MICROSTRUCTURAL EVOLUTION AND STABILITY OF TETRAGONAL PRECIPITATES INY2O3 PARTIALLY-STABILIZED ZRO2 SINGLE-CRYSTALS

The microstructure and morphology of tetragonal precipitates (t-ZrO2) in yttria partially-stabilized zirconia single crystals containing var ious amounts of Y-2 O-3 (3.4, 4.7 and 5.8 mol%) have been studied as a function of aging time at 1600 degrees C in air. The precipitate size and volume fraction of t-ZrO2 phase were determined using transmissio n electron microscopy. The evolution of the precipitate volume fractio n with aging time indicated that the precipitation reaction was comple ted after 24 h of annealing, in agreement with the values of the Y2O3 composition in the matrix measured by analytical electron microscopy. Further aging coarsened the precipitates which joined together forming fiber-like particles of several micrometers in length and remaining u ntransformed despite their large size. The stability of the t-ZrO2 pre cipitates against the tetragonal to monoclinic transformation in the Z rO2-Y2O3 system seems to be related to interactions between precipitat es due to coherency stresses rather than with their morphological char acteristics.