The doping amount and temperature dependence of high temperature superplastic flow in TiO2 and/or GeO2-doped tetragonal ZrO2 polycrystal (TZP) was systematically examined at temperature in a range from 1200°C to 1550°C in air at an initial strain rate of 1.3×10-4 s-1. The amount of dopant cations was in a range of 0.2 - 8 mol%. Ionic transport property in TZP co-doped with TiO2 and GeO2 was also measured by impedance spectroscopy in air in a temperature range from 250°C to 1000°C. Tensile ductility in TZP was highly improved by the co-doping of 2.2 mol% TiO2 and 2.2 mol% GeO2: the elongation to failure reached 1000% at 1400°C. The flow stress decreases with increasing TiO2 and/or GeO2 addition, but levels off for a 2-3 mol% addition for GeO2- or (TiO2-GeO2)-doping. The data for superplastic flow stress and ionic conductivity indicate that the grain boundary segregation of the doped cations works to enhance the grain boundary diffusion of zirconium cations, but retards oxygen-anions’ diffusion. The improved high temperature ductility of the co-doped TZP is likely attributed to the enhanced grain boundary cations diffusion and a reduction in the grain boundary energy. The change in the grain boundary diffusion is affected by the formation of strong covalent bonds and a reduction in the ionicity around Ti4+ and Ge4+ cations in TZP.
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