MICROSTRUCTURAL DEVELOPMENT OF SILICON-CARBIDE CONTAINING LARGE SEED GRAINS

Fine (similar to 0.1 mu m) beta-SiC powders, with 3.3 wt% large (simil ar to 0.44 mu m) alpha-SiC or beta-SiC particles (seeds) added, were h ot-pressed at 1750 degrees C and then annealed at 1850 degrees C to en hance grain growth, Microstructural development during annealing was i nvestigated using image analysis, The introduction of larger seeds int o beta-SiC accelerated the grain growth of elongated large grains duri ng annealing, in which no appreciable beta-->alpha phase transformatio n occurred, The growth of matrix grains in materials with beta-SiC see ds was slower than that in materials with alpha-SiC seeds, The materia l with beta-SiC seeds, which was annealed at 1850 degrees C for 4 h, h ad a bimodal microstructure of small matrix grains and large elongated grains, In contrast, the material with alpha-SiC seeds, also annealed at 1850 degrees C for 4 h, had a uniform microstructure consisting of elongated grains, The fracture toughnesses of the annealed materials with alpha-SiC and beta-SiC seeds were 5.5 and 5.4 MPa . m(1/2), respe ctively, Such results suggested that further optimization of microstru cture should be possible with beta-SiC seeds, because of the remnant d riving force for grain growth caused by the bimodal microstructure.