Silicon carbide (SiC) has advantages, including strength, wear-resistance, and good resistance to creep at high-temperatures because of its covalent nature. On the other hand, even boron, carbon-doped SiC exhibits superplastic elongation of >140% when the average grain size is smaller than ~300nm. This ductility is potentially applicable to superplastic forming of engineering ceramics. The deformation mechanism of fine-grained SiC is the grain-boundary sliding. Therefore the creep and superplasticity of SiC is significantly influenced by the chemical composition and the segregation of impurities at grain boundaries. In this study, we investigated the effect of chemical composition at the grain-boundary on deformation of fine-grained SiC, and revealed that the superplasticity of SiC was similar to that of ZrO2. Superplasticity of two types of silicon carbide was investigated. One is SiC doped boron and carbon (B, C-doped SiC) and the other is SiC doped boron, carbon, and aluminum (Al, B, C-doped SiC). Boron was detected at grain boundaries of both materials, and Al was detected at grain boundaries of Al, B, C-doped SiC. The strain rate of Al, B, C-doped SiC in the low stress region were ~1 order of magnitude faster than those of B, C-doped SiC. It was suggested that Al segregation to grain boundaries promoted grain-boundary diffusion. The plot of strain rate versus stress for superplasticity of metals and ZrO2 is divided into several stress regions. The superplasticity of SiC also exhibited two stress regions, and the stress exponent n varied from 1~2 to >3 with decreasing stress. This transition of the stress exponent is often observed in the superplasticity of metals and ZrO2. In addition, the threshold stress of Al, B, C-doped SiC was smaller than those of B, C-doped SiC, and suggested that the threshold stress was dependent on the chemical composition at the grain-boundary.
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