Superplasticity is a mode of deformation for fine-grained polycrystalline solids at elevated temperatures: included are metals, ionic polycrystals (e.g. oxides) and covalent polycrystals (e.g. non-oxide ceramics). Plastic deformation of covalent crystals by dislocation glide is not easy because of their high Peierls force. This paper describes high ductilities in Si3N4 and SiC, and also reports superplastic elongations in Si3N4/SiC composite.
Fine-grained Si3N4/SiC composites (20 wt% SiC) were fabricated by hot-pressing amorphous Si-C-N powder with 6 wt% Y2O3 and 2 wt% A12O3 as sintering aids. The composites were composed of equiaxed grains (<200 nm) and elongated grains. A composite exhibited a superplastic elongation larger than 150 % at 1600 °C and at an initial strain rate of 4 × 10−5 s−1. The superplasticity of the composite is probably related to the presence of an intergranular liquid phase.