A high-strain-rate superplastic ceramic

High-strain-rate superplasticity describes the ability of a material to sustain large plastic deformation in tension at high strain rates of the order of 10(-2) to 10(-1) s(-1) and is of great technological interest for the shape-forming of engineering materials. High-strain-rate superplasticity has been observed in aluminium-based(1) and magnesium-based(2) alloys. But for ceramic materials, superplastic deformation has been restricted to low strain rates of the order of 10(-5) to 10(-4) s(-1) for most oxides(3,4) and nitrides(5) with the presence of intergranular cavities leading to premature failure. Here we show that a composite ceramic material consisting of tetragonal zirconium oxide, magnesium aluminate spinel and a-alumina phases exhibits superplasticity at strain rates up to 1 s(-1). The composite also exhibits a large tensile elongation, exceeding 1,050 per cent for a strain rate of 0.4 s(-1). The tensile flow behaviour and deformed microstructure of the material indicate that superplasticity is due to a combination of limited grain growth in the constitutive phases and the intervention of dislocation-induced plasticity in the zirconium oxide phase. We suggest that the present results hold promise for the application of shape-forming technologies to ceramic materials.