Ageing behaviour of t′-phase in a hot-pressed ZrO2(4 mol% Y2O3) ceramic
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The ageing behaviour of unequilibrium tetragonal (t′) phase and its resultant effect on the mechanical properties of hot-pressed ZrO2(4 mol% Y2O3) ceramic have been investigated by means of transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffractometry (XRD). Experimental results show that t′-phase which is the product of diffusionless transformation from cubic (c) phase during rapid cooling after sintering is unstable when aged in a temperature range of 1400–1600 °C for up to 80 h in that it decomposes diffusionally into equilibrium tetragonal(t) phase and c-phase. Yttria contents of phases formed during decomposition are basically in agreement with those indicated by phase diagram. The stability of t′-phase characterized by the existence of anti-phase domain microstructure under the microscopic dark field image is significantly associated with the tetragonality(c/a) measured by XRD and the larger the tetragonality, the more unstable the t′-phase. Metastable precipitates of t-phase are triggered by applied stress to transform to monoclinic (m) phase during which the fracture toughness is enhanced and transformability of t-phase 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 demonstrates a peak value that moves to shorter ageing times with increasing temperature while the Vickers hardness decreases monotonically with ageing time regardless of ageing temperature due to grain growth.
KeywordsFracture Toughness Yttria Y2O3 Ageing Time Vickers Hardness
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