An ceru So.8713465-72(20 urna Tetragonal-to-Monoclinic Transformation in Mg-PSZ Studied by in Situ Neutron Diffraction Yuxiang ma and Erich H. Kisi Department of Mechanical Engineering. The University of Newcastle. Callaghan, NSW 2308, Australia Shane J. Kennedy and Andrew J Studer Neutron Scattering Group, Australian Nuclear Science and Technology Organization, Private Mail Bag I Menai NSW 2234, Australia The deformation of 9.4 mol% magnesia-partially-stabilized reviews. 0-1 The dominant mechanism responsible for the high zirconia under compressive loads up to 1225 MPa was studied toughness of Mg-PSZ is widely held to be transformation tough- using mechanical testing with in situ neutron diffraction. The ening from the r-m transformation. -Transformation toughen- material shows obvious plastic deformation at applied stresses ng was first proposed by Garvie et al. in Ca-PSZ. Since then, in excess of an estimated critical stress of 925 t 20 MPa. Most significant new toughening mechanisms, such as micro-crack of the accumulated strain occurred by transient room. shielding and crack deflection have been proposed to make an temperature creep. Plastic deformation was associated with important contribution to the process. The bulk of the experimental considerable stress-induced tetragonal-to-monoclinie transfor work has been conducted by examining samples using TEM and mation. The volume change calculated from the strain gauges XRD after they have been mechanically tested. -Stress- correlates well with the amount of t- m transformation induced martensitic transformations, such as 1-m, are highly observed. Unlike previous studies of Ce-TZP and Y-TZP, dependent on the local stress-strain state of the sample. It is well ferroelasticity was not observed, nor was the t-o transfor- mation observed. Minor microstructural changes were noted, known that different surface preparation techniques give quite cluding an increase in the root mean square internal strain of 0.05%, commensurate with an increase in internal stress of tion, and so it can be used to give the bulk-phase proportions in the -100 MPa. It would appear that transformation selectivity ceramic. Er situ neutron diffraction has bee en success ul in solving was exercised with the transformation occurring first in tet- ragonal crystallites favorably oriented to the applied stress. such problems as the phase composition, low temperature he st strong preferred orientation. Comparison is made with the Mg-PSZs. Room-temperature creep has been observed using strain-gauge techniques in samples of 9. 4Mg-PSZ subjected to other commercially interesting zirconia ceramics, Ce-TZP and tensile stresses. An overall volume increase in the sample Y-TZP, which have been studied using the same techniques. fi.e, negative Poisson's ratio) suggested that the creep and duced t-m transformation were related. This was lat L. Introduction confirmed by ex situ neutron diffraction measurements on regions taken from the gauge volume and the(low stress) gripped region AGNESIA-PARTIALLY-STABILIZED ZIRCONIA (Mg-PSZ) has re- of the samples. When a long period of time had elapsed between ceived considerable attention in the literature for its high the tensile testing and neutron diffraction, discrepancies were fracture toughness. The material is a complex mixture of the noted between the amount of m phase observed and the measured ambient pressure cubic (c), tetragonal (0), and monoclinic (m volume increase in the samp 28.79As the strain gauge and phases of pure zirconia. The active microstructural component is neutron observations were conducted separately, it was suggested lenticular precipitates of the t phase, which is present in a largely that stored elastic stresses in the microstructure had triggered the c phase matrix. Under an applied stress, the I phase is able to reverse m-I transformation on relaxation of the applied stress undergo a martensitic transformation to the m phase, accompanied Hence, in addition to the surface sensitivity highlighted above, the by a volume increase of.9%. In its most refined form, results from ex situ experiments may not reflect the real transient controlled cooling from the solution treatment temperature at behavior of the material. In situ neutron diffraction has the 1700C followed by an aging treatment leads to precipitation of capacity to overcome both problems at once and hence to capture dynamic changes in the phase proportions, structural parameter the anion-ordered 8-phase(Mg2Zr, O 2), which replaces a major and internal stress distribution of each phase at different stresses proportion of the c matrix. The 8-phase precipitates at the the interface, leading to localized strain, metastability, and destabili- Recently, in situ neutron diffraction studies have been con- zation of the I phase. . 9 ducted on the microstructurally simpler tetragonal zirconia poly The phase-equilibrium, microstructure, and mechanical proper- crystals 12Ce-TZP 0.3 and 3Y-TZP. 2 There it was observed that the stress-induced I- m transformation is both time-dependent and reversible in 12Ce-TZP and that both materials show a degree of ferroelasticity under compressive loads. 3Y-TZP did not un- dergo any observable t-m transformation under compressive R. Hannink--contnbuting editor loads up to 2.3 GPa. The results of similar in situ loading experiments have not previously been reported for Mg-PSZ. It is important to comment briefly on the creep observed in Mg-PsZ by ex situ measurements and that observed in 12Ce-TZP 87072 Received March 19, 2002: approved December 30, 202. and 3Y-TZP by in situ diffraction measurements. The high scien y d e hunterian Research Couneil and the Australian temperature creep of ceramics is not a new phenomenon, and it is an Ceramic society well understood using the theoretical framework developed for