Investigation On The Microstructures And Mechanical Properties Of LPSO Containing Mg-Gd-Y-Zn Magnesium Alloys

Mg-RE-Zn alloys is major high strength Mg alloys because of their novel mechanical properties.With the suitable heat treatment and chemical composition,LPSO phase form and enhance the mechanical properties in Mg-RE-Zn alloys.However,the content of RE is so high in almost high performance Mg-RE-Zn alloys,and it make the cost higher and become a problem to improve the application of Mg alloys in industrialization.So it's a meaningful thing to research and develop high performance Mg alloys with low RE content,and study on the mechanism of LPSO phase on microstructure and mechanical properties.After research of literature,we found that variety and volume fraction of the second phase will have changed by control Zn content with RE content invariant in Mg-Gd-Y-Zn alloys.So we plan to change Zn content and adopt heat treatment to control the morphology,variety and volume fraction of second phase,and research the rule of effect of LPSO phase on the microstructure and mechanical properties by some experimental method,like SEM,XRD,TEM,tensile test and so on.Systematic study on the strengthening ability of composite structure with LPSO phase and compound,and the transformation law of morphology,type and distribution of LPSO phases with different chemical composition and heat treatment.Finally,we reveal the mechanism of various morphology LPSO phase on the mechanical properties and recrystallization process,and obtain the fine microstructure and high performance Mg alloys.The main research result as follows:(1)In as-cast Mg-1Gd-1Y-x Zn(x=1.0,1.5,2.0at%)alloys,LPSO phase translate to W phase by the increase of Zn content.The morphology of W phase is large fishbone,so the strengthening ability is low.After solution treatment,part of W phase and LPSO phase solid solution into magnesium matrix,and enhance the mechanical properties.Due to the high solid solution strengthening ability of Gd and Y,Mg-1Gd-1Y-2Zn alloys after solution treatment exhibits high performance,the tensile yield strength(TYS)and ultimate tensile strength(UTS)were 145.5 MPa and 265.0 MPa,respectively.(2)The block LPSO phase found in as-cast alloys can promote recrystallization process.Lamellar LPSO phase found after solution treatment hindered the migration of grain boundaries,and reduce the recrystallization degree,impede grain growth and refined grain.W phase translate from fishbone to particles after extrude,and increase the strengthening ability.Mg-1Gd-1Y-1Zn alloys with solution treatment before extrude have finest grain,strong texture and optimal mechanical properties,tensile yield strength(TYS),ultimate tensile strength(UTS),compressive yield strength(CYS),elongation(?)and yield asymmetry were 262 MPa,360 MPa,302 MPa,6.5%and 1.15.(3)In order to improve microstructure and enhance mechanical properties,Ce or La were choose to addition in Mg-1Gd-1Y-1Zn alloys.?phase was found after Ce or La addition,and solid solution into Mg matrix.Ce(Mg,Zn)₁₂ phase was found in the Mg-Gd-Y-Zn-Ce alloys after solution treatment,and La(Mg,Zn)₁₂ phase exsit at both cast and homogenized Mg-Gd-Y-Zn-La alloys.With the increasement of Ce and La content,the volume fraction of Ce(Mg,Zn)₁₂ phase and La(Mg,Zn)₁₂ phase have sharply increase,and enhance mechanical properties due to second phase strengthening.Mg-1Gd-1Y-1Zn-0.3La alloy exhibited optimal mechanical properties,the TYS,UTS,CYS,?,and yield asymmetry of this alloy are 326 MPa,391 MPa,4.6%,387 MPa,and 1.19,respectively.(4)LPSO phase readily unerwent slip during tensile testing in Mg bar contain LPSO phase,while kink deformation was easily generated with compress testing.Kink band strength is higher then slip,and it leads to CYS is higher then TYS in alloys.Finally,Mg alloys with different volume fraction of LPSO phase were made by controll Zn content and adopt suitable heat treatment.The mechanism of LPSO phase on the microstructure and mechanical proprties was research.(1)The block LPSO phase was found in as-cast alloys because of the exsit of composition segregation,and lamellar LPSO phase formed after high temperature heat treatment.Small dispersion lamellar LPSO phase was easily formed in low Zn alloys,and the length of lamellar LPSO phase increase with Zn content increase while the precipitine were hindered.(2)The block LPSO phase can promote recrystallization process,lamellar LPSO phase in grain have no effect on recrystallization process,while lamellar LPSO phase attached to grain boundary can hinder the migration of grain boundary,induce recrystallization degree.(3)Block LPSO phase play a important role to enhance mechanical properties,and higher content of block LPSO phase,higher mechanical properties.Small dispersion lamellar LPSO phase can hinder the movement of dislocation,and sharply enhance the mechanical properties while the strengthening ability of alone lamellar LPSO phase is low.