Effect Of Zn Content On Microstructure And Mechanical Properties Of Cast Mg-9Gd-3Y-0.5Zr Alloy

As a high strength and heat resistant Mg-RE alloy which developed in recent years,Mg-9Gd-3Y-0.5Zr alloy has good mechanical properties at both room and elevated temperatures.It has a broad application prospect in the field of aerospace.At present,the strength of Mg-9Gd-3Y-0.5Zr alloy has meet the application requirements of aerospace and other fields,however,low elongation limits its further popularization and application,therefore,it's particularly important to explore the method of improving the mechanical properties of the Mg-9Gd-3Y-0.5Zr alloy.A lot of studies have reported that Zn can improve the mechanical properties of Mg-RE alloys effectively,especially the elongation.However,there are few reports about the effect of Zn on the properties of Mg-9Gd-3Y-0.5Zr alloy,not to mention the reports of trace Zn(Zn content ?1.5wt%).Therefore,it is necessary to study the influence of trace Zn addition on the microstructure and mechanical properties of Mg-9Gd-3Y-0.5Zr alloy.In this paper,Mg-9Gd-3Y-xZn-0.5Zr(x=0,0.2,0.5,1.0,1.5wt%)alloys with different Zn content were prepared by gravity metal cast.The changes of microstructure,mechanical properties and tensile fracture behavior of the alloys were systematically studied.The heat treatment process of the alloys was optimized and the content of Zn was further optimized.As the structural parts in aerospace and other fields often need to be subjected to cyclic loading,the high cycle fatigue behavior of the alloy is further explored on the basis of optimizing the content of Zn.The results show that the microstructure of the as-cast alloys is mainly composed of ?-Mg matrix and eutectic phase.With the increase of Zn content,the composition is gradually changed from Mg24(Gd,Y)5 to(Mg,Zn)3(Gd,Y).The addition of Zn brings the fine lamellar-shaped structure in the ?-Mg matrix,and its volume fraction increases with the increase of the Zn content.The addition of Zn can enhance the mechanical properties of the as-cast alloys effectively,when the the content of Zn is 0.5wt%,the properties of the alloy are the best,the yield strength,ultimate tensile strength and elongation are 158 MPa,248MPa and 7.2%,respectively.In this paper,the heat treatment process of alloy with different Zn content was optimized.The heat treatment process of Zn content for 0.5wt% alloy is as follows: 500°C ?6h,225°C?16h.After solid solution treatment,the eutectic phases of the alloys(Zn contents: 0-0.2wt%)are basically dissolved into the ?-Mg matrix,and there are a few X phases in the alloys with 0.5-1.5wt%Zn.The yield strength(YS),ultimate tensile strength(UTS)and elongation(EL)of T4-treated alloys increase first and then decrease with the increase of Zn content.When Zn content is 0.5wt%,the best comprehensive mechanical properties are achieved.Under the peak-aged condition,the YS and UTS of the alloys increase first and then decrease with the increase of Zn content,while the EL keep the trend of increase.When the the content of Zn is 0.5wt%,the alloy exhibits a desirable combination of strength and elongation with 244 MPa in YS,371 MPa in UTS and 3.8% in EL.When the content of Zn is greater than 1wt%,a large amount of LPSO structure in the alloy hinders the aging precipitation of the reinforced phase,which leads to a weaker effect of aging hardening.High cycle fatigue tests were carried out on three alloys(Zn contents: 0-0.5wt%),which had higher strength.It was found that the S-N curves of the as-cast and T6-treated alloys had the same trend.T6 heat treatment can effectively improve the fatigue strength of the alloys.When the the content of Zn is 0.5wt%,the alloy has the highest as-cast and T6-treated fatigue strength,which are 90 MPa and 120 MPa,respectively.In addition,the tensile fracture behavior and fatigue fracture behavior of the alloys are also studied in this paper.Tensile fracture behavior: as-cast alloys have a transgranular fracture mode,eutectic phase is the main crack source.After solid solution and aging heat treatment,the alloys have a quasi-cleavage fracture mode.The mode of fatigue crack propagation is mainly transgranular.