Mutual-alloying And Strengthening Mechanisms Of Mg-8Sn Basedalloys

Magnesium alloys have been used in automotive, aeronautic, electronic,militaryand civil and other industries owing to their low density, excellentspecific strength and stiffness, etc. However, the poor hgh-temperatureproperties and low strength have restricted the wide application of mg alloys.The thermal stability, and mechanical properties of the Mg-Sn alloy recentlydeveloped are better than those of the traditional Mg-Al-ZN (AZ) and Mg-Zn-Zr(ZK) magnesium alloys, which results from the presence of micron reinforcedMg2Sn phase with high melting point(770℃). The addition of element Zn, Al,Cu to Mg-Sn alloys has remarkable effect on the regulation of the microstructureof the aging precipitates. Appropriate content of Zn/Al qualityratiocould lead tothe formation of a nano icosahedral phase(Mg-Zn-Al, I-phase),which hasexcellent strengthening effect on the alloy.In this paper, Mg-8Sn-1Zn-xAl(x=1,2,3wt.%) alloys, Mg-8Sn-1Zn-1Al-xCu(x=0,1.0,1.5,2.0wt.%) alloys and Mg-8Sn-xZn-2Al (x=4,5,6wt.%) alloys wereprepared successfully by the conventional permanent mould casting method.Microstructure and phase composition were investigated by optical microscope,X-ray diffraction, scanning electron microscope, energy dispersive spectroscopyand transmission electron microscopy. The mechanical properties of the alloyswere investigated by hardness tester and electronic material tester. The resultsare as follow:(1) Mg-8Sn-1Al-1Zn (TAZ811) alloy composes of α-Mg and semi-continuous network of Mg2Sn phases with the solidification range of92°C.The dendrite arm spacing(DAS) is about35.1μm. The as-cast Mg-8Sn-1Al-1Znalloy shows strong ISE(indentation size effect). With the further addition of Alcontent to2and3wt.%, the granulated new phase of Mgx(ZnAl)1-xcan beobserved in the matrix and the microstructure of Mg2Sn phases changed fromthe semi-continuous network to the dispersed particle compared with that ofTAZ811. The DAS deceases to24.5μm(TAZ821) firstly and then increases alittle as theAl content increases.(2) Adding different contents of Cu toTAZ811alloy leads to the formationof new phases AlMgCu and Cu3Sn that distributed on and adjacent to the grainboundaries. The average DAS size decreased from35.1μm to24.3μmwith theincrease of Cu content. Tensile tests results indicated that the yield strength andductility of the Cu-containing alloys (1.0and1.5wt.%) are better comparedwith those of Cu-free alloy. Among Mg-8Sn-1Zn-1Al-xCu(x=0,1,1.5，2.0wt.%)alloys, the alloy containing1.5wt.%Cu exhibited the best combinationmechanical properties. The excellent strength and ductility of the Cu-containingalloys were mainly related to the second phase and grain boundariesstrengthening mechanisms. The decrease in ultimate tensile strength andelongation of TAZ811-2.0wt.%Cu alloy at room temperature was ascribed tothe formation of continuous AlMgCu and coarse Mg2Sn phases formed in theliquid state.(3) Icosahedral Zn45Mg42Al13I-phase was identified by TEM and EDS inTZA862alloy. With the nano-scaled massive with micro-void morphology, theI-phase is distributed at the grain boundaries, adjacent to Mg2Sn. Hardness andTensile tests results indicated that the microhardness, ultimate tensilestrength(UTS) and elongation increased as increasing Zn content. This isattributed by the refined microstructure and grains. The TZA862alloy exhibitedthe best combination mechanical properties among TZA842, TZA852andTZA862alloys.