| The kinetics of precipitation of both tetragonal zirconia (ZrO(,2)) and CaZr(,4)O(,9) ((phi)(,1)) from cubic calcia-stabilized zirconia solid solutions were studied using transmission electron microscopy (TEM). A hypo-eutectoid (8.5 mole % CaO) hot-pressed polycrystal was used to investigate precipitation of tetragonal zirconia, and a hyper-eutectoid (16 mole % CaO) single crystal was used to investigate precipitation of (phi)(,1). The hypo-eutectoid material could not be quenched rapidly enough from the solution treatment temperature to retain the cubic solid solution; 1-2 nm particles of tetragonal ZrO(,2) were present in quenched samples. Therefore, the precipitation of tetragonal particles could not be characterized from the nucleation stage. The coarsening of tetragonal particles proceeded by diffusional growth following the Greenwood-Lifshitz-Slyozov-Wagner (GLSW) theory, with an apparent activation energy of 425 KJ/mole, close to the activation energy for diffusion of calcium and zirconium in calcia-stabilized zirconia. The interfacial energy between the cubic solid solution and the tetragonal precipitates was estimated from the GLSW theory as 0.5 J/m('2).; Fine particles generally retained tetragonal symmetry upon cooling to room temperature. However, after extended annealing treatments at 1300(DEGREES)C and 1400(DEGREES)C, some of the coarse particles transformed to either monoclinic zirconia, or to a new phase, designated zeta ((zeta)). Zeta phase may arise as a result of strain-induced transformation during specimen preparation, or may possibly be a transformation product that developed during cooling from the annealing temperature.; The precipitation and growth of (phi)(,1) proceeded by nucleation and diffusional growth, and an isothermal transformation (IT) diagram showing the time required for initiation of the reaction as a function of annealing temperature was constructed. The activation energy for (phi)(,1) coarsening was estimated from the GLSW theory to be 390 KJ/mole, essentially the same as that for the coarsening of tetragonal zirconia precipitates and also corresponding to the diffusion of calcium and zirconium. The interfacial-energy between the cubic solid solution and (phi)(,1) was approximately 0.2 J/m('2). The space group of (phi)(,1) was determined through the use of convergent beam electron diffraction (CBD) to be C2/c, identical to CaHf(,4)O(,9).; Prominent diffuse scattering originated from the cubic solid solution in both hypo- and hyper-eutectoid materials. In hyper-eutectoid material, the diffuse intensity sharpened into discrete (phi)(,1) reflections but was still present in hypo-eutectoid material after precipitation of tetragonal ZrO(,2). The origin of diffuse scattering in the latter case is not fully understood. |
