| Magnesium alloys have wide application prospects as the lightest metal structure material.At present,the most of magnesium alloys witch have better heat resistances containing rare earth elements is not suitable for civilian.In order to optimize the heat resistant magnesium alloy performance and reduce costs,base on Mg-3Sn-1.5Si-0.5Sr-0.5Bi alloy in this thesis,the effect of different Zn contents addition on the micro-structure and phase composition of the as-cast and heat treated Mg-3Sn-1.5Si-0.5Sr-0.5Bi alloy was investigated by the optical microscope(OM),scanning electron microscope(SEM)and X-ray diffraction(XRD).The micro-hardness of Mg-3Sn-1.5Si-x Zn-0.5Sr-0.5Bi(x=0,3,5,8wt.%)alloys was measured by micro-hardness tester.The relationship between the micro-hardness and the aging times of Mg-3Sn-1.5Si-x Zn-0.5Sr-0.5Bi(x=0,3,5,8wt.%)alloys was investigated by micro-hardness tester.The mechanical properties of the as-cast and heat treated Mg-3Sn-1.5Si-x Zn-0.5Sr-0.5Bi(x=0,3,5,8wt.%)alloy at room temperature and 150? was tested by the universal testing machine.The important results are summarized as follows:The main precipitated-phases of the as-cast Mg-3Sn-1.5Si-x Zn-0.5Sr-0.5Bi(x=0,3,5,8wt.%)alloys are Mg2Sn,MgSnSr,MgZn and Mg2Si phases.With the contents of Zn increased from 3%to 8%,the number of eutectic MgZn phase in the alloy increased.Adding 3%,5%and 8%of Zn results in the Mg2Sn phase becoming small in different levels,and makes the Mg2Sn phase distributing uniformly.The volume fraction of the precipitated Mg2Si eutectic phase is also increased due to the extension of the solid solubility limit of Si in the alloy by 5%Zn addition.Furthermore a combined phase,i.e.,Mg2(Si,Sn)phase from Mg2Si,was observed.While the opposite is that the 8%Zn addition reduces the solid solubility limit of Si in the alloy,and such increases the volume fraction of primary Mg2Si precipitation.The precipitation of Mg2Sn can be accelerated and the time for the second hardening peak appeared can be reduced when the Zn content in the alloy is 8%.Under the metal mold casting process,the micro-structure of the Mg-3Sn-1.5Si-x Zn-0.5Sr-0.5Bi(x=0,3,5,8wt.%)alloys become refined obviously.And the size of Mg2Si precipitation decreases from 50-60?m to 20-25?m in the 8%Zn alloy.The line shape MgZn phases which distributing uniformly were observed in the heat treated Mg-3Sn-1.5Si-(5,8)Zn alloys.The hardness of the Mg-3Sn-1.5Si-5Zn-0.5Sr-0.5Bi alloys which were casting by the metal mold increase significantly.The peak hardness and the corresponding aging times for the Mg-3Sn-1.5Si-x Zn-0.5Sr-0.5Bi(x=0,3,5,8wt.%)alloys are 58.5HV/48h,62.5HV/48h,78.5HV/48h and 84.7HV/48h,respectively.The increase of the Zn content from 0 to 8%in the alloys results in the hardness of alloys which were casting by the metal mold increasing.The as-cast 8%Zn alloy have the best mechanical properties of the as-cast Mg-3Sn-1.5Si-x Zn-0.5Sr-0.5Bi(x=0,3,5,8wt.%)alloys,and the tensile strength,yield strength and elongation are 148.7MPa,107.9MPa and 7.8%,respectively at room temperature.And the tensile strength,yield strength and elongation of the as-cast 8%Zn alloy are 130.7MPa,130.7MPa and 8.8%,respectively at 150?.The mechanical properties of the heat treated Mg-3Sn-1.5Si-x Zn-0.5Sr-0.5Bi(x=0,3,5,8wt.%)alloys are improved obviously.And the tensile strength,yield strength and elongation of the heat treated 8%Zn alloy are 176.5MPa,148.5MPa and 8.1%,respectively at room temperature.And the tensile strength,yield strength and elongation of the heat treated 8%Zn alloy are 135.1MPa,114.5MPa and 11%,respectively at 150?.The fracture morphology of the as-cast and heat treated Mg-3Sn-1.5Si-x Zn-0.5Sr-0.5Bi(x=0,3,5,8,wt.%)alloys exhibit quasi-cleavage. |
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