Superplastic Ceramics (With Emphasis on Iron Carbide)

Abstract

Abstract : Ultrafine grained iron carbide material was developed by an atomized powder process, utilizing hipping, pressing and extrusion procedures. The material was made superplastic and behaved like other superplastic ceramics. This observation lead to the following conclusions. Superplastic ceramics and metallic alloys exhibit different trends in tensile ductility in the range where the strain-rate-sensitivity exponent, m, is high. The tensile ductility of superplastic 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 only a function of the strain rate sensitivity exponent, but also a function of the parameter where the steady state strain rate and Qc is the activation energy for superplastic flow. Superplastic ceramic materials exhibit a large decrease in tensile elongation with an increase. 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. 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.