Study For Preparation Of Surface Gradient Nanostructure And Tensile Property Of ZK60 Magnesium Alloy

Magnesium alloys,a lightweight material,it has the characteristics of high specific strength and good corrosion resistance,and it is widely used in industrial fields such as aerospace,high-speed train and automobile manufacturing.However,the low strength and plasticity of magnesium alloys has been an important reason for limiting the application and development of magnesium alloys.In fact,the problem of strength and plasticity has always been a difficult problem in the field of metal materials.Most of plastic deformation of polycrystalline materials is mainly achieved by the dislocation slip.The easier the dislocation slips,the better the plasticity of the material.However,from the perspective of strength,the more difficult the dislocation slips the better the strength of material.So it is difficult to improve the strength and the plasticity at the same time.Even if the strength of material improve,its plasticity will inevitable decrease,and vice versa.In the strengthening mechanism of metal materials,fine-grain strengthening is the only one strengthening mechanism that increase the strength and also improve the plasticity of the materials.Many processing methods of magnesium alloys have used the principle of the fine-grain strengthening,such as equal angle extrusion,rapid rolling methods and equal diameter angular extrusion etc.However,when the grain size reduce less than a certain size,a sharp drop in ductility and toughness is commonly inevitable during further strengthening for the mechanism of fine-grain strengthening,so the processing methods has certain limitation.In this case,how to improve the strength of the magnesium alloys while maintaining enough plasticity is the focus of this study.This article uses the ZK60 magnesium alloy as the research materials.Firstly,different rolling states of ZK60 magnesium alloy are prepared by changing the number of rolling passes.The microstructure is analyzed and compared by means of optical microscope,microhardness tester and tensile test.And the rolled ZK60 magnesium alloys for the excellent strength and plasticity are selected for ultrasonic surface rolling processing(USRP).At the same time,observe the microstructure and mechanical properties of the gradient nanostructured materials prepared by the compound modification,and explore the mechanism of the effect of the gradient nanostructure on the tensile properties.The main findings in this study are as follows:(1)After 9 passes of rolling,the grain size of ZK60 magnesium alloy was 4.7μm,and the yield strength and ultimate tensile strength were 235MPa and 310MPa,respectively,and the elongation was 17.5%.By comparison,the ZK60 magnesium alloy in this rolling state has the best strength-plasticity matching and it is suitable to USRP treating.(2)The thickness of the gradient nanostructure layer prepared by the compound modification(Rolling+USRP)was 300μm,the minimum grain size of the surface layer is50nm,and the average grain size is 100nm.The tensile test measured that the yield strength and ultimate tensile strength of the ZK60 magnesium alloy after the compound modification were 283MPa and 359MPa,respectively,and the elongation was still 14.8%,and the strength plasticity product U_Treached 5313.2MPa·%.(3)The reason for the gradient structure improve the strength while maintaining enough plasticity of the materials was due to the gradient structure possessed excellent uniform elongation and the sharp decline in the plasticity was delayed.(4)After analysis,the increase of the strength in the ZK60 magnesium alloy after the compound modification was determined by the combined effects of three strengthening mechanisms,and the percentages ofσ_g,σ_dandσ_pamong the total strength were 48.02%,40.85%and 11.02%,respectively.