[SCG69-08] 電場印加による酸化物多結晶体の超塑性流動の促進
キーワード：酸化物、粒界、超塑性、電場、拡散
In polycrystalline oxides, high temperature plastic deformation often occurs by diffusional mechanism such as diffusional creep and grain boundary sliding. In fine-grained oxides with the grain size of typically less than 1 μm can exhibit superplastic flow, in which large elongation to failure is achieved in tensile manner.
It has been recently pointed out that diffusional mass transport is highly accelerated by the application of strong electric field. For instance, sintering densification of zirconia (ZrO2) polycrystal is enhanced by the application of DC field of 40 V/cm, and highly enhanced by DC field beyond 60 V/cm; the densification finishes within 10 seconds at 850°C under the field of 120 V/cm, while the densification is completed by conventional sintering at 1400°C for a few hours without electric field (M. Cologna et al., J. Am. Ceram. Soc., 93 (2010) 3556). The abrupt densification under strong electric field is called flash event.
We have demonstrated that by applying a 190 V/cm DC field, conventional Y2O3-stabilized tetragonal ZrO2 polycrystal with the grain size of about 0.4 μm may exhibit superplastic deformation with an elongation to failure of >150%, at a lower furnace temperature of 800°C and a higher strain rate of 2×10−3 1/s compared to previous methods (H. Yoshida and Y. Sasaki, Scripta Mater., 146 (2018) 173). Analysis of the grain growth with and without the applied DC field revealed that the field-activated superplastic flow observed in this study was attributed to enhanced self-diffusion in ZrO2 due to the applied DC field. In addition, high temperature three-point flexural test in the tetragonal ZrO2 polycrystal at lower current density indicated that the electric field and/or current was likely to enhance the grain boundary sliding; even taking into account the specimen temperature, the strain rate was accelerated, and the flow stress was reduced by applying the electric field/current. Since application of electric field enhances diffusional atomic transport in ceramics, it would seem that the electric field and/or current could accelerate the grain boundary sliding of the polycrystalline oxide.
It has been recently pointed out that diffusional mass transport is highly accelerated by the application of strong electric field. For instance, sintering densification of zirconia (ZrO2) polycrystal is enhanced by the application of DC field of 40 V/cm, and highly enhanced by DC field beyond 60 V/cm; the densification finishes within 10 seconds at 850°C under the field of 120 V/cm, while the densification is completed by conventional sintering at 1400°C for a few hours without electric field (M. Cologna et al., J. Am. Ceram. Soc., 93 (2010) 3556). The abrupt densification under strong electric field is called flash event.
We have demonstrated that by applying a 190 V/cm DC field, conventional Y2O3-stabilized tetragonal ZrO2 polycrystal with the grain size of about 0.4 μm may exhibit superplastic deformation with an elongation to failure of >150%, at a lower furnace temperature of 800°C and a higher strain rate of 2×10−3 1/s compared to previous methods (H. Yoshida and Y. Sasaki, Scripta Mater., 146 (2018) 173). Analysis of the grain growth with and without the applied DC field revealed that the field-activated superplastic flow observed in this study was attributed to enhanced self-diffusion in ZrO2 due to the applied DC field. In addition, high temperature three-point flexural test in the tetragonal ZrO2 polycrystal at lower current density indicated that the electric field and/or current was likely to enhance the grain boundary sliding; even taking into account the specimen temperature, the strain rate was accelerated, and the flow stress was reduced by applying the electric field/current. Since application of electric field enhances diffusional atomic transport in ceramics, it would seem that the electric field and/or current could accelerate the grain boundary sliding of the polycrystalline oxide.