466 Journal of the American Ceramic Society-Ma et al. Vol 87. No. 3 creep in metallic materials. The mechanisms involved are not active at room temperature and cannot explain the observed creep in structural zirconia ceramics. The behavior ously observed in Mg-PSZ was transient creep and the strains(both longitudinal and transverse)approximated a power law similar to that proposed by Andrade ∈=Br It was observed that the stress also approximated a power b 38≥ leading to a composite equation of the form III II III In I! III IlIl El Il The creep parameters were quite variabl 2000 It I lI1 1101011IIa sensitively on the processing and thermal history of the samples. Although creep in 12Ce-TZP was also observed to follow a 2e(degree er law such as Eq (1). the observations were made during a stepwise loading experiment and it was accompanied by large Fig. I. Rietveld refinement result for 9.4Mg-PSz(sample 2).The bursts of transformation that prevented a systematic study of the observed data are indicated by (+)and the calculated pattern by the solid stress dependence. Very much smaller creep strains were observed in 3Y-TZP 32.37 and the time constant for reversal was much he observed and calculated patterns on ame scale. The rows of shorter. The only analysis that could be conducted was in the final markers below the pattern show(from the top down) the positions of peaks unloading step from 500 MPa to 0 MPa, during which the strains from cubic. tetragonal, and monoclinic zirconia and the 6 phase decayed to zero in good accord with a creep-relaxation model (Mg2 Zr,O1) This paper presents the results of an in situ neutron diffraction study of time-dependent stress-induced microstructural changes in 9.4 mol% Mg-PSZ during uniaxial compression testing shown in Fig. 1. Observed data are shown as(+) and the calculated profile as a solid line, The curve below is a difference plot between the observed data and the calculation, Reflection IL. Experimental Methods markers are indicated with small bars for the cubic, tetragonal monoclinic, and 8-rhombohedral phases(from the top down). Samples used for this study were Ts grade 9. 4 mol%o Mg-PSZ ICI Advanced Ceramics, Melbourne, Australia). The ceramic preparation is a proprietary process, the precise details of which IlL. Experimental Results are commercially sensitive. The process involves firing at -1700 C followed by controlled cooling and an aging treatment at (1) Stress-Strain Curves 100C. Sample I was cut to a height of 15 10 mm from a The stress-strain curves of the second 9. 4Mg-PSZ are shown in 90-mm-diameter rod with a diamond-impregnated saw Sample Fig. 2. Here, the strains are those at the end of the constant-stress 2 was cut into a prism having a cross section of 9. 70 mm X 9.80 holds and have been corrected for gauge off-set and an apparently mm at both ends. After cutting, the samples were polished with erroneous value in the longitudinal gauge at low stress. In both 1200-grit emery paper to remove surface micro-cracks, which may samples, when the stress was smaller than 800 MPa, the longitu- become the potential source of collapse. In addition the edges and dinal and transverse strains were linear functions of the applied corners were ground with fine SiC paper to have smooth 0.5 mm stress Sample I gave a Youngs modulus of 205+8 GPa, and radius rounded edges. Poissons ratio 0.33+ 0.06 in the range -100-1000 MPa. For The experiments were conducted with the medium-resolution Sample 2, Youngs modulus was determined to be 217+3 GPa powder diffractometer (MRPD) at HIFAR(high-flux Australian and Poissons ratio is 0.35=0.01, measured in the range 200-800 reactor of the Australian Nuclear Science and Technology Orga MPa. When the applied stress was >1000 MPa, nonlinearity nization). The angle range for neutron diffraction was 4-104 2 indicates that the sample had begun to plastically deform. Some with a step size of 0. 1. Neutron wavelengths were 1.6663A for nonlinearity in the transverse strain data were already apparent Sample I and 1. 6676A for Sample 2, as determined from a between 800 and 1000 MPa. We conclude that the critical stress is standard rutile specimen marginally <1000 MPa. Unlike in the elastic region, where the Compression testing of sample I began with a holding stress of transverse strain was much smaller than the longitudinal strain after the onset of plasticity, the transverse strains were much 2. 8 MPa, then 100 MPa, followed by 50 MPa increments up to larger. The plastic strains were estimated by extrapolating the 000 MPa For sample 2, the applied stress sequence was 5.2. 200 400,600.800.1000,100.1200.1225,1200.1100.1000.900 800.700,600.530.480.420.380,250.180.100,0MPa, followed by post-loading neutron diffraction patterns to detect microstruc- tural changes immediately after the load was released Transverse Two strain gauges were glued to the surfaces of the two Horzontal amples. One, placed vertically, measured the longitudinal strain 4000 and the other, placed horizontally, measured the transverse strain. Strain data were recorded every 30 s(Datataker DT50, Data 0 Electronics, Victoria, Australia) and downloaded to a computer. Rietveld analysis was used to extract information from leutron diffraction data as outlined in previous publications. 3/5 Four phases were found in these les, cubic zirconia (c) tetragonal zirconia(n), monoclinic zirconia (m), and the rhomb- hedral 8-phase(Mg, Zr, O,2). The refined parameters were global 1200 parameters and scale factors of all the four phases. In addition, for Applied stress(MPa the major phases, lattice parameters, the breadth of the internal strain distribution, and the March coefficient for preferred orien Fig. 2. Stress-strain curves of a 9. 4Mg-PSZ sample during uniaxial tation were also refined. An example refinement for sample 2 is compression testing