Control of Hierarchical Solid-solution Microstructures by Sintering Melt for Ultra-high-temperature Multiphase Ceramics: Retrospect and Prospect

ZrB₂-based multiphase ceramics are representative ultra-high temperature ceramics (UHTCs). Their service temperature significantly exceeds the sintering temperature, often requiring substantial amounts of SiC as a sintering aid to achieve the densification and enhance the oxidation resistance. The transition metal carbides (MCs) are superior sintering aids for UHTCs, effectively removing the oxygen-impurity and improving high-temperature strength. From some projects supported from the National Natural Science Foundation of China, our studies focus on the effect of MC on controlling the multiphase microstructure of UHTCs. The results obtained reveal the reactive-sintering mechanism engaging the high-viscous liquid-phase and explore the mutual-solution behavior in multiphase ceramics along with the structure-property relationship. The quantitative characterization for microstructures indicates the dominance of bora-carbide sintering-melts on reactive-sintering and densification, and on regulating the multi-level evolution of high-solution microstructures. MC additives are transformed into ZrC grain boundary phases via the sintering-melt, and its exchange-reaction with the primary phase governs the multiphase relationship. The melt as a transient liquid enables a bi-solubility of M to create prevalent core–rim structures. In the later stages of sintering, ZrC second-phase precipitates with a higher solubility of M.

The multi-levelled control of multiphase microstructures by the reactive-melt is analogous to “dissolution–reprecipitation” process for liquid-phase sintering in the transformable microstructures of silicon-based ceramics, with silicate-melts and glassy phases at grain boundaries. In contrast to the monolithic ceramics of high-entropy MB₂ and MC, the multi-levelled solid-solutions and the associated multiphase microstructures of M^ⅠB₂-M^ⅡC UHTCs offer ample and novel routes for comprehensive control, better optimization and further enhancement in high-performance UHTCs. The coherent hetero-interfaces created from the multi-levelled solutions via solid-state phase-separations and their interconnected dislocation networks can further improve the high-temperature strength, and those phase-boundaries, grain-boundaries, and solute-segregates allow a precise control over the multiscale semi-coherent microstructures. The research on this synergistic evolution of intergranular phases and sintering-melts at high temperatures along with the multiphase transformation has a promising potential for future advancements in ceramic genomes and levelled structure-property relationship for multiphase UHTCs governed by solid-solutions as enthalpy-regulation.