Thermodynamic properties of the Co-W-C system

Abstract

The thermodynamic properties of the Co-W-C system have been evaluated by using the CALPHAD method. The properties of the Co-C and W-C systems are taken from recent analyses, whereas the properties of the Co-W system are evaluated in the first part of the present work. In particular, the effect of W in decreasing the Curie temperature of fcc Co (Tc) is evaluated from recent solubility data, by using the Hillert-Jarl model for magnetic Gibbs energy. Our evaluation gives a less dramatic effect of W on Tc than according to early work, but strong enough to produce a magnetic miscibility gap in the fcc phase. A new phase diagram for the Co-W system, showing the immiscibility effects, is presented. The properties of the fcc, hcp, and bcc phases, two ternary carbides (M12C and M6C), and the liquid phase in the Co-W-C system are evaluated in the second part. Various isothermal and vertical sections, and a projection of the liquidus surface of the Co-W-C diagram are reported. Most of the experimental information available is satisfactorily accounted for by the present thermodynamic description.