" compound having no appreciable solubility range and occurring at compositions in excess of 50 at.% Ta, according to [61Rau], is likely to be the same as the Au2Ta3 phase. The transition temperature at ~1600 C observed by [73Cam] is tentatively shown as relating to a polymorphic (or an order-disorder) transition in the Au2Ta3 phase. Likewise, the nominal AuTa3 intermediate phase found by [61Rau] may be identical to the present AuTa5 phase. 61Rau: E. Raub, H. Beeskow, and D. Menzel, Z. Metallkd., 52, 189-193 (1961) in German. 73Cam: M. Camacho, thesis, Univ. Geneve (1973) in French. Published in Phase Diagrams of Binary Gold Alloys, 1987, and Bull. Alloy Phase Diagrams, 6(4), Aug 1985. Complete evaluation contains 2 figures, 3 tables, and 6 references. 1