Microstructure Evolution And Mechanical Properties Of ECAP Deformed Mg-4Al-1Zn-1Sn Magnesium Alloy

As a common commercial magnesium alloy,Mg-Al-Zn(AZ series)alloys have the advantages of low cost and good casting performance,but its application scope is limited by its disadvantages such as poor formability and low strength.It is found that adding Sn into AZ alloys can reduce the stacking fault energy of magnesium alloys and improve the strength and elongation.However,Mg-Al-Zn-Sn alloys usually have high alloy content and high strength at the expense of plasticity.There is no Mg-Al-Zn-Sn alloy with a good combination of strength and ductilityIn this work,Equal Channel Angular Pressing is used for a new type low alloy content Mg-4Al-1Zn-1Sn(AZT411)magnesium alloys.The Mg-Al-Zn-Sn alloys with a good combination of strength and ductility have been fabricated via optimizing process parameters,and the dynamic recrystallization mechanism and the strength-toughness mechanism of ECAP deformed magnesium alloys have been analyzed.The main conclusions are as follows:(1)The influence of ECAP deformation parameters on the microstructure of AZT411 magnesium alloy:with increasing the number of ECAP passes,the area fraction of second phase increases.Moreover,the grain size decreases firstly and increases later.The shear mode of route Bc is conducive to strain accumulation and the second phase precipitation,resulting in the better grain refinement effect at the same strain.However,the minimum grain sizes of different routes are basically the same.(2)Dynamic recrystallization mechanism:dynamic recrystallization occurs in the ECAP processing under the shear stress.Continuous dynamic recrystallization(CDRX)and discontinuous dynamic recrystallization(DDRX)occur at the initial deformation stage.With increasing the number of ECAP passes,DDRX gradually becomes the main mechanism for grain nucleation and growth.(3)After ECAP deformation via route A,the magnesium alloy forms a strong basal texture,and the grains have the hard orientation.After ECAP deformation via route Bc,the diffraction peak strength of{10~-11}crystal gradually increases with increasing the ECAP passes,while the basal texture weakens.(4)The influence of ECAP deformation parameters on the mechanical properties of AZT411 magnesium alloys:the yield strength shows a trend of decreasing firstly and increasing later with increasing the number of ECAP passes.When the number of ECAP passes is 4 via route A,the second phase strengthening,grain refinement and texture strengthening result in the highest yield strength;later,when the number of ECAP passes is more than 4,the coarsening of the second phase particles weakens the solid solution strengthening and the second phase strengthening,decreasing the yield strength.In route Bc,grain refinement and second phase strengthening make the highest yield strength after 2 passes ECAP;and when the number of ECAP passes is more than 2,the yield strength decreases due to the formation of inclined texture.With increasing the number of ECAP passes from 2 passes to 4 passes,the structure of the two routes all become more uniform and the fracture elongation increased.After 4-8 passes,the grain growth happens due to the coarsening of the second phase and SIBM.This leads to stress concentration,which make the fracture elongation decrease.(5)The comprehensive mechanical properties after 4 passes of ECAP deformation via route A is the best.The yield strength,ultimate tensile strength and fracture elongation are 225 MPa,312 MPa and 31.9%,respectively.The high yield strength is mainly resulting from grain refinement,second phase strengthening and texture strengthening.The high elongation is due to the uniform fine equiaxed grains and the dispersed distribution of the second phase particles.(6)The superplastic is obtained at 250?after 4 passes of ECAP deformation via route A.The fracture elongation is 315%due to the uniform fine equiaxed grains and the dispersed distribution of the second phase particles,which has good uniform plastic deformation ability.The calculation results show that the superplastic deformation at 250?is the result of lattice diffusion and grain boundary diffusion.