FERROELASTIC DOMAIN SWITCHING AND PHASE TRANSFORMATIONS IN Ce-TZP. E.H. Kisi, Department of Mechanical Engineering, The University of Newcastle, Callaghan, NSW 2308, Australia; S.J. Kennedy and C.J. Howard, Australian Nuclear Science and Technology Organisation, Menai, NSW 2234, Australia

A ceria-tetragonal zirconia polycrystal (Ce-TZP) ceramic has been studied, by neutron powder diffraction, under uniaxial compressive stresses to 1.6 GPa. At stresses above about 1.2 GPa, ferroelastic switching of tetragonal material and the tetragonal to monoclinic phase transformation were both observed. The stress-strain plot was simultaneously recorded making use of a strain gauge parallel to the applied stress. Neutron diffraction patterns were analysed by the Rietveld method, the parameters of interest being the lattice parameters (responding to the stress), the preferred orientation parameter for the tetragonal phase (a measure of the ferroelastic switching), and the fraction of monoclinic phase. At stresses to 1.2 GPa, the stress-strain plot was essentially linear, and the sample remained entirely in the tetragonal phase. There was some deterioration in the Rietveld fit with increasing stress, attributed to elastic anisotropy in the tetragonal phase. At stresses above 1.2 GPa, there was a change in slope of the macroscopic stress-strain curve, a change in the value of the preferred orientation parameter (onset of ferroelastic switching), and the appearance of monoclinic phase. Lattice parameters showed anomalous variation in the same stress range. Both ferroelastic switching and tetragonal to monoclinic transformation were partly reversed when the stress was removed. It is interesting that tetragonal ferroelastic switching and the tetragonal to monoclinic transformation both occur in Ce-TZP. It may be that one of these processes triggers the other, but it is not clear from the measurements reported here which occurs first. A similar study of Y-TZP (Cain, Bennington, Lewis and Hull, Philos. Mag, 1994, B 69, 499-507) has indicated a higher threshold stress for ferroelastic switching and no monoclinic phase, suggesting that the tetragonal to monoclinic phase transformation in Ce-TZP may act as the trigger for the switching.