High-Entropy Boride-Carbide Two-Phase Ultrahigh Temperature Ceramics Fabricated by Reactive Spark Plasma Sintering

A series of high-entropy boride-carbide two-phase ultrahigh temperature ceramics (UHTCs) are fabricated via a novel reactive spark plasma sintering (SPS) based route, each from five individual commercial powders, including N metal diborides and (5-N) metal carbides, of equimolar amounts. Greater than ~99% of the theoretical densities have been achieved with virtually no native oxides. These two-phase UHTCs consist of a hexagonal high-entropy boride (HEB) phase and a cubic high-entropy carbide (HEC) phase, where a chemical equilibrium between the two phases that drive them away from equimolar compositions. A thermodynamic relation that governs the compositions of the HEB and HEC phases in equilibrium is discovered and a thermodynamic model is proposed. A new observation is that the high-entropy two-phase UHTCs have higher hardness than the weighted linear average of the two single-phase HEB and HEC, which are already harder than the rule-of-mixture (RoM) averages of individual UHTCs. Several intriguing observations about the moduli are revealed with accurate acoustic waves measurements. On the one hand, the Young's moduli of high-entropy two-phase UHTCs are always higher than the theoretical RoM averages. On the other hand, the shear modulus of HEC is higher, but that of HEB is lower, than the theoretical RoM average. The latter unexpected observation is explained by the unique layered structure of the HEB, where an expansion in lattice parameter c is confirmed by X-ray diffraction. This study extends the state of the art by introducing two-phase high-entropy ceramics, which provide a new platform to tailor various properties via changing the phase fraction and microstructure.