HIGH-VOLTAGE ELECTRON-MICROSCOPY IN-SITU STUDY ON THE PLASTIC-DEFORMATION OF PARTIALLY-STABILIZED TETRAGONAL ZIRCONIA

Zirconia single crystals, partially stabilized with 6.6 mol% Y2O3, are annealed at 1600 degrees C for 50 h. These crystals, which were investigated by high voltage electron microscopy, contain few relatively large precipitation colonies and many small precipitates regularly arranged in a tweed structure. The colonies consist of always two twin-related t-ZrO2 variants alternating and sharing the same {110} habit plane. The plate-like precipitates forming the colonies and the small precipitates of the tweed structure have a similar morphology, however, their size is different. At the outer border of the colonies small precipitates are arranged in the same way as the precipitates inside the colonies. In situ straining experiments in the high voltage electron micrscope (HVEM) at 1150 degrees C reveal that the precipitates in the colonies and in the tweed structure are transformed into that tetragonal variant with the c-axis nearest to the tensile direction. This process is very similar to the ferroelastic deformation observed in the so-called t'-ZrO2 polycrystals. It is assumed that after deformation tetragonal precipitates with a low Y2O3 content exist in a t'-matrix with a high concentration of Y2O3, with the c-axes of both having the same orientation.