Effect Of Y And Gd Content On The Microstructure And Mechanical Properties Of Mg-Y-RE Alloys

Mg-Y-RE alloys（such as WE43,WE54）have good room temperature,high temperature mechanical properties and creep resistance.They are widely used in aerospace applications and are successful commercial heat resistant magnesium alloys.However,during the development of a complex thin-walled aerospace component,it was found that the Mg-Y-RE alloy contains a large amount of active rare earth Y element,which is easy to form cerium oxide inclusions during the smelting and casting process.The oxide inclusions make Mg-Y-RE alloy show poor mechanical properties far from the requirement,the reduction of the mechanical properties is even more distinct when it comes to the application in the thin walled complex-precision castings.The problem of oxide inclusion has become a key bottleneck for the development of the aviation product.Therefore,while ensuring the mechanical properties of the alloy,it is necessary to find a low-reactive rare earth element instead of Y,reduce the content of Y element in the Mg-Y-RE alloy,and thus reduce the inclusion of yttrium oxide,and develop a new low Y-content magnesium-rare-earth alloy for the complex thin-walled magnesium rare earth.The development of alloy components is of great significance.Based on Mg-4Y-2Nd-1Gd-0.5Zn-0.5Zr alloy,Mg-Y-RE alloy with different Y/Gd content was prepared by replacing some Y elements with Gd element while ensuring the total mass fraction of rare earth.Using metallographic microscope,X-ray diffractometer,scanning electron microscope and other hardness test,tensile test to study the effect of the Y content and Y/Gd ratio on the microstructure and mechanical properties of the alloy.The influence of the change of Y and Gd content on its toughness was proved,and we developed a low Y content magnesium rare earth alloy with good room temperature and high temperature mechanical properties.Based on optimized alloy composition,the solid solution and aging heat treatment of the alloy were systematically studied,and the optimized T6 heat treatment process parameters were obtained,which laid a foundation for its industrial application.The main research results are as follows:Firstly,the effect of Y content on the microstructure and mechanical properties of the alloy was studied.It was found that the as-cast microstructure of Mg-xY-2Nd-1Gd-0.5Zn-0.5Zr alloy consisted ofα-Mg matrix and Mg₁₂Nd,Mg₁₄Nd₂Y,Mg_5.05（GdY）,GdZn,Mg₂₄Y₅ and other second phase composition.The second phase is mainly distributed in the semi-continuous network structure at the grain boundary;as the Y content decreases,the grain size of the alloy increases,the second phase at the grain boundary decreased and the size of the second phase was remarkably refined.The mechanical properties of the alloy were found to decrease with the decrease of Y content,and the elongation and elongation of the alloy decreased,and the elongation was mainly due to the decrease of Y content.As the grain size increases,the aging precipitation phase decreases and the precipitation strengthening effect decreases.The Mg-xY-2Nd-（5-x）Gd-0.5Zn-0.5Zr alloy was prepared by keeping the total amount of rare earth unchanged and replacing some Y elements with rare earth Gd.The Y/Gd ratio was changed for the alloy microscopic.The results of microstructure and mechanical properties show that the as-cast microstructure of the alloy consists ofα-Mg matrix and eutectic second phase,and the grain size changes little with the Y/Gd ratio;4Y-1Gd,3Y-2Gd and 2Y The mechanical properties of the-3Gd alloy are similar,while the strength of the 1Y-4Gd alloy is significantly reduced.The main reason for the change of mechanical properties is the change of the Y/Gd ratio.The amount of precipitated strengthening phase in the alloy and the composition and composition of the precipitated phase in the precipitated phase are different.As the Y content decreases,the Gd content increases.First,the total number of moles of rare earth elements decreases.Second,the precipitation phase containing Y decreases,and the precipitated phase containing Gd increases,while the precipitated phase containing Gd has a better strengthening effect..Among them,the low-content Mg-2Y-2Nd-3Gd-0.5Zn-0.5Zr alloy has the same room temperature and high temperature mechanical properties as the high Y content of Mg-4Y-2Nd-1Gd-0.5Zn-0.5Zr alloy,but its Y The content is reduced by 50%,and the tendency to form cerium oxide inclusions is greatly reduced.The heat treatment process of Mg-2Y-2Nd-3Gd-0.5Zn-0.5Zr alloy was optimized.It was found that when the solution was solid at 500℃,the second phase solid solution was slow due to the low temperature.When the temperature of solid solution is 550℃,the crystal grains grow up seriously,and at the same time,it is easy to produce over-burning.Considering the solid solution temperature and solid solution time,525℃×8 h is a suitable solid solution process The peak aging strength of the alloy decreases with the increase of the aging temperature,and the strength after the aging treatment also shows the same trend.Considering the toughness of the alloy,250℃×8 h is a suitable aging process.In summary,the optimized heat treatment process is as follows:solution treatment 525℃×8 h,aging treatment 250℃×8 h,the yield strength,tensile strength and elongation of the alloy after treatment are 198 MPa,288 MPa and 10%respectively.The new low Y content Mg-2Y-2Nd-3Gd-0.5Zn-0.5Zr alloy developed is expected to replace the high Y content WE43 and WE54 alloys,which can be used in the production of complex thin-walled parts of aviation,it effectively reduces the oxide inclusion in castings and improves the quality of the alloy and pass rate of castings,and has broad application prospects.