Effects Of Zn Additions On Microstructure And Mechanical Properties Of Mg-Sn-Zn-Mn Alloy

In order to save costs,the research of rare earth element-free Mg alloys has gradually become the mainstream.The Mg-Sn-based alloys have received more attention due to their significant age hardening effect,high temperature resistance and low cost.Besides,the addition of single element has been unable to cover the performance requirements for magnesium alloys.Scientists have begun to study the combined effects of two or more elements,which include both mechanical properties and microstructure.In this study,0.5wt.%and 1wt.%Bi were added to Mg-6Sn-3Zn-0.5Mn alloys,and the effects of Bi addition on mechanical properties and microstructure are studied.The technologies such as SEM,HAADF-STEM and EDS-STEM were used to characterize the type,shape,size and number density of precipitates in Mg-6Sn-3Zn-xBi-0.5Mn(x=0,0.5,1)alloys in the as-cast,solution-treated,peak-aged and extrusion conditions.The mechanical properties were tested,and the relationship between the microstructure and mechanical properties was discussed.The mechanical properties and microstructure of peak-aged Mg-6Sn-3Zn-xBi-0.5Mn alloys were studied.It was found that the age-hardening response of Mg-6Sn-3Zn-xBi-0.5Mn increased with an increase in Bi content.The peak-aged hardness value increased to 90HV during isothermal aging at 200?,which was 17HV higher than that of Bi-free alloy.The increased hardness value is attributed to the formation of composite precipitates consisting of prismatic?'₁ rods and poly-shaped Mg₂Sn,and a Bi-enriched core was observed in Mg₂Sn.The composite precipitates in Bi-containing alloys were fine and with high number density.The mechanical properties and microstructure of the Mg-6Sn-3Zn-xBi-0.5Mn extrusion alloys were also studied.With the addition of Bi,the precipitates were relatively fine and uniform after extrusion,and the grain size was significantly reduced.The ultimate tensile strength of Mg-6Sn-3Zn-1Bi-0.5Mn extruded alloy is 291MPa,which is 21MPa higher than that of Bi-free alloy.The results show the relationship between the structure and mechanical properties of the alloy,analyzed the effect of different microstructures on the mechanical properties,which is the great significance for the development of high-strength magnesium alloys by adjusting the morphology of the precipitated phase.At the same time,it's also very important for the development of non-rare earth magnesium alloys and reducing the cost of magnesium alloys.