Study On Fine Grain Strengthening Of Mg-Y-Sm-Zn Magnesium Alloy

Magnesium alloys have attracted extensive attention due to their excellent advantages such as shock absorption,low density,high specific strength and stiffness,and easy recovery.However,magnesium alloys obtained under conventional casting conditions have some problems,such as the serious tendency of coarsening microstructure,the existence of coarse second phases at grain boundaries,and poor strength,which limit the further application of magnesium alloys.Adding rare earth elements can improve the microstructure and mechanical properties of magnesium alloys.Fine grain strengthening is an effective way to further improve the microstructure,mechanical properties and the application scope of magnesium alloys without changing the amount of rare earth elements.In order to develop cast magnesium alloys with low cost and high strength,Mg-3Y-3.5Sm-2Zn alloy in Mg-Y-Sm-Zn series alloys studied by our research group is selected in present work.In this paper,Mg-3Y-3.5Sm-2Zn alloy has been studied by OM observation,DSC thermal analysis,X-ray diffraction analysis,scanning electron microscopy observation and tensile test at room temperature.The effects of Zr adding manners?K₂ZrF₆ salts mixture and Mg-Zr master alloy?and ultrasonic treatment parameters?temperature and time?on the microstructure and mechanical properties of Mg-3Y-3.5Sm-2Zn alloy are studied.The refining mechanisms are also discussed,and the main results are as follows:The microstructure of the untreated alloy is coarse dendrite.For the alloy refined by the K₂ZrF₆ salt mixture with the composition of60wt.%K₂ZrF₆-20wt.%NaCl-20wt.%KCl at 780°C,the microstructure changes into equiaxed structure,and its room temperature mechanical properties are improved.Compared with the alloy refined by K₂ZrF₆ salt mixture,average grain size of the alloy refined by Mg-Zr master alloy decreases by 21.78%,and its ultimate tensile strength and tensile yield strength at room temperature are increased by 8.70%and 8.67%,respectively.The grain refinement mechanism of K₂ZrF₆ salt mixtures can be explained as follows:two kinds of Zr?fine Zr particles disperse in the matrix and Zr-rich aggregates distribute on the matrix?are obtained by the reduction reaction between K₂ZrF₆ salts mixture and Mg melt,which can play a role in grain refinement and improve the mechanical properties of the alloy.Ultrasound treatment?UST?can not only refine the grains,but also change the continuity of the second phases and improve the room temperature mechanical properties of the alloy.The optimum temperature and time parameters for ultrasonic treatment are 700°C and 90s,respectively.Compared with the alloy refined by Mg-Zr master alloy,the room temperature ultimate tensile strength and tensile yield strength of the alloy treated under the optimum parameters are decreased by 13.11%and 32.50%,respectively.However,the room temperature strength of the alloy under the optimum parameters is higher than that of the alloy without ultrasonic treatment.The supercooling nucleation caused by cavitation effect and the effective inclusions nucleation substrates produced by the high-pressure pulse caused by the collapse of cavitation bubbles,alone or in combination,can be used as the refinement mechanism of UST.After Zr-ultrasonic compound treatment,the microstructure of the alloy is further refined,and the concentration of solute elements Y,Sm and Zn at grain boundary decreases.Compared with the alloy refined by Mg-Zr master alloy,the room temperature ultimate tensile strength,tensile yield strength of the alloy are increased by32.50%and 6.88%,respectively.The refining mechanism of Mg-Zr master alloy can be explained as follows:dissolved Zr inhibits grain growth through growth factor,and undissolved Zr acts as nucleation core to promote nucleation.The strengthening effect of compound treatment can be explained as the result of the interaction between Mg-Zr master alloy and ultrasonic treatment.After compound treatment,the mechanism of UST has not changed,but the role of undissolved Zr as nucleation core to promote nucleation has been further enhanced.Therefore,the effect of Zr-ultrasound compound treatment on the microstructure and mechanical properties of the alloy is superior to that of Zr refining and ultrasonic treatment alone.