EFFECT OF MICROSTRUCTURE ON THE CREEP OF SILICONIZED SILICON-CARBIDE

Mechanisms of creep deformation have been investigated for a commercial grade of siliconized carbide containing almost-equal-to 33%/silicon. Microstructural studies of both tensile and compressive test specimens indicate dislocation damage generation in both the silicon carbide and the silicon phases as a consequence of creep. In the silicon carbide, dislocation damage was normally restricted to contact sites between the silicon carbide grains resulting from high intergranular contact stresses during deformation. Dislocation damage was also observed in the silicon. Although dislocation damage was heavy in some regions of the specimens, most regions were free of dislocations. This result is consistent with the hypothesis that deformation occurs by the motion of clusters of grains during deformation. In tension, creep at high strain rates, epsilon greater-than-or-equal-to 1 x 10(-8) s-1, was accompanied by the formation of cavities at Si/SiC interfaces within the intergranular silicon phase. As cavities were not associated with dislocations, their growth was probably controlled by diffusional processes. Based on observations of the microstructure, a model of deformation is proposed to explain the fact that siliconized silicon carbide creeps faster in tension than in compression, at the same applied stress. The model is based on soil mechanics concepts. It is suggested that creep is controlled by intergranular friction between aggregate particles of the composite.