Yu-ichi Yoshizawa

The microstructure and mechanical properties of yttria-stabilized zirconia prepared by arc-melting

The microstructure of ZrO2-Y2O3 alloys prepared by arc-melting was examined mainly by electron microscopy. It was found that the microstructure changed markedly with yttria content between 0 and 8·7 mol%. Pure zirconia was a single monoclinic phase, while ZrO2-8·7 mol% Y2O3 alloy was single cubic phase as expected from ZrO2-Y2O3 phase diagram. Tetragonal phase was found in alloys with 1 to 6 mol% Y2O3 together with monoclinic or cubic phase. The tetragonal phase found in present alloys normally had a lenticular shape with a length 1 to 5μm and a width 0.1 to 0.3μm, which is much larger than that formed by annealing. The phase with a herring-bone appearance was found in alloys with Y2O3 between 2 and 3 mol%, which was recognized to be a metastable rhombohedral phase. The structure of the present alloys is likely to be formed by martensitic or bainitic transformation during fairly rapid cooling from the melt temperature. The change in hardness and toughness with yttria content of the alloys is discussed on the basis of microstructural observations.

The modulated structure formed by isothermal ageing in ZrO2-5.2 mol % Y2O3 alloy

The modulated structure produced by isothermal ageing of ZrO2-5.2 mol % Y2O3 alloy was examined mainly by electron microscopy. It was found that the modulated structure was formed at ageing temperatures between 1400 and 1600° C, but not at 1700° C. The structure is developed by spinodal decomposition, which produces compositional fluctuation in the elastically soft 〈111〉 direction in cubic zirconia. The hardness increase caused by the development of modulated structure during ageing is larger than the hardening by precipitation of tetragonal phase in the cubic matrix.

The metastable two-phase region in the zirconia-rich part of the ZrO2-Y2O3 system

ZrO2-Y2O3 alloys with yttria contents between 2.0 and 6.3 mol% were prepared by arcmelting. The microstructure of the alloys after isothermal ageing was examined by electron microscopy. It was found that a modulated structure was formed in alloys aged at appropriate temperatures. The modulated structure resembles the structure of spinodally decomposed metallic alloys and ceramics. The range in which the modulated structure is developed is inside the cubic-tetragonal two-phase region of the ZrO2-Y2O3 system. The modulated structure is associated with metastable phase decomposition.