Mechanical Properties And Microstructure Evolution Of Cu-rich Nano-precipitate Strengthened Steel At High Strain Rates

The Cu-rich nano-precipitate strengthened steels possess good comprehensive mechanical properties under static loading conditions because of its unique alloy design and strengthening method.But the force situation,deformation mechanism and static loading will become different when it is subjected to high strain rate loading.Therefore,investigating the mechanical properties and microstructure evolution of Cu-rich nano-precipitate strengthened steel under dynamic loading has an important guiding significance for evaluating its high-speed impact properties and failure mechanism.In this paper,the influence of matrix phases（type and grain size）and Cu-rich nano-precipitates on the dynamic mechanical properties of nano-precipitate strengthened steel are studied,and the morphology and formation mechanism of adiabatic shear zone are characterized and discussed.The following conclusions are drawn:The yield strength and maximum flow stress of the quenched martensite Cu-rich nano-precipitate strengthened steel are higher than those of the tempered martensite when the strain rate is 2500 s^-1～4500 s^-1,and then with the increase of strain rate,the maximum flow of these two states are almost the same.When the strain rate reaches 6000 s^-1,a transition shear band appears in the quenched state,while the tempered state remains the deformed shear band.In the dynamic deformation process,compared with the maximum flow stress,the effect of grain size mainly affects the yield strength.The small grain size can effectively increase the yield strength of the Cu-rich nano-precipitate strengthened steels under high strain rate impact.During the high temperature tempering process,when the tempering time is less than 1 h,the strength of the Cu-rich nano-precipitate strengthened steels at high strain ratesreaches a peak value.As the tempering time increases,the strength continues to decline first,and eventually reaches a stress platform.The maximum flow stress of the over-aged Cu-rich nano-precipitate strengthened steel（preset nano-precipitates）at the same strain rate is higher than that of the solid solution state（without nano-precipitates）,indicating that the nano-precipitates can significantly increase the strength of the Cu-rich nano-precipitate strengthened steel during dynamic deformation by hindering the dislocation slip during dynamic deformation.When the strain rate is lower than4000 s^-1,the increased strength of the peak aged Cu-rich nano-precipitate strengthened steel is not obvious.With the increase of the strain rate,the contribution of precipitates to the dynamic mechanical properties increases significantly,and the strength of copper-rich nano-precipitate strengthened steel increases by 726 MPa at the strain rate of 6000 s^-1 and has not reached the strength peak,indicating that precipitates can still further strengthen the strength of the Cu-rich nano-precipitate steel at higher strain rates.The strain rate sensitivity coefficient of the over-aged state first increases rapidly and then slows down,and finally changes to a negative value when strain rate is greater than 5200 s^-1.The strain rate sensitivity of the peak aging state rises slowly at the initial stage,and when the strain rate is greater than 4000 s^-1,it the strain rate sensitivity coefficient sharply increases,showing a relatively large positive strain rate sensitivity.After high-speed impact,the Cu-rich nano-precipitate strengthened steels in all the states have a significant increase in hardness compared to that before impacting.The change of hardness value with strain rate is consistent with the strength change law at corresponding state.There is no obvious structural change in each state when the strain rate is lower than 5200 s^-1.The adiabatic temperature in the sample also gradually increases as the strain rate increases,and the adiabatic temperature rise formula is corrected.After the correction,the critical strain rate generated by the adiabatic shear band of the cylindrical sample is 5200 s^-1,corresponding to the critical temperature in the local area is 835 K.Later,the shear bands in the samples in each state become more obvious as the strain rate increases.The shear band provides a preferential way and direction for crack generation and propagation.The interior of the shear band of Cu-rich nano-precipitate strengthened steel is composed of substructures and fine equiaxed crystals.The dislocation density is low and dynamic recrystallization occurs.The severely elongated grains have high dislocation density and large amounts of plugging.No element enrichment or depletion occur in the center of the adiabatic shear zone and the transition zone.Due to the combined effect of highly concentrated stress and adiabatic temperature rise,a rotating recrystallization mechanism occurs inside the shear zone,resulting in the grain refinement and the generation of nano-scale equiaxed grains.Afterwards,with the decrease of stress concentration and adiabatic temperature rise,the formation mechanism of the transition zone changes into a dynamic recovery mechanism.Under the combined action of dynamic recrystallization and dynamic recovery,the Cu-rich nano-precipitate strengthened steel forms an adiabatic shear band.