Investigation On The Cyclic Deformation Behavior Of Pure Copper After Surface Mechanical Attrition Treatment

Most of the damage of metal materials originates from the surface,and strengthening the surface plays a vital role in improving the service life of metal materials.The traditional materials with the uniform structure all show an inverted relationship of strength and toughness.Coarsegrained materials have strong toughness,but the strength is low,while ultra-fine-grained materials are exactly the opposite.Materials with the gradient nano-grained(GNG)structure can overcome the relationship.The GNG is a structure with nanocrystalline grains on the surface,and the closer to the core,the larger the grain size.It is the special structure with a gradient distribution of grain sizes that makes the material exhibit excellent mechanical properties.Thus,the gradient nanostructure is introduced into the material by surface treatment,which can effectively improve the overall service performance of the material.In this paper,the metal surface nanometer test machine was used to perform surface mechanical attrition treatment(SMAT)on pure copper to prepare gradient nanostructured pure copper.Through macro and micro experiments and finite element modeling,the cyclic deformation behavior of pure copper before and after SMAT and the micro-mechanisms that cause such differences have been studied.The results have certain guiding significance for improving the fatigue life of materials and revealing the principles behind them.The specific research content and results are as follows:(1)The pure copper with the GNG structure was prepared by using the SMAT technology,and uniaxial tensile experiments were performed on it.The experiment results show that the SMAT technology has greatly improved the yield strength and in a short processing time range,nor is there much loss of toughness.In addition,the pure copper after SMAT was tested for microhardness,and the test found that its surface hardness has been greatly improved,and it shows a gradient trend of gradually decreasing from the surface to the inside.(2)Microscopic experimental researches were carried out,including electron backscatter diffraction(EBSD)analysis,X-ray diffraction(XRD)analysis,scanning electron microscope(SEM)observation and metallographic observation.These studies found that the average grain size of the material after SMAT decreases,the average dislocation density increases,small-angle grain boundaries increase and large-angle grain boundaries decrease.In addition,the grain size,total dislocation density,density of geometrically necessary dislocation and small-angle grain boundaries all show a trend of spatial gradient distribution from the surface to the inside.(3)Cyclic experimental studies under asymmetric stress control show that the SMAT technology has an inhibitory effect on the ratcheting strain of pure copper materials and prolongs the fatigue life of pure copper materials.(4)The finite element simulation was used to conduct a qualitative comparative analysis of pure copper's uniaxial tension and cyclic response before and after SMAT.The study found that the selected constitutive model can well describe the uniaxial tensile response of the material after SMAT,and it can qualitatively reflect the inhibitory effect of the SMAT technology on ratchet strain evolution.The simulation results are consistent with the trend reflected in experiments.