Research On Microstructure Evolution And Mechanical Properties Of Mg-Gd-Y-Zn-Al Alloy

Casting Mg-RE alloy has the advantages of light weight,high strength,heat resistance,which is increasingly used in industries such as aerospace and military.However,due to the low absolute strength and heat resistance temperature,its application in key components is limited.Therefore,it is of great significance to develop high-strength and heat-resistant casting Mg-RE-Zn alloy,and to reveal microstructure evolution law and strengthening mechanism.Multi element alloying is one of the most effective means to regulate the microstructure and improve mechanical properties of casting Mg-RE alloy.In this paper,Mg-Gd-Y-Zn-Al alloy is prepared with rare earth elements Gd and Y selected,Al and trace Zn added,and gravity casting method applied.By means of optical microscopy（OM）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,X-ray diffractometer（XRD）,thermometer,universal tensile testing machine and Vickers hardness tester,the effect of Al on the microstructure evolution and mechanical properties of Mg-11Gd-2Y-1Zn alloy has been studied systematically.The strengthening mechanism of Al alloyed Mg-11Gd-2Y-1Zn alloy has been revealed.On the base of Al content optimization,the effect of Gd/Y ratio on the microstructure evolution and mechanical properties of alloys has been studied.The strengthening mechanism of Gd/Y ratio has been revealed.The effects of cooling rate and heat treatment on the microstructure evolution and mechanical properties of fixed composition Mg-Gd-Y-Zn-Al alloy have been investigated.The main research results are as follows:Through observation and analysis of microstructure of Mg-11Gd-2Y-1Zn-x Al（x=0,0.5,1,2,3,4 and 5）alloy,it has been found that Al can refine grains,and the refinement effect is related to Al content.With the increase of Al content,the grain size first increases,then decreases and finally increases again.When Al content is 1wt.%,the refinement effect is the best,with the grain size of 28.9μm（without Al added,the grain size is 164.2μm）.The phase analysis results indicate that without Al addition,the second phase is Mg₃（RE,Zn）.As Al content increases,Mg₃（RE,Zn）content gradually decreases,LPSO content first increases and then decreases,and Al₂RE content gradually increases.When Al content is 5 wt.%,the second phase only consists of Al₂RE.The addition of Al inhibits the formation of Mg₃（RE,Zn）and promotes the formation of LPSO,granular Al₂RE and acicular Al₂RE.The test results of mechanical properties show that with the increase of Al content,the ultimate tensile strength and yield strength first increase and then decrease,while the elongation first decreases,then increases and finally decreases.When Al content is 1wt.%,the tensile strength reaches the maximum at either room temperature or275°C.The ultimate tensile strength,yield strength and elongation at room temperature are 225.6 MPa,159.0 MPa and 7.8%,respectively,and at 275°C they are 199.2 MPa,140.7 MPa and 21.1%,respectively.By observing and analyzing the microstructure of Mg-13（Gd,Y）-1Zn-1Al alloy with different Gd/Y ratios（Gd/Y ratios are 5/8,7/6,9/4 and 11/2）,it is found that the microstructure is composed ofα-Mg,granular Al₂RE,acicular Al₂RE,LPSO and eutectic phase.Gd/Y ratio mainly affects the type,content and size of the second phase,but has little effect on the grain size ofα-Mg.The eutectic phase transforms from Mg₂₄（RE,Zn）₅ to Mg₃（RE,Zn）with the increase of Gd/Y ratio.In addition,with the increase of Gd/Y ratio,the size of granular Al₂RE and acicular Al₂RE increases,and the content of acicular Al₂RE increases,while the content of eutectic phase,granular Al₂RE and LPSO decreases.The test results show that the ultimate tensile strength,yield strength and elongation all increase first and then decrease with the increase of Gd/Y ratio.When Gd/Y ratio is 9/4,the comprehensive mechanical properties are the best.The ultimate tensile strength,yield strength and elongation at room temperature are 232.7 MPa,162.0 MPa and 9.1%,respectively,and at 275℃they are 198.3 MPa,145.1 MPa and 21.2%,respectively.By studying the effect of cooling rate on the microstructure of Mg-9Gd-4Y-1Zn-1Al（GWZA9411）alloy,it has been found that the grain size decreases with the increase of cooling rate.When the cooling rate is 5.84°C/s,the grain size is 97.4μm.When cooling rate increases to 32.50°C/s,the grain size decreases to 30.6μm.The cooling rate does not change the phase composition,but it can significantly affect the phase content.Under different cooling rates,GWZA9411alloy is always composed ofα-Mg,granular Al₂RE,acicular Al₂RE,LPSO and Mg₃（RE,Zn）.With the increase of cooling rate,the content of granular Al₂RE decreases,while the content of other second phases increases.The test results of mechanical properties indicate that the faster the cooling rate is,the higher the ultimate tensile strength and yield strength are,and the greater the elongation is.When the cooling rate is 32.50°C/s,the ultimate tensile strength,yield strength and elongation at room temperature are 228.2 MPa,158.2 MPa and 8.8%,respectively,and at 275℃they are 194.6 MPa,140.1 MPa and 20.9%,respectively.Through the research of solution treatment on GWZA9411 alloy,it has been found that Mg₃（RE,Zn）decomposes,and the type of LPSO changes from 18R to14H.At the same time,14H-LPSO is precipitated along（0001）_α-Mg base plane inside grains,granular Al₂RE does not change,while acicular Al₂RE is melted.With the increase of solution temperature,the total content of the second phase and the content of streaked 14H-LPSO decrease,while the solute content in the matrix increases.When the solution temperature exceeds 570℃,overburning occurs,therefore 560℃is the optimal solution temperature.Through studying on the aging treatment of GWZA9411 alloy,it has been found that aging at 200℃～300℃has the effect of aging strengthening.With the extension of aging time,it undergoes underaging,peak aging and overaging stages.With the increase of aging temperature,the peak hardness decreases,but the time to reach peak aging is shortened.The aging hardening effect is related to the formation of nanometer sized precipitates.The precipitation sequence of low temperature aging（200℃～275℃）is:supersaturated solid solution→β"→β’→β₁→β;while the precipitation sequence of aging at 300℃is supersaturated solid solution→β.The peak hardness of aging at 200℃is the highest,reaching 123.5 HV.The ultimate tensile strength,yield strength and elongation at room temperature are 304.9 MPa,268.7 MPa and 6.2%,respectively,and at 275℃they are 248.2 MPa,216.4 MPa and 17.1%,respectively.