Phase transformation and texture in hot-forged or annealed liquid-phase-sintered silicon carbide ceramics

Starting from three powder mixtures of 80 vol% SiC (100alpha, 50alpha/50beta, 100beta) and 20 vol% YAG, liquid-phase-sintered silicon carbide ceramics were prepared by hot pressing at 1800degreesC for I h under 25 MPa, and then by hot forging or annealing at 1900degreesC for 4 h under an applied stress of 25 MPa in argon. The phase transformation and texture development in the as-hot-pressed, hot-forged, and annealed SiC ceramics were investigated via X-ray diffraction (XRD) and the pole figure measurements. The 6H --> 411 polytypic transformation was observed in samples consisting of both alpha- and beta-SiC phases when subjected to compressive deformation but absent in the case of annealing, suggesting the deformation-enhanced solubility of aluminum in SiC. Deformation was also found to enhance the 3C --> 414 transformation in the sample containing entirely beta-phase, which is due to the accelerated solution-precipitation process assisted by grain boundary sliding. The current study showed that the beta- --> alpha-phase transformation had little effect on texture development in SiC. Hot forging generally produced the strongest texture, with the calculated maximum of 2.2 times random in samples started with pure alpha-SiC phase. The mechanism for texture development was explained based on the microstructural observations.