| Wakai was the first to show (1986) that a polycrystalline ceramic material, an yttria-stabilized tetragonal zirconia, can be made superplastic. Since then, high tensile elongations have been achieved in a number of ultra fine-grained ceramics. In this research, it is shown that a fine-grained iron-carbide material (Fe{dollar}sb3{dollar}C-20 vol.% Fe) prepared from rapidly solidified powders exhibits superplastic properties over a wide range of strain rates and temperatures. Tensile elongation as high as 610% have been achieved. Based on the knowledge obtained from the iron-carbide material, the tensile elongation behavior of superplastic ceramics was compared with that of superplastic metallic alloys in the temperature and strain rate range where high strain-rate-sensitivity exponents are observed. Tensile ductility of superplastic ceramics in the high strain-rate-sensitivity region is shown to be a strong function of flow stress or Zener-Hollomon parameter. On the other hand, the tensile ductility of superplastic metallic alloys is primarily a function of the strain-rate-sensitivity exponent. Thus, high m values are a necessary but not sufficient condition for observing large tensile ductility in ceramic materials. A "fracture mechanics model" was developed which predicts quantitatively the trend observed between tensile ductility and the applied flow stress. 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{dollar}gamma{lcub}rm s{rcub}{dollar}-{dollar}gamma{dollar}g b), where {dollar}gamma{dollar}s is the surface energy and {dollar}gamma{dollar}gb is the grain boundary energy. A quantitative evaluation of cavitation damage and crack formation as a function of flow stress and plastic strain was made to assess the phenomenological relation of crack growth assumed for "fracture mechanics model" and to determine the influence of stress on crack growth. The possibility of refining an ingot-processed iron carbide material by using thermomechanical routes is explored and the promising results obtained are discussed. Tensile elongation behavior of superplastic ceramics are reexamined with newly available tensile elongation data and compared with that of superplastic intermetallic compounds. |
