Influence of annealing treatments on microstructure and toughness of liquid-phase-sintered silicon carbide

The possibility of an in situ toughening through the beta --> alpha phase t ransition was evaluated on liquid-phase-sintered SiC (with Al2O3 and Y2O3 a s additives). Dense hot-pressed materials with an equiaxed morphology were annealed at 1850, 1900 and 1950 degreesC for 1 to 4 h so the effect of time , temperature, composition, and amount of second phase could be investigate d. Microstructure features revealed that 1900 degreesC was the best tempera ture in producing a high percentage of elongated grains with limited grain coarsening and reduction of second-phase pockets. Mean grain size and aspec t ratios increased from 0.5 to 1.5 mum and to 5, respectively, during the f irst 2 h of treatment at 1900 degreesC and then maintained a constant value . The amount and composition of second phase influenced the rate of transfo rmation from equiaxed to elongated grain morphology. Toughness increased fr om 3.3 to 5.5 Mpa . m(1/2) in 1900 degreesC annealed samples due to a crack -deflection mechanism.