Phase equilibria, isothermal reactions, and a thermodynamic study in the Co-W-C system at 1150°C

The phase equilibria in the Co-W-C system at 1150°C were studied using a technique which enables heat treatments to be carried out at controlled carbon activities. The specimens were investigated by means of X-ray diffraction, metallographic techniques and microprobe analysis. The final carbon activities were obtained through carburizing as well as decarburizing reactions. Accurate X-ray and microprobe measurements were used to determine the homogeneity ranges and compositions of the M6C and M12C carbides as well as the cobalt phase. The experimental data were analysed in terms of a thermodynamic model. Free energies of formation of the M6C, M12C, WC, and μ (C07.3W5.7) phases as well as their solubilities in the cobalt phase have been calculated. The Gibbs free energy of formation obtained for the individual phases were:∆°GGM6C = − 77.2 kJ mol−1,∆°GM12C = − 130 kJ mol−1,∆°Gwc = −37.7 kJ mol−1,and∆°Gμ(C07.3W5.7) = − 26.6 kJ mol−1,respectively. The homogeneity range of the M6C carbide was found to be Co2.2 W3.8C-Co2.0W4.0C (11.236-11.25 Å) and for the M12C carbide C06.0W6.0C-CO5.9W6.1C (10.896–10.901 Å). The solubility of tungsten in cobalt was found to be Yw = 0.018 (ac = 1) and Yw = 0.14 (ac = 0.013).