The effect of yttrium on the transition of the stress exponent in the deformation of nanocrystalline Y—TZP at elevated temperature

Abstract

Ultrafine-grained monoclinic ZrO₂ polycrystals (MZP) and 3mol% Y₂O₃-stabilized tetragonal ZrO₂ polycrystals (3Y-TZP) were obtained by hot isostatic pressing (HIP). Both MZP and TZP were “high purity” materials with impurities less than 0.1 wt%. The deformation behavior was studied at 1273 K, which was lower than the monoclinic ⇔ragonal transition temperature. The stress exponent of 3Y-TZP with grain size of 60 nm was 2.7 in higher stress region, and increased with decreasing stress. The transition of the stress exponent with stress was similar to the deformation of high-purity Y-TZP with grain size of 200 - 400 nm at 1673 K - 1723 K. The deformation of MZP was characterized by a stress exponent of 2.5 over a wide stress range. The transition of stress exponent, which is peculiar to superplasticity of high-purity Y-TZP, is probably a result of yttrium segregation at grain boundary or the solution of yttrium into ZrO₂ lattice.