March 2004 Tetragonal-to-Monoclinic Transformation in Mg-PSZ Studied by in Situ Neutron Diffraction simply, for the tetragonal phase s13 >$12. This behavior contrasts load, were averaged over every 30 s. The averaged strain data were with 3Y-TZP and 12Ce-TZP37 with simpler phase compositions. hen plotted as a function of time at each applied stress. An where c/a was unchanged below the critical stress At >1100 MPa example of such curves is given in Fig. 6(a), showing th the c/a ratio in Mg-PSZ decreased by approximately three standard accumulation of vertical, horizontal, and volume strains at 800 deviations. Reasons for the decrease may be: (i) removal from the MPa, below the gross critical stress. Creep curves for samples diffraction pattern by transformation of the most highly stressed I loaded >800 MPa were fitted to functions using the model given articles, causing the average to shift to a lower value: (i)removal by Finlayson et al. 20 rom the diffraction pattern of the least-stabilized t particles, so Unlike the strain from the tensile creep samples. the strain in again the average shifts to lower values: or(ii) residual strain this compression test was not a direct function of applied stress imposed on I-phase particles by the transformed monoclinic phas from the step-wise loading. The fitting was to a modified Andrade If the latter effect is responsible, the residual stresses can be function estimated to be of order I 10 MPa (B) Internal Strain Distribution in the t Phas ∈=∈n+Brn uare(rms) width of the internal strain dis lculated from the tane component of the The fitted creep curves are shown in Fig. 6(b). Uobs. using the following equation: <A The volume strains caused by creep have been used to estimate amount of m phase formed from creep at each load on the right-hand scale in Fig. 6(b). The resemblance of the terminal em=m-bm×18042n2 values from Fig. 6(b)to those in Table I suggest that an overwhelming majority of the observed volume change in 9. 4Mg- where U. is the standard width from the instrument(0.31 when PSZ was from creep MRPD is used in high-intensity mode at a wavelength of 1. 67A The results shown in Fig. 5(b) indicate that the internal strain IV. Discussion distribution is not constant, even during elastic loading and unloading at <1000 MPa. although the graph in these regions is Mg-PSZ loaded in uniaxial compression shows almost purely parallel, Elastic anisotropy is believed to be responsible for the elastic deformation <800 MPa and well-developed plastic defor slope in this region. An increase in internal strain at >1000 MPa mation >1000 MPa. The plastic deformation is accompanied by a accompanied the r-m phase transformation. The overall increase volume expansion. In situ neutron diffraction has shown that the in internal strain of 0.05% is equivalent to an increase in rms plastic deformation is accompanied by a substantial amount of internal stress of-lo0 MPa stress-induced t- m transformation within the ceramic. No (C) Preferred Orientation: Previous uniaxial compression evidence was found in this material for ferroelastic switching of t ests with in situ neutron diffraction 1.2 on 3Y-TZP and 12Ce- crystallites nor was the competing t-o transformation that has TZP have shown considerable changes to the orientation distribu- been previously reported in tensile room temperature creep sam convenient measure of the degree of preferred orientation and from the volume change (assuming a 4.9% local volume change on density function due to March"obtained during the Rietveld tron diffraction data. The two estimates for the total amour refinements, In both cases referred to above, the ferroelastically transformed agree to within two standard deviations. It can switched domains oriented to diffract from the shorter a crystal therefore be concluded that the plastic strains are primarily due to lographic axes out of the neutron beam and switched the longer c the t-m transformation. coefficient of the t phase above the critical stress. By contrast, for as The transition from elastic to plastic deformation in Fig. 2 is not axes into the beam. This resulted in a strong decrease in the March 9. 4Mg-PSZ, the March coefficient of the t phase began very close stress takes. A value in excess of 1000 MPa is indicated by the to unity, indicating an initially random orientation of t crystallites. longitudinal strain in Fig. 2. whereas the transverse strain gives a The influence of the compressive stress is shown in Fig. 5(c)to be slight increase in the March coefficient. It can be concluded from this that ferroelastic reorientation of the I phase does not occur in observable amounts in 9.4Mg-PSZ during uniaxial compression. It is believed that the slight increase in the March coefficient is due to slight transformation-induced texture. I42 The March coefficient for a monoclinic structure is often x nadequate, as there is no a priori reason to select a particular pole axis or preferred orientation vector. It does, however, often serve as a useful correction to the calculated intensities by use of a vector chosen based on the observed intensities In these experiments, it was noted that the stress-induced m phase has a more prominent 2500 (II reflection than a random orientation. The number of other phases present and the initially small amount of m phase present made the refinement of meaningful preferred orientation coeffi cients as a function of applied stress difficult. Instead, a March coefficient was refined at the maximum m phase content, and this >1000 was used as a constant in all other refinements above the critical stress and during unloading. The preferred orientation was con firmed by one of the post-loading neutron diffraction patterns that was taken after the sample was turned 90 to put the compression 120160 axis of the sample in the diffraction plane. This pattern showed a Time(min far smaller( 11 1) peak in agreement with the above observations (5) Time-Dependent Strain/Creep Fig. 6. (a) Transverse strain. 800 MPa of 9 4Mg-PSZ showi As noted in the introduction, strains in zirconia ceramics have fitting of the volume strain at been reported to have a strong time-dependence. .- To assess the monoclinic phase fraction this, the strain data, recorded while the sample was held at each by 0. 049% for each I% increase in volume