Yttria superplasticity

The first observation of superplasticity in a 3 mol% yttria-stabilized tetragonal zirconia polycrystal ceramic (YTZP) with a grain size of 0.4 pm was reported by Wakai et al in 1986 (Fig. 16.1). Since then, a large number... 

For instance, dislocations have been shown to play a key role in the accommodation process in YTZP, justifying the threshold stress in YTZP, in contrast with the hypothesis that this threshold stress is due to the electric field created by impurity segregation. However, dislocations are not systematically observed in YTZP furthermore it was shown that in yttria-stabilized tetragonal zirconia single crystals, the stress necessary to activate dislocations at 1400°C was over 400 MPa, one order of magnitude higher than the stresses used during superplastic deformation of YTZP at the same temperature. It will be necessary to conduct a systematic study of the microstructure of the monolithic ceramics such as YTZP before and after deformation and to correlate their relationship with the superplastic features. 

Wakai, F., Sakaguchi, S., and Matsuno, Y., Superplasticity of yttria-stabilized tetragonal Zr02 poly crystals , Adv. Ceram, Mater., 1986, 1, 259-63. 

Jimenez-Melendo, M., Dominguez-Rodriguez, A., and Bravo-Leon, A., Superplastic flow in fine-grained yttria-stabilized zirconium poly crystals constitutive equation and deformation mechanisms , J. Am. Ceram. Soc., 1998, 81, 2761-76. 

Owen, D.M., and Chokshi, A.H., The high temperature mechanical characteristics of superplastic 3 mol% yttria stabilized zirconia , Acta Mater, 1998, 46, 667-79. 

Jimenez-Melendo, M., and Dominguez-Rodriguez, A., High temperature mechanical characteristics of superplastic yttria stabilized zirconia. An examination of the flow process , Acta Mater, 2000, 48, 3201-10. 

Berbon, M.Z., and Langdon, T.G., An examination of the flow process in superplastic yttria-stabilized tetragonal zirconia , Acta Mater, 1999, 47, 2485-95. 

Dominguez-Rodriguez, A., Bravo-Leon, A., Ye, J.D., and Jimenez-Melendo, M., Grain size and temperature dependence of the threshold stress for superplastic deformation in yttria-stabilized zirconia polycrystals , Mater. Sci. Eng, 1998, A247, 97-101. 

Morita, K., and Hiraga, K., Reply to comment on the role of intragranular dislocations in superplastic yttria-stabilized zirconia , Scripta Mater, 2003, 48, 1403-7. 

Mimurada, J., Nakano, M., Sasaki, K., Ykuhara, Y., and Sakuma, T., Effect of cation doping on the superplastic flow in yttria-stabilized tetragonal zirconia poly crystals , J. Am. Ceram. Soc, 2001, 84, 1817-21. 

Figure 15.1 As-received and tensile-deformed specimens of superplastic yttria tetragonal zirconia polycrystals (YTZP), as displayed by Wakai et al., in 1986. Illustration courtesy of Prof Wakai.

To conclude this section on superplasticity in yttria stabilized zirconia, it may be of interest to show the amount of strain expressed as elongation in superplastic 5 wt% Si02 TZP, in which the extent of elongation is 1038 % at 1400 °C (Fig. 6.49). 

F. Wakai, S. Sakaguchi, and Y. Matsuno, Superplasticity of Yttria-Stabilized Tetragonal ZrOj Polycrystals, Adv. Ceram Mater., 1,259-263 (1986). 

Y, Ma and T. G Langdon, A Critical Assessment of Flow and Cavity Formation in a Superplastic Yttria-Stabilized Zirconia, Acta Metall. Mater., 42,2753-2761 (1994). 

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