A SUPERPLASTIC COVALENT CRYSTAL COMPOSITE

Superplasticity is defined phenomenologically as the ability of a material to exhibit exceptionally large elongations during tensile deformation1. It is a property of some poly crystalline solids, and is well established for metals and alloys2. Superplasticity has also been observed in some ionic crystals, such as Y2O3-stabilized tetragonal ZrO2 polycrystals3,4, but has not been found previously for covalent crystals. Here we report superplastic elongation (by more than 150%) of a covalent crystal composite, Si3N4/SiC. The superplasticity is probably related to the presence of an inter-granular liquid phase. Combined with its hardness, this property suggests several useful applications for the novel material: for example, to form engine components - superplasticity will make it readily mouldable at high temperatures. © 1990 Nature Publishing Group.