TENSILE DUCTILITY OF SUPERPLASTIC CERAMICS AND METALLIC ALLOYS

Superplastic ceramics and metallic alloys exhibit different trends in tensile ductility in the range where the strain-rate-sensitivity exponent, m, is high (m greater-than-or-equal-to 0.5). The tensile ductility of super-plastic metallic alloys (e.g. fine-grained zinc, aluminum, nickel and titanium alloys) is primarily a function of the strain-rate-sensitivity exponent. In contrast, the tensile ductility of superplastic ceramic materials (e.g. zirconia, alumina, zirconia-alumina composites and iron carbide) is not a function of the strain-rate-sensitivity exponent, but also a function of the parameter epsilon exp(Q(c)/RT) where epsilon is the steady-state strain rate and Q(c) is the activation energy for superplastic flow. Superplastic ceramic materials exhibit a large decrease in tensile elongation with an increase in epsilon exp(Q(c)/RT). This trend in tensile elongation is explained based on a "fracture-mechanics" model. The model predicts that tensile ductility increases with a decrease in flow stress, a decrease in grain size and an increase in the parameter (2-gamma(s) - gamma-gb), where gamma(s) is the surface energy and gamma-gb is the grain boundary energy. The difference in the tensile ductility behavior of superplastic ceramics and metallic alloys can be related to their different failure mechanisms. Superplastic ceramics deform without necking and fail by intergranular cracks that propagate perpendicular to the applied tensile axis. In contrast, superplastic metallic alloys commonly fail by intergranular and transgranular (shearing) mechanisms with associated void formation in the neck region.