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

Abstract

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 (C0_7.3W_5.7) 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:

∆°G_GM6C = − 77.2 kJ mol⁻¹,

∆°G_M12C = − 130 kJ mol⁻¹,

∆°G_wc = −37.7 kJ mol⁻¹,

and

∆°G_{μ(C07.3W5.7)} = − 26.6 kJ mol⁻¹,

respectively. The homogeneity range of the M₆C carbide was found to be Co_2.2 W_3.8C-Co_2.0W_4.0C (11.236-11.25 Å) and for the M₁₂C carbide C0_6.0W_6.0C-CO_5.9W_6.1C (10.896–10.901 Å). The solubility of tungsten in cobalt was found to be Y_w = 0.018 (a_c = 1) and Y_w = 0.14 (a_c = 0.013).