Experimental Research On Dynamic Shear Properties And Softening Mechanism Of ECAP Ultra-Fine Grained Copper

With the progress of science and technology and economic development,the comprehensive performance of materials is more and more demanding.Grain refinement is one of the mechanism of material strength,usually the grain size is 100-1000 nm material,called ultra-fine crystal material,because the ultra-fine crystal material has fine microstructure,have unique mechanical properties and application prospects.The preparation methods and mechanical properties of ultrafine crystalline materials have become a hot topic in the field of materials science.In order to study the thermal stability of ultra-fine grained copper microstructure prepared by equal channel angular pressing(ECAP)and its effect on dynamic mechanical properties of materials.Using Hopkinson pressure bar and MTS hydraulic servo testing machine of ECAP ultrafine grained copper and annealed pure copper cap type shear strain control sample loading,combined with the digital image correlation method and "frozen" recovery sample micro and XRD analysis,the dynamic shear deformation behavior and microstructure evolution of research.Results showed that(1)The superfine copper after ECAP has the characteristics of strain hardening under quasistatic shear,but the shear stress-strain strain curve shows a softening characteristic under high strain rate.Dynamic recrystallization is the cause of strain hardening rate under high loading rate.The recrystallization temperature of ultra-fine crystal copper based on plastic work is only 325 k,so it is easy to produce adiabatic shear deformation under high strain rate.(2)The width of the recrystallization of the superfine copper shear zone under the same strain rate is linearly dependent on the strain;With the increase of strain rate of ultra-fine crystal copper yield strength and flow stress is becoming more and more big,the strain hardening rate increased with the increase of strain rate are getting smaller,when the strain of 3000 for the critical strain rate,strain hardening and strain softening of strain rate is lower than the 3000 did not produce recrystallization,when more than 3000,began to appear recrystallization.(3)The superfine copper has the strain hardening characteristic under quasi-static shear,but the shear stress-strain strain curve shows the softening characteristic under high strain rate.The adiabatic shear zone producing dynamic recrystallization at high loading rate is the reason why the strain hardening rate is negative.Under the same strain rate,the width and strain of recrystallization in the shear zone of ultrafine crystal copper are linear.With the increase of strain rate,the yield strength and flow stress of ultrafine crystalline copper become larger and larger,and the strain hardening rate decreases with the increase of strain rate(4)With the increase of strain rate,the width of shear band becomes wider and wider.Under the same strain rate,the width of the absolute shear band is wider and wider with the increase of strain.The higher the strain rate is,the narrower the shear band width is.It is also proved that recrystallization takes precedence in the middle position of shear band and expands to both sides with the development of strain.