Research On Superplastic Deformation Of 3YSZ Ceramics Under DC Electric Field

Structural ceramics have many excellent properties,such as high strength,low density,high temperature resistance,wear resistance and oxidation resistance.They are the preferred materials for long-term work in various harsh environments,and their application fields are very wide.However,the intrinsic brittleness of ceramic materials makes it difficult to form large and complex components by plastic processing.In addition to the extremely high deformation temperature required,the processing efficiency is usually low.Moreover,there are often many defects in the deformed components,which greatly reduces the performance of the components.These aspects have always been the bottleneck restricting the development of ceramic plastic processing.In this dissertation,3YSZ is subjected to constant load tensile and constant speed tensile by applying a DC electric field at low temperature（900°C）.The effects of current density,strain rate and stress on the superplastic deformation behavior and microstructure of 3YSZ were studied,and the deformation mechanism was explored.The main results are as follows:The application of a DC electric field can significantly improve the plasticity of 3YSZ ceramics at low temperature,which not only reduces the flow stress during high strain rate deformation by an order of magnitude,but also greatly increases the elongation after fracture.When the current density is 200 mA/mm²,an elongation of 355%can be obtained at an initial strain rate of 10^-2 s^-1,and the elongation is even as high as 425%at an initial strain rate of 5×10^-3 s^-1.Since the electric field can effectively suppress the development of cavity,the steady-state strain can reach 300% under certain conditions when the 3YSZ is subjected to constant load tensile.It indicates that the application of DC electric field to plastic processing of 3YSZ at low temperature has a good application prospect.Comparing the superplastic deformation behavior exhibited by 3YSZ under different current density conditions,it is found that the improvement of plasticity by electric field is not only due to the temperature rise of the sample caused by Joule heat,but also the promotion effect of electric field on mass transport,and the latter plays a major role.However,the electric field also changes the grain growth rate,thereby suppresses deformation to some extent.By comparing the evolution of microstructure under different current densities,it is found that the greater the current density,the more obvious the inhibition effect on the cavity nucleation,and thus the steady-state strain.In addition,it is also found that the static and dynamic grain growth rates are proportional to the current density.Under the same current density,the rate of dynamic growth is much faster than that of static growth,and both the static and dynamic growth show that the grain size of the negative electrode region is slightly larger than that of the positive electrode region.The deformation activation energy of 3YSZ ceramics measured at initial stresses of 1.5 MPa and 2.5 MPa in the range of 1270¹400℃ is 275 kJ/mol and 280 kJ/mol,respectively,which is close to the grain boundary diffusion activation of Zr⁴⁺in tetragonal zirconia.It can be concluded that the low temperature superplastic deformation rate of 3YSZ ceramic under DC electric field is controlled by the grain boundary diffusion of Zr⁴⁺.The stress exponent n at 160 mA/mm² and 220 mA/mm²current density is 1.62 and 1.46,respectively.Combined with the microstructure characteristics after deformation,it is considered that the low temperature superplastic deformation mechanism of 3YSZ ceramics under DC electric field is grain boundary sliding accommodated by diffusion.In addition,the movement of dislocation may also play a partial coordinating role in grain boundary sliding during deformation.