High-temperature tensile ductility in TZP and TiO2-doped TZP

The role of TiO2 on the superplastic flow in tetragonal zirconia polycrystal (TZP) stabilized by Y2O3 is examined with a special interest in tensile ductility at temperatures between 1200 and 1550°C. The initial grain size of 0.66 μm in TZP-5wt.% TiO2 is about twice as large as the size of 0.30 μm in TZP, but the flow stress is lower in TZP-5wt.% TiO2 than TZP. The tensile elongation is improved by the addition of TiO2 at temperatures below about 1500°C. The grain size at the time of failure, df, is much larger in TZP-5wt.% TZP than TZP at all temperatures. Assuming that df is 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 ϵf vs. T relationship can reasonably be expressed by the estimation. The present analysis reveals that there is an optimum temperature for superplastic flow at a certain strain rate in each material. The temperature region of superplastic flow and the optimum temperature are mainly dependent on the activation energies for superplastic flow and grain growth. The observed effect of 5 wt.% TiO2 addition on the tensile elongation is explained from a slight reduction in the two activation energies.