Transition-layer of core-rim structures and beta ->alpha transformation in SiC ceramics

Similar to Si3N4 ceramics, b->a 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, a-lamellae were revealed as stacking-faults (SF) and twin-boundaries (TB) in b-grains, co-existing with the core-rim structures as a/b->a'/b' transformation by sintering. The structural transformation can proceed much further by gas-pressuresintering than hot-pressing with only RE2O3 agents, while the latter retain a high-density of SF/TB in the metastable b-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 b->a 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.(c) 2022 The Authors. Published by Elsevier B.V. on behalf of The Chinese Ceramic Society. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).