Effects Of Bi Additions On Age-hardening Behavior And Microstructure Of Mg-6Sn(-1Zn)-0.5Mn Alloys

Age-hardenable Mg-Sn based alloys have attracted extensive attention in recent years due to their potential to develop low-cost and high-strength magnesium alloys.In Mg-Sn binary alloy,the precipitate phase is mainly lathβ-Mg₂Sn lying on the basal plane of magnesium matrix.Therefore,the aging hardening respone of the alloy is moderate.In this paper,Mg-6Sn-0.5Mn and Mg-6Sn-1Zn-0.5Mn（wt.%）alloys were used as the base alloys to study the effects of Bi additions on the aging behavior and microstructure of the base alloys.In this paper,the microstructure of the as-cast and solution-treated Mg-6Sn-xBi-0.5Mn and Mg-6Sn-1Zn-xBi-0.5Mn alloys were characterized by SEM and microhardness tester,and the age-hardening curves were drawn.The results show that the additions of Bi can reduce the proportion of eutectic compounds formed along grain boundaries.The additions of Bi only slightly enhance the age-hardening response of Mg-6Sn-0.5Mn alloy,but remarkably improve the age-hardening response of Mg-6Sn-1Zn-0.5Mn alloy.The peak hardness of Mg-6Sn-1Zn-2Bi-0.5Mn alloy after stage aging treatment is up to 67.5 HV,which is about 17%higher than that of the base alloy,and further increased to 70.5 HV after double aging treatment.The enhanced hardness in Mg-6Sn-1Zn-xBi-0.5Mn alloys was mainly attribute to an increased volume fraction and significant of the precipitates.The types,morphology,structure and orientation of precipitates in the two alloys at different aging conditions were analyzed in detail by using HAADF-STEM and EDS-STEM techniques.The distribution of Bi element in theβ-Mg₂Sn phase of the two alloys was found by EDS-STEM.Bi-rich layers andα-Mg₃Bi₂ precipitates were formed on the（111）βplane of theβ-Mg2Sn phase.In Mg-6Sn-1Zn-2Bi-0.5Mn alloy,β’₁precipitates were observed in direct contact with theβ-Mg2Sn precipitates.Theβ’₁ phase was composed of C14 structureβ’₁-Mg Zn2,C15 structureβ’₁-Mg Zn2 and elongated hexagonal Mg₆Zn₇.In addition,the segregation of Zn betweenβphase and magnesium matrix were detected in aged Mg-6Sn-1Zn-2Bi-0.5Mn alloy.Zn segregation can improve the nucleation rate ofβphase and reduce the coarsening rate,thus improving the age-hardening response of the alloys.