Research On Mechanical Properties Of Gradient Structure Copper By Surface Mechanical Grinding Treatment

The gradient structured materials can effectively solve the strength-ductility“trade-off”relationship of traditional materials to a certain extent,and exhibit good strength-ductility matching,as well as fatigue resistance,surface wear resistance and corrosion resistance.At present,the research on the controllable preparation,strengthening and toughening mechanism s of the gradient structured materials are still not perfect enough,and further research is still needed.In this paper,gradient structured pure copper samples were prepared by surface mechanical attrition treatment（SMAT）under liquid nitrogen conditions（77 K）.The existence of gradient structure can effectively improve the strength of the ma terial while ensuring good ductility.When the SMAT time is less than 30 min,the surface hardness,gradient layer thickness and yield strength（YS）of the sample increase with the increase of SMAT time.When the processing time is 60 min,the surface hardness,gradient layer thickness and YS of the sample are not obviously improved,that is,there is a limit value for SMAT treatment of pure copper under liquid nitrogen conditions.Among them,the SMAT10 min sample showed the best combination of strength-ductility,which was related to the low exhaustion of mobile dislocation density and the high strain hardening ability,and the relative mo bile dislocation density showed a trend of decreasing first and then increasing.The fracture morphology of the sample shows that the surface layer and the core of the gradient structured materials have two different fracture modes,and the toughness-brittle fracture matching method makes the gradient structured sample obtain high strength and ductility as much as possible.The best combination of high strength and ductility is obtained.In the tensile test,as the strain rate increases(from 5×10^-4 s^-1 to 5×10^-2 s^-1),the dynamic recovery of dislocations in the gradient structured sample is suppressed,the strain hardening rate of the material is enhanced,and the strength-ductility is simultaneously improved,wherein the ultimate tensile strength（UTS）and the uniform elongation（UE）are more sensitive to the strain rate.The strain rate sensitivity and mechanism of gradient structured samples and coarse-grained（CG）samples were studied by strain rate jump tests.It was found that the strain rate sensitivity（m value）increased with the increase of strain rate,and compared with SMAT10 min and SMAT30 min samples,SMAT60 mi n sample is more sensitive to the strain rate.The CG sample is not sensitive to the strain rate,so the strain rate is increased and the yield stress of the CG sample is not significantly improved.The high strength of the samples is contributed by two parts,namely grain refinement and back stress.Compared with the CG sample,the gradient structured samples have more pronounced bauschinger effect and the corresponding back stress is larger.Increasing the strain rate,a large number of dislocation intera ctions in the gradient structured sample,the geometrically necessary dislocations at the interface must be accumulated,and the dislocation density is increased,which makes the back stress significantly improved.