Фазовая диаграмма системы Ag-Pt

К оглавлению: Другие диаграммы (Others phase diargams)


Ag-Pt (Silver-Platinum) I. Karakaya and W.T. Thompson The assessed (partially calculated) Ag-Pt phase diagram is topologically similar to [Hansen]. The phase boundaries shown by the solid lines have been established mainly on the basis of thermodynamic modeling by the present authors. The dashed boundaries associated with intermetallics are reproduced from [Hansen], but the intermetallic at about 90 at.% Pt has been disregarded. The equilibrium condensed phases of the Ag-Pt system above 965 C are a liquid, L, and an fcc solid solution, with a large degree of immiscibility. The existence of intermetallic phases (b, g, and g›) at lower temperatures was reported by [30Joh]. Other phases (a›, a››, and b›) and possibly another intermetallic phase (with about 90 at.% Pt) were reported by [43Sch]. However, other studies [27Kur, 57Nov] do not indicate the presence of any intermetallics. The solid solubilities from X-ray studies near the peritectic [30Joh, 43Sch] are in agreement with the boundaries in the assessed diagram. Furthermore, the maximum volume change in the sintered Ag-Pt alloys at about 40 at.% Pt [76Sol] also may be interpreted as the result of the invariant reaction. Thermodynamic modeling of the liquid phase and the solid solutions was performed to obtain the smooth and self-consistent phase boundaries shown by solid lines in the assessed diagram. Emphasis was given to the liquidus points of [07Doe] and the X-ray data from [43Sch] for the solid solubilities. The intermetallic phases a›, b, and g were not included in the modeling because of controversy over their existence. In general, the calculated diagram is in agreement with liquidus measurements, but it was not possible to accommodate the positioning of the Ag-rich solidus without grossly affecting the liquidus. In order to represent this solidus, the partial excess Gibbs energy of Ag in the range of 0 to 20 at.% Pt must vary from 0 to 2000 J/mol. Such large positive deviation from ideality is unlikely, particularly when there is the suggestion that intermetallics may form near this composition range. Moreover, it is possible that the solidus points obtained from general analysis [07Doe] may be due to the proposed (Ag) = a› transformation [43Sch]. The present evaluators consider the experimental points on this solidus [07Doe] to be unreasonably low. A complete and systematic experimental study of this system is required to resolve the uncertainties surrounding the intermetallic phases. In view of the gross difference between the calculated and measured Ag solidus, it is suggested that this feature be redetermined by liquation experiments using modern techniques for phase analysis. 07Doe: F. Doerinckel, Z. Anorg. Chem., 54, 338-344 (1907) in German. 27Kur: N.S. Kurnakow and W.A. Nemilow, Z. Anorg. Chem., 168, 339-348 (1927) in German. 30Joh: C.H. Johansson and J.O. Line, Ann. Phys., 6, 458-486 (1930) in German. 43Sch: A. Schneidner and U. Esch, Z. Elektrochem., 49, 72-89 (1943) in German. 57Nov: O.A. Novikova and A.A. Rudnitskii, J. Inorg. Chem. USSR, 2, 208-221 ( 1957). 76Sol: S.M. Solnin, Poroshk. Metall., 4, 31-34 (1976) in Russian. Published in Bull. Alloy Phase Diagrams, 8(4), Aug 1987. Complete evaluation contains 6 figures, 6 tables, and 13 references. Special Points of the Ag-Pt System