Microstructure And Mechanical Properties Of Mg-Sn-Y Alloys

Magnesium（Mg）alloys have high potential for improving the fuel efficiency and reducing the CO₂ emission of vehicles because of their high specific strength and stiffness.There is an increasing demand for mass produced Mg alloy sheets with high performance for industrial applications.Thus,it is urgent to develop more new type of magnesium alloys with high performance.Recently,there is an increasing interest in Mg-Sn system alloys because of its high strength,excellent corrosion resistance,high temperature superplastic deformation and low extrusion temperature.Unfortunately,Mg-Sn alloy system exist serious segregation,such as the second phase to be coarsened and long aging time.Therefore,Mg-Sn alloy systems need further research.In this paper,the Mg-xSn-yY（x=0.2,0.4,0.7;y=0.7,1.1,1.4）series alloys were prepared in a mild steel crucible placed in an electric resistance furnace protected by a gas mixture of CO₂ and SF₆（99:1）.Alloying design and extrusion deformation temperature were jointly used to control the second phase content,distribution,grain size and texture intensity,which achieved to the optimal balance of strength and plasticity of Mg-Sn-Y alloys.The effect of Sn and Y micro-alloying addition and extrusion temperature on the microstructure evolution and mechanical properties of Mg-Sn base alloys is performed,with inductively coupled plasma atomic emission spectroscopy（ICP）,optical microstructure（OM）,X-ray diffractometer（XRD）,scanning electron microscopy（SEM/EBSD）,X-ray energy dispersive spectroscopy（EDS）and mechanical testing machine.Firstly,microstructure and mechanical properties of Mg-0.2Sn-xY alloys were studied.The addition of Y refined the grains of as-cast Mg-0.2Sn alloy.And with the increase of the Y,the amount of the second phase gradually increased and the distribution of the second phase was gradually dispersed.And,the grain size of as-extruded bar significantly also was refined,the volume fraction of recrystallized grains increased,the typical basal texture was transformed into the splitting one tilted to the extrusion direction,and the elongation has improved significantly with increasing Y content.Compared with as-extruded bar,all as-extruded plates undergo dynamic recrystallization after extrusion.Moreover,with the addition of Y,the strength of the alloy is almost constant,but the plasticity increases and the mechanical properties of the plate are better than that of the bar.Secondly,the effect of Sn content on microstructure,texture and mechanical properties of Mg-1.4Y alloys was studied.It was found that the grain size was refined,the amount of the second phase gradually increased and the distribution of the second phase was transformed into a network distribution of as-cast Mg-Sn-Y alloy.All as-extruded bars undergo dynamic recrystallization after extrusion,the size of recrystallized grains was uniform and the second phase was broken and dispersed.The typical RE texture was transformed into the typical basal texture,and the strength increased with the increase of Y content.Finally,the as-extruded Mg-0.2Sn-1.4Y alloy with high plasticity and relatively weak strength was selected to optimize the balance by the appropriate extrusion temperature.The experimental results showed that the grain size of recrystallization was coarsened,the composition of the second phase was unchanged,but the distribution was changed.The texture type remained unchanged but the maximum polar density presented first increase and then decrease,meanwhile the plasticity of the alloy remained unchanged but the strength presented first increase and then decrease,with the extrusion temperature increases.Extrusion temperature can effectively change the microstructure and mechanical properties of Mg-0.2Sn-1.4Y alloy.Mg-0.2Sn-1.4Y alloy exhibited excellent comprehensive properties after extrusion at 350℃,namely YS=157.31 MPa,UTS=303.35 MPa,EL=28.91%.