Similar to Si₃N₄ ceramics, β→α phase transformation in SiC ceramics plays a key role in tailoring the microstructures thus optimizing related properties. SiC microstructures are dominated with the core–rim structures by AlN-solution, and by EBSD analysis, α-lamellae were revealed as stacking-faults (SF) and twin-boundaries (TB) in β-grains, co-existing with the core–rim structures as α/β→α'/β' transformation by sintering. The structural transformation can proceed much further by gas-pressure-sintering than hot-pressing with only RE₂O₃ agents, while the latter retain a high-density of SF/TB in the metastable β-SiC grains. By high-angle secondary-electron imaging, nanoscale transition-layer (TL) was observed as an inter-phase to fully separate the core and rim, which is created by a transitory equilibrium in the solution–reprecipitation process. The enrichment of AlN or RE in TL demonstrates their segregation to core surface until reaching the super-saturation and before the growth of rims. With higher AlN or RE solution and after sintering, a shear stress can develop from TL contour to drive the expansion of SF/TB in Martensitic transition, especially under an external isotropic pressure. The combinations of β→α transformation, core–rim structures and viscous liquid-phase enable the comprehensive assessment of sintering–microstructure–property–performance relationship of SiC ceramics, as demonstrated for their creep behaviors and fracture toughness.

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.

The A₂B₂O₇ series of ternary oxides are derivatives of fluorite structure over a wide range of r_A/r_B. Competing by two rare-earths the A-site, La_2-xLu_xZr₂O₇ ceramics were found transparent only in pore-free microstructures with similar grain sizes of pyrochlore (PY) and defective fluorite (DF) phases. Mutual solubilities of Lu and La in both phases were found by imaging and energy-dispersive spectroscopy analysis in scanning electron microscope. The dual-phase microstructures were developed with liquid-phase resulted from the exothermal reactions, creating a miscibility gap between two structures to moderate their competing grain growth. Change in grain growth behaviors in liquid-phase is described by a nucleation line in La₂Zr₂O₇‒Lu₂Zr₂O₇ phase diagram. Variations of solution levels in DF grains and co-existing of dual-phase grain clusters in common orientation were revealed in transparent ceramics by electron backscattered diffraction, resulted by epitaxial relation of two phases promoted by the liquid-phase. Oxygen vacancies and various hole states common in both phases were revealed by characteristic cathodoluminescence peaks. The collective effects of pores, phase and grain boundaries, oxygen vacancies on scattering or absorption of visible light enables to establish a hierarchical microstructure‒transparency relationship in such complex oxide ceramics, which could be tailored or further optimized by controllable sintering.

Correlated phase and microstructural evolution are systematically investigated by electron microscopies in Sr-deficient Sr(Ti, Nb)O₃ (STNO) thermoelectric ceramics incorporated with different fraction of reduced graphene oxide (RGO). It is found that while no impurity except for very few Ti₃O₅ precipitates are observed in monolithic STNO, the Nb-enriched rutile TiO₂ appears in RGO/STNO composites. With increasing RGO content, the amount of precipitates increase at first and then decrease when RGO content becomes high, which can be ascribed to the formation of local Magnéli phase. In addition, the energy-dispersive X-ray spectra combined with cathodoluminescence characterization indicates that the variation of Sr deficiency experiences the opposite trend with respect to the precipitates content. These findings clearly reveal the unique reducing effect of RGO on the microstructure of doped SrTiO₃ with Sr deficiency, which can greatly facilitate the design of perovskite based thermoelectric materials of hierarchical structure.