AGING BEHAVIOR OF T'-PHASE IN A HOT-PRESSED ZRO2(4 MOL-PERCENT Y2O3) CERAMIC

The ageing behaviour of unequilibrium tetragonal (t') phase and its re sultant effect on the mechanical properties of hot-pressed ZrO2(4 mol % Y2O3) ceramic have been investigated by means of transmission electr on microscopy (TEM), scanning electron microscopy (SEM) and X-ray diff ractometry (XRD). Experimental results show that t'-phase which is the product of diffusionless transformation from cubic (c) phase during r apid cooling after sintering is unstable when aged in a temperature ra nge of 1400-1600 degrees C for up to 80 h in that it decomposes diffus ionally into equilibrium tetragonal(t) phase and c-phase. Yttria conte nts of phases formed during decomposition are basically in agreement w ith those indicated by phase diagram. The stability of t'-phase charac terized by the existence of anti-phase domain microstructure under the microscopic dark field image is significantly associated with the tet ragonality(cla) measured by XRD and the larger the tetragonality, the more unstable the t'-phase. Metastable precipitates of t-phase a re tr iggered by applied stress to transform to monoclinic (m) phase during which the fracture toughness is enhanced and transformability of t-pha se is critically dependent upon the solute content as well as size. It is found that when t'- and m-phase coexist with adequate fractions of c- and t-phase, the fracture toughness of the aged specimen demonstra tes a peak value that moves to shorter ageing times with increasing te mperature while the Vickers hardness decreases monotonically with agei ng time regardless of ageing temperature due to grain growth.