Effect of the Strength of the Parent Phase on the DrivingForce for the Martensitic Transformation of Partially Stabilized Zirconia

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1. The effect of the composition and the grain size on the strength of the parent phaseTZP (Tetragonal Zirconia Polycrystals) specimens were prepared with five different composition, namely 3Y,0.75Y-3Ce, 1.5Y-6Ce, 2.25Y-9Ce, and 12Ce, where Y and Ce denote mol% of Y_2O_3 and CeO_2 in zirconia, respectively. The grain size of the specimens was modified by post sinter annealing.The strength of the tetragonal phase as measured by Vickers hardness tester decreased monotonically with increasing ceria content (or decreasing yttria content), but the rate of the decrease became increasingly steep with increasing ceria content. The grain size dependence was reasonably well approximated by the Hall-Petch relation. Thus, the hardness of the two extreme compositions was represented by the following empirical equations :Hv=1250-0.90 (T-293) +100 (d^<-0.5>-1.21) for 3y-TZPHv= 950-0.68 (T-293) +430 (d^<-0.5>-0.72) for 12Ce-TZPwhere T denote temperature in K and d grain size in micrometer.2. The effect of the composition on the transformation temperatureThe specimens used in the above experiment did not transform by cooling as low as liquid nitrogen temperature except a few. Thus, specimens with lower yttria content were prepared for the Ms and As temperature measurement. These temperatures were represented by the follwing equations :As=1459-269Y for0<Y<3Ms=1279-364Y for0<Y<1.5Since the Ms of 2Y was below 77K,the driving force as measured by As-Ms rapidly increased when Y content exceeds 2mol%. This is unlikely to be explained by the free energy change or hardness increase with the increasing Y content. A further study is necessary to explain this drastic change of Ms temperature.