Critical Assessments of Tensile Ductility in Superplastic TZP and TiO2-doped TZP

The effect of TiO2 doping on the superplastic flow in tetragonal zirconia polycrystal (TZP) stabilized by Y2O3 was examined with a special interest in tensile ductility at strain rates between 1.3×10−4 and 1.3×10−2 s−1. TZP-5 mass%TiO2 exhibits improved tensile ductility in comparison with TZP at high strain rates. Flow stress is markedly reduced by TiO2 addition into TZP at all strain rates examined in this study in spite of the fact that the grain size is larger in TZP-5 mass%TiO2 than in TZP. Assuming the grain size at the time of failure, df, as a parameter to describe a limit of an accommodation process for superplastic flow, the elongation to failure is estimated from the grain growth behavior during plastic flow. The observed strain rate dependence of the elongation to failure can reasonably be described by the estimation. The present analysis reveals that there is an optimum strain rate at a constant temperature for superplastic flow as well as an optimum temperature at a certain strain rate in each material. It is clarified that the optimum strain rate or temperature for superplastic flow is dependent mainly on the activation energies for superplastic flow and grain growth, and additionally on the initial grain size.