Compression Deformation Behavior Of Ultrafine Grained Pure Zirconium Processed By ECAP And Rotary Swaging At Room Temperature

Ultrafine grained pure zirconium（UFG pure Zr）with the grain size of 200-250 nm was prepared by ECAP+rotary swaying at room temperature.The mechanical behavior and microstructure evolution of UFG pure Zr under quasi static and dynamic compression were studied.The internal temperature in the shear zone was estimated.Finally,the Arrhenius model and the neural network model were used to establish the UFG pure Zr constitutive model respectively.The quasi static compression tests were conducted in the temperature range of 298-723K and strain rates of 10⁰s^-11 to 10^-4s^-1.The results show that true stress-true strain curves of UFG pure Zr exhibit the obvious steady-state flow characteristics.The flow stress increases rapidly with the strain at the initial stage followed to be constant after a peak value.The flow stress in the steady-state decreases with the increase of temperature and decrease of strain rate.Strain rate sensitivity value,m,of the UFG pure Zr is in the range of 0.028-0.132 and higher than that of the coarse-grained counterpart.The low strain rate and high temperature are beneficial to improve the plasticity of UFG pure Zr.The dislocation density in the grain of UFG pure Zr gradually reduces and the grain boundaries became clear and sharp and the grains grow up with the increasing the temperature and decreasing the strain rate,which shows that obvious dynamic recrystallization occurs.The dynamic compression tests were deformed dynamically in different strain rates of800⁴000 s^-1 at room temperature.The results show that dynamic true stress and true strain curves of UFG pure Zr can be divided into four stages such as high working hardening rate stage,stress collapse stage,continuous hardening stage and adiabatic shear failure stage.The formation of the adiabatic shear bands is the main reason for the failure of UFG pure Zr.The highest temperature within the shear bands of UFG pure Zr is 592K with thermal diffusion.The artificial neural network（ANN）and Arrhenius constitutive equation were used for establishing constitutive model of UFG pure Zr,respectively.The optimal configuration of the ANN model is 3×12×12×1.The Arrhenius constitutive equation of UFG pure Zr is as followed.ε′=79261.68[sinh（0.003902σ）]^17.18751 exp（-74.06993/RT）The average absolute relative errors obtained from the ANN model and Arrhenius constitutive equations are 0.556%and 4.99%,respectively.The correlation coefficient of the ANN model and Arrhenius constitutive equations are 0.999 and 0.966,respectively.It means that the ANN model can more accurately describe the constitutive relations of UFG pure Zr.