Thermodynamic properties and phase equilibria for Pt-Rh alloys
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J/mol
DS E 5 XRh (1 2 XRh)[23.80 1 1.55 3 1022 XRh] J/molzK
The negative enthalpy of mixing obtained in this study is in qualitative agreement with predictions of semiempirical models of Miedema and co-workers and Colinet et al. The results of this study do not support the solid-state miscibility gap suggested in the literature, but are consistent with liquidus data within experimental uncertainty limits.
I.
INTRODUCTION
Alloys of platinum and rhodium are extensively used in thermocouples for high-temperature measurement and as clean and inert heating elements in experimental high-temperature furnaces. The phase diagram for the Pt-Rh system is inadequately characterized.[1] The liquidus has been measured by Mu¨ller[2] and Acken.[3] The results have been normalized using more recent values for the melting points of the two pure components.[1,4] The liquid-solid two-phase region in this isomorphous system is displaced upward with respect to the straight line joining the melting points of the pure metals Pt and Rh,[1,5] implying that the solid solution is relatively more stable than the liquid alloy. Since Pt and Rh belong to the same group of the periodic table, the behavior of the liquid phase is expected to be close to ideal. The nature of the two-phase lens would then imply negative deviation from ideality for the solid solution. However, Raub[6] has suggested the onset of solid-state immiscibility below ;1033 K, based on information for other binary systems involving platinum-group metals such as Pt-Ir, Pd-Ir, and Pd-Rh. Magnetic susceptibility data of Darling,[7] which show a sharp inflection in the susceptibility as a function of composition at XRh ≅ 0.23, have been cited in support of this hypothesis.[8] The miscibility gap has been retained in recent phase diagram compilations,[1,5] although Raub and Falkenburgh[9] were unable to confirm the existence of a miscibility gap even after annealing for 150 days at 873 K.T. JACOB, Professor, and SHASHANK PRIYA, Graduate Student, are with the Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India. YOSHIO WASEDA, Professor and Head, Research Center for Metallurgical Process Engineering, is with the Institute for Advanced Materials Processing, Tohoku University, Sendai 980-8577, Japan. Manuscript submitted August 15, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
and 1073 K. The presence of a miscibility gap would require positive deviation from ideality. Thus, there is an apparent inconsistency between the measured liquidus and the inferred miscibility gap. Since studies on phase separation at low temperatures are time consuming and sometimes inconclusive because of kinetic factors, thermodynamic activities in the solid solutions are measured at temperatures above the suggested miscibility gap. Thermodynamic data can provide decisive evidence for or against solid-state immiscibility. An important limiting factor in the use of Pt-Rh alloys is the temperature below which Rh2O3 can form as the stable phase. In air, pure Rh2O3 is stable belo
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