Phase diagrams in the system Boron-Carbon-Hafnium-Zirconium

The group IV transition metal borides and carbides, and ternary compounds of these materials, are important ultra-high temperature ceramics used in the nuclear and aeronautical industries (hypersonic flight components) due to their very high melting temperatures and material characteristics. For these applications, precise knowledge of the phase stability of materials as a function of composition, temperature and even pressure is required, considering all experimental and accurately modelled properties.

We review the previously assessed phase diagrams within the systems containing boron, carbon, hafnium, and zirconium, and the related experimental data. Within this, regions are identified where the understanding of these compounds is limited due to scarce or inconsistent data.

This review consists of examinations of the equilibrium thermodynamics of the binary and ternary subsystems containing these elements, which have been considered extensively in the literature. A review of the literature reveals that the phase diagrams for these systems are often conflicting, and some are based on very little experimental data.

Using the CALPHAD approach, a quaternary phase diagram is produced by extrapolation from currently accepted lower-order assessments of the subsystems, and regions of interest within this system are identified. Based upon comparison with existing experimental data, new experiments are suggested, and changes to the thermodynamic models are proposed, considering insights from first principles density functional theory investigations.