On Effect Of Alloying On Modification, Elevated Properties And Corrosion Resistance Of Mg-Zn-Si Alloys

Magnesium alloys are widely applied in the fields which are strongly driven towardsweight-reduction and vibrate-reduction, such as automotive, aeronautic and astronautic industries, etc,because of their high ratio of strength and density and high ratio of stiffness to desity. The in-situmagnesium matrix composite, in which the coarse phase modified though alloying, is one of theresearch directions. The hypereutectic Mg-6Zn-4Si alloys which have attracted increasing attentionhave been researched in this paper. Aiming at improving the elevated properties of the material, themodification effect and mechanisms of primary and eutectic Mg₂Si in Mg-Zn-Si alloys wereinvestigated by the single element（Y，Sr，Sb） and their complex alloying. Furthermore, the tensileproperties at ambient and elevated temperature, wear resistance, corrosion resistance and creepproperties of the composites were investigated by means of OM, EDS, XRD, SEM, image processingsoftware, electrochemical workstation, the friction-wear test machine and electronic creep testingmachine in this thesis.The appropriate level of Y or Sr addition could remarkably modify the morphology and size ofthe primary and eutectic Mg₂Si in hypereutectic Mg-6Zn-4Si alloys. The morphology of primaryMg₂Si in the alloys changed from coarse dendrites to polygonal or quadrangle particles with theaddition of Y or Sr addition, and the morphology of eutectic Mg₂Si changed gradually from theChinese script morphology to fine clavate. The influence mechanism of Y or Sr alloying on theprimary and eutectic Mg₂Si was as follows: the Y or Sr element could change the solidificationcondition by absorbing in the growth front of the Mg₂Si phase, this results in the preferred growthhabit and the growth rate of the Mg₂Si phase being supressed. The Y or Sr atoms on the growth frontof the Mg₂Si caused a “poisoning effect”. The multi-element alloying, such as Sr+Sb or Y+Sr+Sb,leaded to a better refine-effect on the primary and the eutectic Mg₂Si phase. The primary Mg₂Si waschanged to particles with a size of15μm in the composites. The multi-element alloying hadcombinative effect on the refinement of Mg₂Si. However, Y and Sb combinative addition weaken themodification effect. The reason may be due to the preferential formation of a hard Sb3Y5particle.The appropriate level of Y or Sr elements addition could remarkably improve the tensileproperties, wear resistance and creep properties of the in-situ magnesium matrix composite. When theY addition was0.5%, the maximum of the ultimate tensile and the elongation was achieved, whichwas50.5%and67.8%higher than that of the base alloy respectively, and the friction coefficient and the wear rate were the lowest, and the corrosion rate was reduced by63.6%, the corrosion currentdensity was reduced by a magnitude, too. With0.5%Sr addition, the ultimate tensile and theelongation was53.3%and55.2%higher than that of the base alloy respectively, and the corrosion ratewas reduced by47.7%, the corrosion current density was reduced by three magnitudes, the wear ratewas the lowest at the high loads. The multi-element alloying, such as Y+Sb, Sr+Sb, or Y+Sr+Sb,leaded to a better tensile properties, and the ultimate tensile of the alloy with0.5Y+0.5Sr+0.5Sbaddition was193MPa, which was83.8%higher than that of the base alloy. The improvement oftensile properties of the alloys with Y and Sb combinative addition was mainly attributed to theformation of a hard dispersion Sb3Y5phase and the Mg₂Si phase modified. The corrosion propertiesof the alloy with0.5Sr+1.0Sb addition was45.6%higher than that of the base alloy.The single element and multi-element alloying modification could effectively improve the tensileproperties, wear resistance and creep properties of Mg-6Zn-4Si alloys at elevated temperature. With0.5%Y or0.5%Sr addition, the ultimate tensile at elevated temperature was46.2%and64.5%higherthan that of the base alloy respectively, and the ultimate tensile at elevated temperature of the alloywith0.5Y+0.5Sr+0.5Sb was182MPa and95.7%higher than that of the base alloy. With0.5%Y or0.5%Sr addition, the wear rate at elevated temperature was the lowest （5.3×10-6g/m）. The steady creeprate of Mg-6Zn-4Si alloy modified with0.5Y+0.5Sr+0.5Sb was4.23percent of the commercialAZ91D alloy, which was remarkable lower than that of the base alloy and the commercial AZ91Dalloy. The mechanism was as follows: the fine Mg₂Si phase effectively modified the stress distributionof the grain broundary, and reduced the formation of the micro-cracks, hold up the slippage of thegrain broundary.