Effects of residual pores and optical birefringence on transmission through translucent polycrystalline alumina have been studied with the Mie scattering theory. The in-line and total forward transmissions of the translucent polycrystalline alumina were simulated as a function of porosity, pore radius, and grain size. The model revealed that porosity has a significant effect on both the total forward scattering and the in-line transmission, whereas grain size affects only in-line transmission. The calculated transmittance is in good agreement with the experimental values. The in-line transmission gradually increased with a decrease in grain size, and the effect of birefringence on the total forward transmission is negligible in the visible spectrum. The total forward transmission in the visible spectrum is mainly governed by residual pores.

Static grain growth behavior in 1 mol% of GeO 2 , TiO 2 , MgO or BaO-doped ZrO 2 -3 mol%Y 2 O 3 (3Y-TZP) was examined at 1400 C with a special interest in dopant effect on superplastic flow stress in fine-grained 3Y-TZP. The static grain growth can be described as normal grain growth in single-phase ceramics, and growth constant K for each material is in the order of 10% flow stress of the superplastic flow. The value of K in cation-doped TZP is correlated well with dopant cation's ionic radius. Assuming activation energy for diffusivity of constituent ion can be given as a linear function of strain caused by difference in the ionic size of dopant cation, the dependence of the growth constant and the flow stress on the ionic radius can be described as a function of the ionic radius of the dopant cation. The activation energy for the diffusivity in cationdoped TZP estimated from the calculation is in good agreement with the experimental data. The small dopant effect on the superplastic flow stress is well described by the activation energy as the function of the dopant cation's ionic size.

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Superplastic behavior in a fine-grained, GeO 2 -doped 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) with the dopant level of 0.2-3 mol% was examined at 1400 C under an initial strain rate of 1:3 Â 10 À4 s À1 . Microstructure and chemical composition at the grain boundaries was examined by high-resolution electron microscopy (HREM) combined with an X-ray energy dispersive spectrometer (EDS). No secondary phase was observed along the grain boundaries, though EDS analysis indicated the segregation of Ge cations along the grain boundaries. The Ge content at the grain boundaries tends to increase with increasing the total amount of GeO 2 addition, but saturate over the doping level of 2 mol%. Dependence of flow stress reduction on the total amount of GeO 2 addition has a good correlation with Ge content at the grain boundaries. This fact indicates that the GeO 2 -doping effect on the flow stress in 3Y-TZP is caused mainly from the grain boundary segregation of Ge cations.

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