DIFFUSIONLESS CUBIC-TO-TETRAGONAL PHASE-TRANSITION AND MICROSTRUCTURAL EVOLUTION IN SINTERED ZIRCONIA YTTRIA CERAMICS

Microstructural changes associated with the diffusionless cubic-to-tetragonal phase transition (c-t') in sintered ZrO2-Y2O3 ceramics with 4 to 20 mol% Y2O3 are investigated. Ceramics containing 4 to 7 mol% Y2O3 that are quickly cooled from high temperatures experience cubic-to-tetragonal transformation by a diffusionless mechanism. The TEM diffraction pattern of the transformation product, t'-ZrO2, reveals (112) reflections that are suppressed in the c-ZrO2 phase. The microstructural features of the t'-ZrO2 phase include antiphase boundaries and a twinned substructure in the (112) dark-field images. The (112) reflections result from the displacement of oxygen atoms in the lattice during phase transformation. With increasing Y2O3 content, the antiphase domain size, the tetragonality of the lattice, and the intensity of the (112) reflections decrease. When the Y2O3 content reaches 8 mol% or more, the (112) reflections appear, but the dark-field image shows no antiphase boundary, and the tetragonality decreases to 1. In ceramics with 14 to 20 mol% Y2O3, no suppressed reflection appears because the c-t transformation is fully suppressed. A model of the lattice structure is calculated and the intensity of diffraction determined experimentally; on this basis, the mechanism of diffusionless c-t' transformation is discussed.