ENHANCEMENT OF SUPERPLASTIC FLOW IN TETRAGONAL ZIRCONIA POLYCRYSTALS WITH SIO2-DOPING

Superplasticity in SiO2-doped tetragonal zirconia polycrystal (2.5Y-TZ P) is investigated by means of tensile testing in a temperature range 1200-1500 degrees C. The grain boundary SiO2 phase reduces the flow st ress and greatly enhances the tensile ductility in TZP. The stress and grain size exponents lake a value close to 2 and 3, respectively, in this temperature range, but there is an abrupt change in activation en ergy at 1380 degrees C in SiO2-doped TZP. The activation energy for su perplastic flow above and below this temperature is estimated to be 18 2 kJ/mol and 635 kJ/mol. The enhancement of superplasticity due to SiO 2-doping is explained in terms of the accelerated plastic flow in the grain boundary SiO2 phase to accommodate the stress concentration gene rated by grain boundary sliding. The elongation to failure in the SiO2 -doped TZP is phenomenologically described as a function of the Zener- Hollomon parameter. At high temperatures and low strain rates, the cav ities tend to align and interlink parallel to the tensile axis, and th e huge elongation in excess of several hundred percent is obtained.