Thermodynamic description and quaternary miscibility gap of the C-Hf-Ti-W system

The C-Hf-Ti, C-Hf-W and C-Hf-Ti-W systems have been assessed by means of the CALPHAD (CALculation of PHAse Diagram) approach. All of the experimental phase diagram and thermodynamic data available from the literature are critically reviewed. According to the available experimental data, no ternary or quaternary compound has been found in these systems. The liquid is modeled as a substitutional solution phase, while the solid solution phases including βHf, αHf, βTi, αTi, W, HfC1-x, TiC1-x, WC1-x, WC, W2C and HfW2 are described by sublattice models. The modeling of C-Hf-Ti and C-Hf-W systems covers the whole composition range. A set of self-consistent thermodynamic parameters for the C-Hf-Ti and C-Hf-W systems are then obtained. Comprehensive comparisons between the calculated and experimental data show that the experimental information is satisfactorily accounted for by the present thermodynamic description. The liquidus projection and reaction scheme of the C-Hf-Ti and C-Hf-W systems are generated by using the present thermodynamic parameters. Due to limited experimental data, the simple Hf-Ti-W system is extrapolated from its sub-binary systems. Based on the present work and the previous assessment of the C-Ti-W system, a thermodynamic description of the quaternary C-Hf-Ti-W system is extrapolated, and the experimental phase equilibria can also be well reproduced by the present thermodynamic modeling. For the first time, the miscibility gap of the quaternary cubic carbides is calculated in the quaternary C-Hf-Ti-W system. A high potential in industrial applications such as in bulk cemented carbide, cermet cutting tools and hard-coatings is discussed mainly in terms of the miscibility gap.