Microstructure and phase changes in yttria-partially stabilized zirconia crystals annealed with electric current

4.5 mol% yttria-partially stabilized zirconia crystals were annealed between 1500°C and 1800°C (c-ZrO₂ and t-ZrO₂ co-region) with d.c. electric current in air and microstructure and phase changes were studied using SEM, EPMA and XRD. Although no significant microstructure and phase changes were observed in samples annealed without current, an opaque tongue-shaped zone was formed at the cathode in samples annealed at high temperature with current and then cooled to room temperature without current. This zone grew through the sample toward the anode with annealing time. The opaque zone was almost the single phase of tetragonal t' phase and contained many microcracks and some large cracks. On the other hand, when annealed at 1650°C with current and cooled to about 1000°C with current and then to room temperature without current, the single crystal turned brown and did not show microstructure change and transformation to t' phase. When this sample was annealed again without current, an opaque zone of t' phase was formed at the cathode. It is presumed that the transformation to t' phase occurs when reduced samples are oxidized at high temperature.