LOW-TEMPERATURE PHASE-EQUILIBRIA BY THE FLUX METHOD AND THE METASTABLE-STABLE PHASE-DIAGRAM IN THE ZRO2-CEO2 SYSTEM

Low-temperature phase equilibria ranging from 1000-degrees to 1200-deg rees-C in the ZrO2-CeO2 system were investigated by annealing composit ionally homogeneous ZrO2-CeO2 solid solutions in a Na2B2O7.NaF flux. T he 5 mol% CeO2 samples decomposed into monoclinic (m) and tetragonal ( t) phases during annealing at 1100-degrees and 1120-degrees-C, and the t-phase transformed diffusionlessly into monoclinic (m') symmetry dur ing quenching. A eutectoid reaction, t --> (m + c), was confirmed to o ccur at 1055-degrees +/- 10-degrees-C, where the equilibrium compositi ons of the t-, m-, and c-phases were 11.2 +/- 2.8, 0.9 +/- 0.9, and 84 +/- 1 mol% CeO2, respectively. The equilibrium phase boundaries were almost independent of the annealing time and/or the flux:sample ratio, which indicates that the flux accelerates the reaction rate without a ffecting the equilibration. The previous data are discussed using meta stable-stable phase diagrams. The discrepancies of the low-temperature phase diagram in the literature are attributable to either regarding the metastable phase boundaries as stable ones or ignoring the sluggis h kinetics.