The solubility of cubic carbide formers in liquid cobalt

Accurate phase diagram and thermodynamic information are essential for the understanding and modeling of microstructure evolution. An important example is the growth of a binder phase enriched and cubic carbonitride free functional gradient in cemented carbides. The growth mechanism of such a gradient is controlled by diffusion in the liquid binder and the kinetics depends an gradients in chemical potentials, diffusivities and the solubility in the liquid binder phase. Available experimental data an the solubility of cubic carbide formers, e.g. Nb, Ta, Ti, and Zr, in the liquid binder phase of cemented carbides is insufficient for accurate predictions of microstructure evolution. Therefore, an experimental technique based on heat treatment of a compound of a straight WC-Co grade and a grade further containing a cubic carbide phase has been developed. The carbide forming metal will diffuse into the liquid binder of the straight grade without forming any cubic carbide. A state close to the equilibrium can be reached by long time holding. Thus, after cooling the solubility in the liquid binder can be determined by standard methods. Making sure that graphite or M6C carbide is present in the compound during the heat treatment controls the carbon content. The experimental method can also be used to study the kinetics of diffusion and surface tension driven viscous flow of the liquid by interrupting the process before an equilibrium state is reached and analyzing gradients in the WC-Co part of the compound. (author)