New Aspects of the martensitic Transformation of Zirconia

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Zirconia doped with a small amount of yttria exhibits both isothermal and athermal burst type transformation depending on being heated at around 300 ℃ or subzero cooled. In this work, the effect of yttria content, grain size and the specimen size were quantitatively measured for both modes of transformation to look insight into the nucleation of martensite. Small spherical specimens of 0.3〜0.8 mm diameter containing 1.4〜1.6 mol%Y_2O_3 and the range of grain size 0.5〜1.0 μm were prepared. The isothermal transformation kinetics was studied by micro-dilatometer during isothermal holding and Mb temperature was measured by video recording the burst transformation together with temperature while the specimens were continuously cooled.Isothermal transformation started after some incubation time and followed a sigmoidal curve until its completion in a relatively short time. A transformation start time exhibited a typical C-curve in a TTT diagram. The C-curve shifted toward short time side with decreasing yttria content and with increasing grain size. The effect of aging during the incubation period was found cumulative, suggesting a continuous growth of nucleation during the incubation period. The Mb temperature was also significantly increased with decreasing yttria content and with increasing grain size, suggesting a significant destabilizing effect of the reduction of yttria content and grain growth. On the other hand, Mb temperature decreased only slightly with decreasing specimen size. The Weibull modulus for Mb was 80, which was considerably higher than that for the fracture strength of a similar material (m〜6). This suggests that the embryos of the martensite have an essentially different distribution from those defects responsible for the fracture.