Microstructural Development of Silicon Carbide Containing Large Seed Grains

Fine ({approximately}0.1 {micro}m) {beta}-SiC powders, with 3.3 wt% large ({approximately}0.44 {micro}m) {alpha}-SiC or {beta}-SiC particles (seeds) added, were hot-pressed at 1,750 C and then annealed at 1,850 C to enhance grain growth. Microstructural development during annealing was investigated using image analysis. The introduction of larger seeds into {beta}-SiC accelerated the grain growth of elongated large grains during annealing, in which no appreciable {beta}{yields}{alpha} phase transformation occurred. The growth of matrix grains in materials with {beta}-SiC seeds was slower than that in materials with {alpha}-SiC seeds. The material with {beta}-SiC seeds, which was annealed at 1,850 C for 4 h, had a bimodal microstructure of small matrix grains and large elongated grains. In contrast, the material with {alpha}-SiC seeds, also annealed at 1,850 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{center_dot}m{sup 1/2}, respectively. Such results suggested that further optimization of microstructure should be possible with {beta}-SiC seeds, because of the remnant driving force for grain growth caused by the bimodal microstructure.

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