Microstructural analysis of liquid-phase-sintered β-silicon carbide

The microstructures of fine-grained beta-SiC materials with alpha-SiC seeds annealed either with or without uniaxial pressure at 1900degreesC for 4 h in an argon atmosphere were investigated using analytical electron microscopy and high-resolution electron microscopy (HREM). An applied annealing pressure can greatly, retard phase transformation and grain growth. The material annealed with pressure consisted of fine grains with beta-SiC as a major phase. In contrast, the microstructure in the material annealed without pressure consisted of elongated grains with half alpha-SiC. Energy-dispersive X-ray analysis showed no differences in the amount of segregation of aluminum and oxygen atoms at grain boundaries, but did show a significant difference in the segregation of yttrium atoms at grain boundaries along SiC grains for the two materials. The increased segregation of yttrium ions at grain boundaries caused by the applied pressure might be the reason for the retarded phase transformation and grain growth. HREM showed a thin secondary phase of 1 nm at the grain boundary interface for both materials. The development of a composite grain consisting of a mixture of beta/alpha polytypes during annealing was a feature common to both materials. The possible mechanisms for grain growth and phase transformation are discussed.