Core‒rim structure, bi-solubility and a hierarchical phase relationship in hot-pressed ZrB2‒SiC‒MC ceramics (M=Nb, Hf, Ta, W)

Core‒rim structures were identified as a common feature in hot-pressed ZrB2‒SiC‒MC ceramics (M = Nb, Hf, Ta and W) by a combination of X-ray diffraction, scanning and transmission electron microscopies. Quantitative analyses associate them with the bi-solubility of M in ZrB2 phase, in which transition of solubility across the core/rim boundary is abrupted, signifying their creation via dissolution‒reprecipitation process facilitated by transient liquid-phase. The cores were retained from starting powder after surface melting and the rims were grown from the liquid-phase to incorporate more solutes, leaving the residual liquid to turn into ZrC phase with higher solubility of M. We propose g-point scheme in the ZrB2‒MB2 diagrams to combine the bi-solubility and the core‒rim structures into an intra-phase relationship created by sintering, leading further to a hierarchical phase relationship. The temperature dependence of flexural strength in the ZrB2‒SiC‒MC ceramics varies with MC additions, which can be respectively strengthened by the strain energy created in the core‒rim structures and metal segregation to grain boundaries.