Freezing and melting in the peritectic W–C–X

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The tungsten–carbon (WC) peritectic, with a liquidus temperature near to 3020 K, shows promise as a high-temperature reference fixed point. It appears that the WC phase is a discrete phase with full stoichiometry of the components W and C, in which impurities cannot be dissolved. In this paper we describe the use of Thermo-Calc software to derive the liquidus and solidus temperatures versus the impurity concentration for the impurities V and Si. It is found that the liquidus is univariant, whereas the solidus is invariant. Additionally, the transition curves between the solidus and the liquidus are modeled. The liquid fraction at the liquidus point is found to be less than 1; in contrast, it is unity for metal–carbon eutectics. Measurements are presented for a series of melting and freezing curves for two WC fixed-point cells. The Gibbs phase rule requires segregation of a fourth compound phase, involving the impurity in question, at the solidus point. The observations confirm that during supercooling, impurities are trapped in compound phases consisting of the impurity together with W and/or C, encapsulating the impurity, and resulting in a flat freezing plateau characteristic of the pure system. Finally, an expression for the melting temperature, as a function of the liquid fraction, is derived, spanning the temperature range from an initial melting temperature to the liquidus temperature. Within this range just free impurities are involved.