Core‒Rim Structures and Hierarchical Phase Relationship for Hot-Pressed ZrB <sub>2</sub>‒SiC‒MC (M=Nb, Hf, Ta, W) Ceramics

Abstract

Core‒rim structure emerges as a common microstructural feature of hot-pressed ZrB2‒SiC‒MC (M=Nb, Hf, Ta and W) ceramics. A combination of X-ray diffraction and microscopic analyses reveal that it originates from the re-distribution of transition-metals into their bi-solubility in the primary phase, hence turning the core‒rim structures into an intra-phase relationship. Solution‒reprecipitation process rationalizes their formation via a transient liquid-phase of Zr‒M‒B‒C‒(O). It dictates the special transformation process by enabling an exchange of metal with cores to reach the lower solubility and a transition to grow rims into higher solubility, driven by an over-saturation of boron in the liquid. A third and even higher solubility in the minor ZrC phase finalizes the liquid-phase sintering process, leading to hierarchical phase relationship. We propose further a scheme of g-point to syndicate the bi-solubility as solidus and liquidus limits of metal in ZrB2 phase, hence to schematize the coordinated evolution of multiphase microstructures. The core/rim boundaries can strengthen the ceramics by storing and re-distributing strain energy within the grains, which leaves the microstructure‒property relationship to renew and further optimize for UHTC.