Research About Microstructures And Mechanical Properties Of Mg-Gd-Dy Based Casting Magnesium Alloys With High Strength

Casting magnesium alloys which are thought as the light engineering structural materials,have great potential for the applications in automotive,aerospace and other areas.However,the mechanical properties for the available casting magnesium alloys do not meet the needs of high strength and toughness for industrial production.Therefore,based on the previous investigations about high strength magnesium alloys,the research and development of the new casting magnesium alloys with high strength and toughness are very necessary.Mg-Gd based magnesium alloys have received widespread attention at home and overseas due to their great potential in developing casting magnesium alloys with high strength.However,at present the research and development of Mg-Gd based casting magnesium alloys mainly focus on the Mg-Gd-Y based alloys,and the investigations of other Mg-Gd based alloys such as Mg-Gd-Dy are very limited.Since Y and Dy elements have similar physical and chemical properties and relatively good solid solution strengthening effects,Mg-Gd-Dy based alloys are considered another promising casting magnesium alloys with high strength.Therefore,it is very necessary to design and develop new Mg-Gd-Dy based alloys and then further investigate the effects of alloying and/or micro-alloying,and heat treatment on the microstructures and mechanical properties of Mg-Gd-Dy based alloys,which are predicted to has very important theoretical significance and practicable value in developing and widening the application for the Mg-Gd based casting magnesium alloys with high strength.Based on the previous investigations about Mg-Gd-Y based casting magnesium alloy and combined with using X-Ray diffraction（XRD）,differential scanning calorimetry（DSC）,optical microscope（OM）,scanning electron microscope（SEM）and EDS analysis,micro-hardness and tensile properties testing,the new type Mg-6Gd-2Dy-（0.4-2）Zn/0.4Zr和Mg-（6-8）Gd-（1-3）Dy-0.4Zn（wt.%）based casting magnesium alloys were designed and prepared,and then the effects of an change in Gd,Dy and Zn contents and heat treatment on the microstructures and tensile properties of these alloys,were investigated,and the following main results were obtained in the paper:1)The as-cast microstructures of the Mg-6Gd-x Dy-0.4Zn（x=1.0,2.0,3.0 wt.%）experimental magnesium alloys are mainly composed ofα-Mg and Mg₂₄（Gd,Dy,Zn）₅phases.Although the change of Dy content has no obvious influence on the type of the secondary phases in the alloys,the morphology of the alloying phases greatly change.At the same time,an increase in Dy content from 1.0 wt.%to 3.0 wt.%,the grains of the alloys gradually coarsen.2)Among the Mg-6Gd-xDy-0.4Zn（x=1.0,2.0,3.0 wt.%）experimental magnesium alloys,the as-cast alloy with 1.0 wt.%Dy,whose tensile and yield strengthes,elongation at room temperature（RT）are 185 MPa,88 MPa and 5.3%,respectively,exhibits the relatively higher tensile properties at RT than other alloys.The variation of the tensile properties at RT for the alloys with different Dy contents is possibly related to the influence of the change of Dy content on the strength of the solid solution matrix and the second phase morphology and grain size.In addition,T6heat treatment may improve the tensile and yield strengthes at RT but decrease the elongation.Among the T6-treated alloys with different Dy contents,the tensile properties at RT for the T6-treated alloy with 1.0 wt.%Dy are relatively higher than other alloys.After being T6 heat-treated（solutionized at 430℃for 12 h+water cooling and followed by aged at 200℃for 24 h+air cooling）,the tensile and yield strengthes,elongation at RT for the T6-treated Mg-6Gd-1Dy-0.4Zn alloy reach 257MPa,123 MPa and 2.5%,respectively.3)The as-cast microstructures of the Mg-6Gd-2Dy-x Zn（x=0.4,1.0,2.0 wt.%）experimental alloys are mainly composed ofα-Mg and eutectic Mg_x（Gd,Dy,Zn）_y phases,thereinto the Mg_x（Gd,Dy,Zn）_y is Mg₂₄（Gd,Dy,Zn）₅ phase for the alloy with0.4 wt.%Zn but the Mg_x（Gd,Dy,Zn）_y is Mg₃（Gd,Dy,Zn）phase for the alloys with 1.0wt.%or 2.0 wt.%Zn,and an increase in Zn content from 1.0 wt.%to 2.0 wt.%causes the amount and size of the Mg₃（Gd,Dy,Zn）phase to increase.4)Among the Mg-6Gd-2Dy-xZn（x=1.0,2.0,3.0 wt.%）and Mg-6Gd-2Dy-0.4Zr experimental magnesium alloys,the as-cast alloy with 2.0 wt.%Zn,whose tensile and yield strengthes,elongation at room temperature（RT）are 195 MPa、89MPa和5.5%,respectively,exhibits the relatively higher tensile properties at RT than other alloys,which is possibly related to the formation of the Mg_x（Gd,Dy,Zn）_y phase and its change in the amount and size resulted from Zn contnent.In addition,T6 heat treatment may improve the tensile and yield strengthes at RT but decrease the elongation.Among the T6-treated alloys with different Zn contents,the tensile properties at RT for the T6-treated alloy with 2.0 wt.%Zn are relatively higher than other alloys.After being T6 heat-treated（solutionized at 460℃for 12 h+water cooling and followed by aged at 200℃for 24 h+air cooling）,the tensile and yield strengthes,elongation at RT for the T6-treated Mg-6Gd-1Dy-2Zn alloy reach 288MPa,127 MPa and 3.9%,respectively.5)The as-cast microstructures of the Mg-xGd-1Dy-0.4Zn（x=6.0,8.0 wt.%）experimental alloys are different,thereinto the alloy with 6.0 wt.%Gd is mainly composed ofα-Mg and Mg₂₄（Gd,Dy,Zn）₅ phases,and the alloy with 8.0 wt.%Gd is mainly composed ofα-Mg,Mg₂₄Gd and Mg₁₂GdZn phases.6)The as-cast tensile properties at RT for the Mg-xGd-1Dy-0.4Zn（x=6.0,8.0wt.%）experimental alloys are different,thereinto the tensile and yield strengthes of the alloy with 8.0 wt.%Gd is relatively higher than those of the alloy with 6.0 wt.%Gd.However,the elongation of the former is slightly lower than that of the later.In addition,T6 heat treatment may improve the tensile and yield strengthes at RT but decrease the elongation.Among the T6-treated alloys with different Gd contents,the tensile properties at RT for the T6-treated alloy with 8.0 wt.%Gd are relatively higher than another alloy.After being T6 heat-treated（solutionized at 430℃for 12h+water cooling and followed by aged at 200℃for 24 h+air cooling）,the tensile and yield strengthes,elongation at RT for the T6-treated Mg-8Gd-1Dy-0.4Zn alloy reach303 MPa,155MPa and 2.0%,respectively.