Research About Microstructure And Mechanical Properties Of Mg-10Gd-xY-1.0Zn-0.5Zr Casting Magnesium Alloys

Magnesium alloys containing rare earth elements,which are thought to be a promising magnesium alloy with high performance,have broad application prospects in aerospace and other fields.Aging strengthening effects of magnesium alloys can be greatly improved by the additions of both Gd and Y elements,which makes Mg-Gd-Y system magnesium alloys become one of the research hotspots of rare earth magnesium alloys nowadays.It is well known that adding Zn into Mg-Gd-Y system magnesium alloys can form the long-period ordered structure phases?LPSO?,and LPSO structure phases have excellent thermal stability and plastic deformation ability,which can ensure that the Mg-Gd-Y system magnesium alloys containing Zn have both high strength and good toughness.Therefore,the research and development of Mg-Gd-Y-Zn based rare earth magnesium alloys have been paid great attention at home and abroad,and many positive studies have been carried out.Considering that the research about the effects of Y contents and heat treatment on the microstructure and properties of Mg-Gd-Y-Zn based alloys is not very systematic,it is of great significance and practical value to take the further study about the effects of alloying and heat treatment on the microstructures and mechanical properties of Mg-Gd-Y-Zn based rare earth magnesium alloys for the development and application of Mg-Gd-Y and/or Mg-Gd-Y-Zn based rare earth magnesium alloys with high performance.In this paper,the Mg-10Gd-x Y-1.0Zn-0.5Zr?x=1.0-2.4 wt.%?casting magnesium alloys were first designed and prepared by changing the content of Y element on the basis of the previous Mg-Gd-Y-Zn based casting magnesium alloys with high strength and toughness,and then by using optical microscope?OM?,X-Ray diffractometer?XRD?,scanning electron microscope?SEM?and energy dispersive spectrometer?EDS?,differential scanning calorimetry?DSC?,micro-hardness and tensile properties testing,the differences in the microstructure and mechanical properties for the experimental magnesium alloys with different Y contents and different heat treatment states were investigated.The following main research results were obtained:1)The as-cast microstructures of the Mg-10Gd-x Y-1.0Zn-0.5Zr experimental magnesium alloys with different Y contents are mainly composed of?-Mg,skeletal Mg₃?Gd,Y,Zn?eutectic phase,striped Mg₁₀?Gd,Y,Zn?phase?LPSO structure phase?and a few square MgRE phase,thereinto the eutectic phases and LPSO structure phases with a continuous network distribution alternate at grain boundaries,while the MgRE phases are scattered around the LPSO structure phases.In addition,with the increase of Y content from 1.0wt.%to 2.4wt.%,the volume fraction of the second phases and the dendrite size in the alloys gradually increases and decreases,respectively.2)The tensile properties at room temperature for the Mg-10Gd-x Y-1.0Zn-0.5Zr experimental magnesium alloys with different Y contents are different.With the increase of Y content from 1.0 to 2.4 wt.%,the tensile strength of the experimental magnesium alloys increases gradually but the yield strength and elongation first increase and then decrease.Amongst the experimental magnesium alloys with different Y contents,the alloy with 2.0wt.%Y has relatively higher comprehensive tensile properties at room temperature than other alloys,and its tensile strength,yield strength and elongation reach 212.4 MPa,174.1MPa and 7.1%,respectively.3)The eutectic phases in the Mg-10Gd-x Y-1.0Zn-0.5Zr experimental magnesium alloys with different Y contents are almost completely decomposed after being solutionized at 480?for 12-16 h.At the same time,the lamellar and massive Mg₁₂?Gd,Y,Zn?phases?LPSO structure phases?with discontinuous network distributions precipitate at the grain boundaries in the alloys but within the grains the acicular LPSO structure phases which are surrounded by a large number of dispersing distributed square MgRE phases,precipitate and penetrate the whole grains.In addition,after being further aged at 200?for 24 h,a large number of dispersed??phases precipitate in the aged alloys,and the massive LPSO structure phases at grain boundaries gradually decompose and transform into the lamellar shapes.At the same time,the acicular LPSO structure phases within the grains in the aged alloys almost disappear and the amount of the MgRE phase decreases.4)The tensile properties at room temperature for the Mg-10Gd-x Y-1.0Zn-0.5Zr experimental magnesium alloys with different Y contents can be obviously improved by solution treatment,and an increase in Y content causes more obvious improvement in the tensile properties at room temperature for the experimental alloys.Amongst the experimental magnesium alloys with different Y contents,the alloy with 2.4 wt.%Y has the relatively better comprehensive mechanical properties after being solutionized at 480?for 16 h,and the tensile strength,yield strength and elongation at room temperature for the solutionized alloy respectively reach 229.0 MPa,183.9 MPa and 9.7%,which are 3.3%,17.7%and 36.6%higher than those of the corresponding as-cast alloy,respectively.5)The tensile strength at room temperature for the Mg-10Gd-x Y-1.0Zn-0.5Zr experimental magnesium alloys with different Y contents can be obviously improved by aging treatment,but the yield strength and elongation of the aged experimental alloys significantly decrease.Amongst the experimental magnesium alloys with different Y contents,the alloy with 2.4 wt.%Y has the highest tensile strength?290.6 MPa?after being solutionized at 480?for 16 h and followed by aging at 200?for 24 h,however,the yield strength and elongation of the aged alloy are only 177.9 MPa and 3.2%,respectively.In addition,although the alloy with 2.0 wt.%Y after being solutionized at 480?for 16 h and followed by aging at 200?for 24 h,obtains the highest elongation?5.2%?and relatively high yield strength?213.8 MPa?,its tensile strength is only 278.4 MPa.