The grain size dependency of compressive deformation of a superplastic 3 mol% yttria stabilized tetragonal zirconia

The compressive behavior of a medium purity 3 mol% yttria stabilized tetragonal zirconia (3YTZ) was investigated with three different initial grain sizes at the temperature of 1723 K. It was found that the strain rate sensitivity exponent increases from 0.45 to 0.55 when the initial grain size increases from 0.5 to 1.1 μm and that the grain size exponent is about 2.0. Microstructural measurements revealed that grains retained the essentially equiaxed nature even after large plastic strain and that the instantaneous grain size had a linear relationship with true strain at the specified initial strain rates. Transmission electron microscope observation demonstrated that an amorphous phase concentrated at multiple points and that no amorphous phase formed at two-grain boundaries. Like Al2O3, the SiO2 content had a significant effect on superplastic 3YTZ characteristics. It is considered that grain boundary sliding (GBS) should play a dominant role in 3YTZ superplasticity and that the presence of an amorphous phase at triple or multiple points could effectively release the stress concentration caused by GBS and thereby facilitate superplastic deformation.