SOLID-STATE REACTION, MICROSTRUCTURE AND PHASE-RELATIONS IN THE ZRO2-RICH REGION OF THE ZRO2-YB2O3 SYSTEM

Phase relations below 1700-degrees-C in the ZrO2-rich region of the zi rconia-ytterbia system have been established using thermal expansion, room-temperature X-ray diffraction, precision lattice parameter measur ements and microscopic observations. The solubility limits of ytterbia in both monoclinic and tetragonal zirconia were determined. A eutecto id reaction, tetragonal zirconia solid solution-->monoclinic + cubic z irconia solid solutions at 400 +/- 20-degrees-C and 2.4 mol% ytterbia was found. The left-hand boundary of the cubic zirconia solid-solution field was redetermined between room temperature and about 1700-degree s-C. Long-range ordering was present at 40 mol % ytterbia and the form ation of an ordered phase, Zr3Yb4O12, isostructural with M7O12-type co mpounds was found. Its thermal stability was established between room temperature and 1630 +/- 10-degrees-C, in which it decomposes into cub ic zirconia solid solution by an order-disorder reaction.