Research About Microstructures And Mechanical Properties Of Mg-10Gd-xY-yZn-0.5Zr Casting Magnesium Alloys

Magnesium alloys are currently the lightest metal structural material in industrial applications.It has a low density,high specific strength and specific rigidity,recyclability,good machinability and casting performance,and has a wide range of applications in electronics,transportation,aerospace.As we all know,Mg-Gd-Y-Zn-Zr series high-strength casting magnesium alloys have obvious advantages in the fields of aerospace and military industry.However,comprehensive mechanical properties of Mg-Gd-Y-Zn-Zr series magnesium alloys currently researched and developed still are difficult to meet the needs of industrial production.Therefore,it necessary to further develops Mg-Gd-Y-Zn-Zr series high-strength casting magnesium alloys.Considering that alloying and heat treatment strengthening are the most effective methods to improve the mechanical properties of rare earth magnesium alloys,especially considering that Y and Zn elements in Mg-Gd-Y-Zn-Zr magnesium alloys can form the long-period stack ordered structure enhancement phase(LPSO phase)which is beneficial to the improvement of mechanical properties,in this paper the effects of the change in Y and Zn contents and the heat treatment process on the microstructure and mechanical properties of Mg-Gd-Y-Zn-Zr high-strength casting magnesium alloys are investigated in order to provide theoretical guidance for the research and development of new Mg-Gd-Y-Zn-Zr high-strength casting magnesium alloys through composition design and heat treatment optimization.Based on the design and preparation of the Mg-10Gd-x Y-y Zn-0.5Zr(x=1,2 wt.%;y=0-1.5 wt.%)experimental magnesium alloys in this paper,through optical microscope(OM),scanning electron microscope(SEM),energy spectrometer(EDS),X-ray diffractometer(XRD)for microstructure analysis,DSC differential thermal analysis to determine the second phase transition,and microhardness tester and tensile testing machine for mechanical performance testing,etc.,the effects of the change in the Y and Zn contents and heat treatment process on the as-cast and heat-treated microstructure and mechanical properties of the experimental magnesium alloys were investigated,and the following main research results have been obtained:1)The as-cast microstructures of the Mg-10Gd-1Y-0.5Zr and Mg-10Gd-2Y-0.5Zr experimental magnesium alloys are mainly composed of a-Mg,Mg₅(Gd,Y)and Mg₂₄(Y,Gd)₅ phases.After adding 0.5-1.5 wt.%Zn,the microstructures of all the experimental alloys containing 1wt.%and 2wt.%Y change to a-Mg,Mg₅(Gd,Y),Mg₂₄(Y,Gd)₅ and18R-LPSO,and with the increase of Zn content from 0.5wt.%to 1.5wt.%,the grain size of the experimental alloys containing 1wt.%and 2wt.%Y gradually decreases,and the number of second phases gradually increases.2)For the Mg-10Gd-1Y-0.5Zr and Mg-10Gd-2Y-0.5Zr experimental magnesium alloys,the as-cast tensile strength and yield strength of the alloy containing 1wt.%Y are lower than those of the alloy containing 2wt.%Y,but the elongation of the former is greater than in the latter.After adding 0.5 and 1.0wt.%Zn to the experimental alloys containing 1wt.%and 2wt.%Y,respectively,the as-cast tensile properties of the alloys,especially the tensile strength and yield strength are improved,thereinto the improvement for the alloys with the addition of 0.5wt.%Zn is the most obvious.On the contrary,the addition of 1.5 wt.%Zn is not beneficial to the as-cast tensile properties of the experimental alloys containing 1 wt.%and 2 wt.%Y.3)After being solution-treated at 480?×12h/24h for the Mg-10Gd-1Y-0.5Zr and Mg-10Gd-2Y-0.5Zr experimental magnesium alloys,the coarse eutectic Mg₅(Gd,Y)and Mg₂₄(Y,Gd)₅ phases in the alloys gradually decompose and dissolve into the a-Mg matrix,and after adding 0.5-1.5 wt.%Zn,during solution treatment at 480?×12h/24h the Mg₅(Gd,Y),Mg₂₄(Y,Gd)₅ and 18R-LPSO second phases in the alloys containing 1wt.%and 2wt.%Y also decompose gradually and turn into the 14H-LPSO phase with high temperature stability.In general,the solution treatment gets the obvious and beneficial effects on the tensile properties of the Mg-10Gd-2Y-0.5Zr experimental magnesium alloys containing Zn,thereinto the alloy with the addition of 0.5wt.%Zn exhibits relatively high tensile properties after solution treatment at 480?×24h,its tensile strength,yield strength and elongation reached 233MPa,199MPa and 15.26%respectively.4)After being aged at 480?×24h+200?×24h for the Mg-10Gd-1Y-0.5Zr and Mg-10Gd-2Y-0.5Zr experimental magnesium alloys are,the tensile properties of the experimental alloys are obviously improved,but the tensile properties of the aged alloy with 1wt.%Y are comparatively higher than those of the aged alloy containing 2wt.%Y.Furthermore,after 0.5-1.5 wt.%Zn was in addition to the experimental alloys containing1wt.%and 2wt.%Y,respectively,the tensile properties of the aged alloys containing 1wt.%Y show the decreasing trend,however,the tensile properties of the aged alloys containing 2wt.%Y are raised significantly,especially the Mg-10Gd-2Y-0.5Zr experimental magnesium alloys containing 0.5wt.%and 1.5wt.%Zn show relatively high tensile properties,and their tensile strength,yield strength and the elongation reach278MPa,235MPa and 5.47%and 286MPa,247MPa and 9.38%,respectively.5)After solution treatment for the Mg-10Gd-1/2Y-x Zn-0.5Zr(x=0-1.5wt.%)experimental magnesium alloys,their micro-hardness decreases significantly,which is possibly related to the reasons that the net-like eutectic phases decompose and dissolve into the matrix.When the experimental alloys are aged at the different aging temperatures(175?,200?,225?,250?)for 24h,with an increase in the aging temperature,the micro-hardness of the experimental alloys first decreases,increases and then decreases,and the peak micro-hardness is obtained for all the experimental alloys aged at 200?x24h.