Dynamic Deformation Behavior And Constitutive Model Of 25CrMo4 Steel

25CrMo4 steel is widely used in high-speed train axles.An important aspect of high-speed train movement is that the axle inevitably needs to withstand impact loading with complex loading conditions.Therefore,it is imperative to investigate the dynamic mechanical properties of the material used to construct the axle at impact load conditions.In this study,a PRL100 material testing machine and split-Hopkinson pressure bar(SHPB)experimental system were used to conduct quasi-static and impact compression experiments on 25CrMo4 steel.The experimental results indicate that there is no strain rate effect on 25CrMo4 steel during quasi-static loading.However,25CrMo4 steel exhibits work hardening and apparent strain rate effects during impact compression.The yield strength and flow stress display a significant increase upon impact from the quasi-static state and with increase in the strain rate.The strain rate sensitivitylis independent of the strain.When the strain rate reaches approximately 4000 s^-1,the material exhibits a marked softening behavior,implying plastic deformation.In the impact compression experiment,25CrMo4 steel exhibited adiabatic temperature rise during the deformation process.Therefore,the Johnson–Cook constitutive model was modified by introducing adiabatic temperature rise as a parameter,based on the relationship between strain,strain rate,and adiabatic temperature rise.The coupling relation between the strain,strain rate,and temperature in the Johnson–Cook constitutive model was optimized so that the improved model can characterize the plastic deformation process of the material under impact load.The experimental results proved that the model can effectively describe the mechanical properties of 25CrMo4 steel during impact loading.In this paper,the strain rate correlation of the yield strength and flow stress of 25CrMo4steel during impact compression is explained based on the thermal activation theory.The internal plastic flow of the metal is essentially a thermal activation process,wherein the dislocation attempts to cross the short-range Peierls–Nabarro barrier.The thermal activation motion of the dislocation to overcome the Peierls–Nabarro stress is analyzed.The results illustrate that the thermal activation stress that must be overcome in the dislocation movement increases with increase in strain rate,resulting in a corresponding increase in yield strength and flow stress.The metallographic microstructure of 25CrMo4 steel at different loading strain rates was observed using a Zeiss Axio scope.A1.The experimental results combined with the observed microstructure indicate that 25CrMo4 steel,which is generally regarded as a BCC metal structure,in fact possesses both BCC and FCC metal structure characteristics;the plastic deformation behavior of 25CrMo4 steel is related to the structural characteristics.It is observed that the grains of 25CrMo4 steel are severely elongated and broken into deformation zones in the direction perpendicular to the compression during impact loading.The softening effect of the material at high strain rates is caused by damage in the form of deformation zones.Considering the characteristics of25CrMo4 steel with both BCC and FCC metal structures,the Z-A(BCC)and Z-A(FCC)constitutive models were unified.A damage variable was introduced into the constitutive model to characterize the softening effect caused by thermal activation damage evolution at high strain rates.Finally,the improved Z-A constitutive model was obtained,which can accurately characterize the dynamic mechanical properties of 25CrMo4 steel well.