Study On Microstructure And Mechanical Properties Of Mg-Sm-Y-Zn-Zr Alloys

In the 21st century,as the lightest structural material,magnesium alloys have received widespread attention due to their excellent properties and are used in various industries.At the same time,some shortcomings of magnesium alloy materials also hindered its further industrial application.Studies have shown that adding trace rare earth elements can effectively improve the microstructure and mechanical properties of magnesium alloys.Rare earth magnesium alloy has better engineering application value than traditional magnesium alloy.Therefore,it is of great significance to research and develop more rare earth magnesium alloy series.In this paper,Mg-4Sm-xY-0.6Zn-0.5Zr magnesium alloy was studied and designed,x=4,5,6 and 7wt.%.The effects of different Y content on the microstructure and mechanical properties of the as-cast alloy and the alloy after heat treatment were analyzed by means of optical microscope,XRD,SEM,microhardness and tensile properties.The results show that the as-cast Mg-4Sm-xY-0.6Zn-0.5Zr alloy gradually becomes finer with the increase of the content of Y element,and Y element has a significant grain refining effect on the alloy.The as-cast structure of the alloy is mainly composed of magnesium matrix,eutectic phase Mg₂₄Y₅,eutectic phase Mg₄₁Sm₅,14H-LPSO phase Mg₁₂YZn,and layered6H-LPSO structure inside the grain.As the content of Y element increases,the number of precipitated phases at the grain boundaries of the alloy increases.The existence of LPSO structure and LPSO phase greatly improve the strength and toughness of the alloy.And the formation mechanism of Mg₁₂YZn phase may be the solidification transition related to the diffusion mechanism.The solution treatment process of Mg-4Sm-xY-0.6Zn-0.5Zr alloy?x=4,5,6,7?is as follows:525?/4h,500?/10h,500?/10h and 510?/10h.After the solution treatment,the eutectic phase at the grain boundary of the alloy is basically dissolved back,and no compound exists at the grain boundary.However,there are Mg₁₂YZn phases inside the grains,which are formed in two ways,one is that the Mg₁₂YZn phase originally at the grain boundary is dissolved back into the grain by the"diffusion-migration"mechanism during the solution treatment,and re-precipitated;the second is that the Mg₂₄Y₅ eutectic phase is transformed into a more stable Mg₁₂YZn phase with 14H-LPSO structure during the resolving process.A small amount of white bulk phase exists near the grain boundary of the solid solution alloy,and its main components are Y and Zr elements.The aging treatment process of Mg-4Sm-xY-0.6Zn-0.5Zr alloy is 200?/30h.After peak aging treatment,a large amount of Mg₄₁Sm₅ phase and Mg₁₂YZn phase precipitated along the grain boundary of the alloy.With the increase of Y element content,the number of precipitated phases increased slightly.The hardness,strength and elongation of the as-cast alloy increase with the increase of the Y content.The alloy has the best mechanical properties when the Y content reaches 7wt.%.The ultimate tensile strength is 193Mpa,the yield strength is 118Mpa,and the elongation is10.5%.The ultimate tensile strength,yield strength and elongation of the alloy increase with the increase of Y content in the alloy after solution treatment and aging treatment.When the Y element content reaches 7wt.%,the best mechanical properties of the alloy are tensile.The ultimate tensile strength is 289Mpa,the yield strength is 195Mpa,and the elongation is 5.7%.In general,the high strength and toughness of the alloy are the result of the combined effects of composite strengthening and toughening factors such as LPSO structure?and stack fault?strengthening and toughening,aging precipitation strengthening,and fine grain strengthening.The fracture mechanism of the as-cast Mg-4Sm-xY-0.6Zn-0.5Zr alloy is cleavage brittle fracture.After T6 heat treatment,the fracture mechanism of each alloy is ductile fracture.The presence of Y element and LPSO structure has a significant effect on the fracture behavior of magnesium alloys.