Low-temperature superplasticity in nanocrystalline tetragonal zirconia polycrystal (TZP)

Nanocrystalline tetragonal ZrO ₂ polycrystals (TZP) have been fabricated by the pressureless sintering of recently developed tetragonal ZrO ₂ powder containing 5.69 mol% YO _1.5 and 0.60 mol% AlO _1.5. The average grain sizes were 160 nm in the TZP sintered at 1150°C for 10 h and 150 nm in the 0.25 mol% GeO ₂-doped TZP sintered at 1100°C for 100 h. The TZP and Ge ⁴⁺-doped TZP-sintered bodies were essentially single-phase materials, and neither the amorphous layer nor the second-phase particle was observed along the grain boundary faces. High-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM), and nanoprobe energy-dispersive X-ray spectrometer (EDS) measurements revealed that the Y ³⁺, Al ³⁺ and Ge ⁴⁺ cations tend to segregate in the vicinity of the grain boundaries in the TZP-sintered bodies. The TZP and Ge ⁴⁺-doped TZP exhibited an elongation to failure of more than 100% in the temperature range of 1150°C-1300°C and initial strain rate range of 1.4 × 10 ^-5 s ^-1 to 1.0 × 10 ^-2 s ^-1. For instance, an elongation to failure in the Ge-doped TZP reached about 200% at 1150°C and 1.4 × 10 ^-5 s ^-1. The nanocrystallization reduced the lower limit of the superplastic temperature of conventional, submicron-grain TZP materials by 150°C. The improved ductility of the TZP at low temperatures was essentially attributed to the reduced grain size.