Study On Inclusion And Microstructure And Properties Of High Strength And Toughness Casting Mg-Gd-Y-Zn-Nd-Zr/Mn Alloy

In the field of modern industry,lightweight is a global development trend,which is of great significance to improve environmental pollution and alleviate the energy crisis.Magnesium alloy,as the lightest metal structural material internationally recognized,has a great demand in national defense construction and national economy.Therefore,the development of high-quality magnesium alloys with excellent mechanical properties is of great significance to expand the application of magnesium alloys.However,inclusions seriously affect the internal quality of magnesium alloys and restrict the wide application of magnesium alloys.Therefore,this subject study the type,morphology,size and content distribution of inclusions in the semi-continuous casting Mg-Gd-Y-Zn-Nd-Zr/Mn alloy.On this basis,the heat treatment process of the alloy was optimized,and the influence of different thermal treatment technology on the microstructure and mechanical properties of the alloy was explored.Firstly,The type,morphology,size and content distribution of inclusions in Mg-Gd-Y-Zn-Nd-Zr/Mn alloy were studied.The morphology of inclusions in the alloys are linear,cluster,lump and globular inclusions,which are dispersed in the alloy matrix.The main components of inclusions are oxides of Mg,Y and Gd.Among them,Mg O is the main component of non-metallic inclusions,and flux inclusions remain.The volume fraction of inclusions in the two alloys is very close.The volume fraction of inclusions at 1/2R of Mg-9.55Gd-2.27Y-1.28Zn-0.11Nd-0.55Zr(wt.%)alloy is 0.116%,and that at R is 0.098%;the volume fraction of inclusions at 1/2R of Mg-8.93Gd-1.96Y-1.25Zn-0.11Nd-0.79Mn(wt.%)alloy is 0.083%,and that at R is 0.107%.Then,the microstructure and mechanical properties of the two alloys were compared and studied.The phase composition of the two alloys includes?-Mg,LPSO phases and a small Mg₅(Gd,Y,Zn)phases.The grain size of as-cast Mg-9.55Gd-2.27Y-1.28Zn-0.11Nd-0.55Zr(wt.%)alloy is smaller,and the mechanical properties is better:the ultimate tensile strength is 270.3MPa,tensile yield strength is 151.4MPa and elongation to failure is 7.3%.The fracture mode of the alloy is typical quasi-cleavage fracture,and the fracture surface mainly includes many cleavage planes,tearing edges,secondary cracks and a small amount of dimples.Finally,the effects of heat treatment process on the microstructure and properties of the two alloys were studied.The phase composition of the two alloys treated at different solution temperatures is mainly?-Mg matrix and LPSO phase.After 480?×12h solution treatment and 200?×48h aging treatment,the comprehensive mechanical properties of the Mg-9.55Gd-2.27Y-1.28Zn-0.11Nd-0.55Zr(wt.%)alloy are excellent:the tensile strength(UTS)is 383 MPa,the yield strength(TYS)is 222.1 MPa,and the elongation after fracture is 3.5%;and Mg-8.93Gd-1.96Y-1.25Zn-0.11Nd-0.79Mn(wt.%)alloy has the best mechanical properties after the same heat treatment process:the tensile strength(UTS)is 331.6 MPa,the yield strength(TYS)is 199.3 MPa,and the elongation after fracture is 1.4%.For the mechanical properties at ambient temperature,although the formation of fine precipitates of Gd,Y and Zn has better strengthening effect than the formation of LPSO phase,the reduction of LPSO phase content will reduce the plasticity,which makes the alloy fracture prematurely and aging can not achieve strengthening before yielding.