Thermodynamic analysis of the stable and metastable Co–Cu and Co–Cu–Fe phase diagrams
Introduction
Some alloys with a large positive enthalpy of mixing and nearly flat liquidus curves present a metastable miscibility gap in the undercooled liquid state. This behaviour has been studied extensively in both the Co–Cu and Cu–Fe systems and more recently in the ternary Co–Cu–Fe system, also under microgravity conditions, because of the fascinating prospect of producing materials containing finely dispersed phases for various applications [1].
Several thermodynamic assessments are available in the literature for both the binary Co–Cu and the ternary Co–Cu–Fe systems. The first reliable calculations of the binary Co–Cu phase diagram date back to the works by Kaufman [2] and Hasebe et al. [3]. Elder et al. [4] revised the phase diagram including experimental data on the composition of phases quenched from the liquid, as obtained with electron microprobe analysis. Gente et al. [5] reported the parameters derived in an unpublished assessment mainly based on the work of Hasebe et al. [3]. More recently, Kubista et al. [6] and Hari Kumar et al. [7] revised the thermodynamic description of the system. Thermodynamic optimizations of the ternary phase diagram have been published by Bamberger et al. [8] and by Wang et al. [9]. Cluster Variation Method (CVM) calculations have also been carried out by Bein et al. [10] and Antoni-Zdziobek et al. [11].
New accurate data on the metastable miscibility gap in the binary Co–Cu [12], [13] and ternary Co–Cu–Fe [14] systems have been determined, showing that descriptions of the miscibility gap of previous calculations were no longer satisfactory. Thus, in this work a reassessment of both the Co–Cu and Co–Cu–Fe phase diagrams has been carried out including the newly available experimental data, and special care has been paid to the description of the miscibility gap in the liquid phase.
Section snippets
Binary Co–Cu system
Early measurements of phase diagram points of the binary Co–Cu system were performed by Konstantinov [15], Sahmen [16] and Hashimoto [17] using different techniques. Liquidus data from Konstantinov show large scatter and are probably rather inaccurate. Later, liquidus/solidus points were determined by Nakagawa [18] (magnetic susceptibility), Hasebe et al. [3] (electron probe microanalysis) and Taskinen [19] (e.m.f. measurements). Two sets of solidus data in the Co-rich side of the diagram
Elements
The pure solid elements in their stable phases at 298.15 K were chosen as a reference state (SER, Standard Element Reference), unless otherwise specified. The Gibbs free energy of the pure elements (lattice stabilities) and the magnetic contributions have been taken from the PURE database [38].
Binary boundary systems
Thermodynamic modelling of binary and higher order phases has been performed according to the compound energy formalism [39]. The Gibbs free energy of a phase is thus given by the following equation:
Results and discussion
All calculations have been performed using the ThermoCalc software package and the PARROT module for optimization.
Conclusions
A new assessment of both the binary Co–Cu and ternary Co–Cu–Fe systems has been performed using the CALPHAD approach.
A significant improvement in the description of the miscibility gap in the liquid phase has been achieved in the binary Co–Cu system. In the Co–Cu–Fe system, ternary parameters had to be introduced in the description of the liquid phase in order to predict a metastable miscibility gap.
A fairly good agreement has been achieved between our thermodynamic description and experimental
Acknowledgements
This work has been supported by the European Space Agency within the project “CoolCop” (ESA MAP Contract No. AO 99 - 010).
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