Microstructure And Properties Of Mg-6Zn-3Al-xSn And Mg-6Al-3Sn-yZn Alloys

In this paper,Mg-6Zn-3Al-xSn（x=0,2,4 wt.%）and Mg-6Al-3Sn-y Zn（y=0,0.5,1.0,1.5,2.0 wt.%）alloys have been designed with the requirements of non-rare earth element and high comprehensive mechanical properties.The effects of addition elements on the structure and properties of the alloys were analyzed by means of optical microscopy（OM）,scanning electron microscopy（SEM）,electron backscatter diffraction（EBSD）and tensile test at room temperature.As-cast alloys show obvious dendritic structure.The addition of Sn and Zn elements can refine secondary dendrites,increase the second phase and gradually aggregates into a network structure.After solution treatment,many second phases still exist in Mg-6Zn-3Al-xSn alloys.So,the alloy would extruded directly without solid solution.After two-stage solid-solution treatment,massive second phases of Mg-6Al-3Sn-yZn alloys can be completely dissolved in the matrix.Therefore,the alloys would extruded after solid-solution treatment.The addition of Sn can refine the grain size and uniform grain distribution of extruded ZA63 alloy,but some unrecrystallized grains would appear.Sn addition can change the morphology and distribution of the second phase in as-extruded alloys.With the increase of Sn content,Al-Mn and MgAlZn phases are refined,Mg₂Sn phases begin to grow,and the number of the second phases increases and the dispersion is more uniform.The room temperature tensile test results of Mg-6Zn-3Al-xSn show that the ultimate tensile strength（UTS）,yield strength（YS）and elongation（EL）of the alloys improved obviously with the increase of Sn content.When Sn content reaches 4wt.%,the alloy has the best mechanical properties,the UTS is 328 MPa,the YS is 244MPa,and the EL reaches 21.2%.The addition of Zn can change the grain size and distribution of extruded AT63alloy,reduce the structure of fine grain zone and uniform grain distribution,but increase the grain size.The addition of Zn can change the morphology and distribution of the second phases in as-extruded alloys.The fragmented massive second phases decrease,the precipitated fine particles increase,and the second phases in the alloys disperse more evenly.In addition,the recrystallization quality of the alloy can be improved by adding Zn.When the content of Zn is 0 wt.%,the recrystallization fraction is only82.42%.But when the content of Zn is 2.0%,the recrystallization fraction reaches97.47%.The tensile test results of Mg-6Al-3Sn-yZn alloys at room temperature show that with the increase of Zn content,the UTS and YS increase obviously.When the addition of Zn reaches 2.0wt.%,the alloy has the highest UTS and YS,reaching 346MPa and 206 MPa,respectively,while the elongation decreases to 21.6%.After pre-aging at 75℃for 24 h and aging at 150℃for 18 h,the yield strength of Mg-6Zn-3Al-4Sn alloy increased from 244 MPa to 288 MPa.The tensile strength hardly changed,and the elongation decreased from 21.2%to 16.4%.The YS and UTS of Mg-6Al-3Sn-2Zn alloy increased significantly from 206MPa and 346MPa to243MPa and 359MPa after aging at 150℃for 26h,while the elongation decreased significantly from 21.6%to 16.7%.Comparing the properties of the two designed alloys,the alloy with the highest YS is aged Mg-6Zn-3Al-4Sn（288MPa）,and the alloy with the best comprehensive properties of elongation and yield strength is extruded Mg-6Zn-3Al-4Sn（YS 244MPa,EL 21.2%）.The alloy with the highest ultimate tensile strength is aged Mg-6Al-3Sn-2Zn（359MPa）.The alloy with the best comprehensive properties of elongation and tensile strength is extruded Mg-6Al-3Sn-2Zn alloy（UTS 346MPa,EL21.6%）.