Microstructures And Mechanical Properties Of Extruded And Rolled Mg-8Gd-4Y-1Zn-0.5Al Rare Earth Magnesium Alloy Sheets

As the lightest metal structural material,magnesium alloy has the advantages of high specific strength and specific stiffness,which can be widely used in the fields of electronic communication,aerospace and transportation.Magnesium alloy profiles with excellent comprehensive properties can be obtained by suitable hot working processes such as extrusion and rolling.In this thesis,Mg-8Gd-4Y-1Zn-0.5Al rare earth magnesium alloy was used as the research object.The alloy sheets were prepared by extrusion and rolling respectively.The evolution on the microstructures and properties of the alloy sheets under different processes and heat treatments were studied.The deformation behavior of LPSO structure in the sheet and its influence on dynamic recrystallization behavior were investigated.Meanwhile,the strengthening mechanism of rare earth magnesium alloy sheet containing LPSO structure was analyzed in detail.The main conclusions are as follows:(1)The as-cast microstructure of Mg-8Gd-4Y-lZn-0.5Al alloy mainly consists of α-Mg matrix,Mg5(RE,Zn)eutectic phase,(Mg,Al)3RE phase and 14H-LPSO structure.After homogenization treatment,a large number of 18R-LPSO structure with the composition of Mg10(Gd,Y)(Zn,Al)appear in the matrix.After hot extrusion,18R-LPSO structure in the alloy sheet with extrusion ratio of 10(ER10)developes a kink deformation,and formes a strong extrusion fiber texture.Because of a larger extrusion stress,the 18R-LPSO structure in the alloy sheet with extrusion ratio of 22(ER22)breaks and separates,and the average size of recrystallization grain is smaller due to the morphology and distribution of LPSO structure.(2)During the extrusion process,the 18R-LPSO structure goes through the process of kink deformation→ breaking into pieces→ bending-delamination→ separating→ kink.In ER22 alloy sheet,the separated smaller 18R-LPSO structure can improve the strength of the sheet.After peak-aged treatment,a large amount of nanoscale β’ phases are precipitated in the extruded sheet,and then the strength of sheet is enhanced significantly by precipitation strengthening effect.After aging treatment,the yield strength YS,ultimate tensile strength UTS and elongation EL of ER10 and ER22 sheets are 295 MPa,417 MPa,6.5%and 328 MPa,440 MPa and 7.0%,respectively.(3)The microstructure of homogenized sample after cold rolling consists of deformed grains,twins and deformation bands,and 18R-LPSO structure becomes gradually gathered.After annealing at 500℃ for 10 minutes,all the deformed grains are transformed into static recrystallized grains.After aging treatment,a large number of stacking faults and β’ precipitate within some grains for cold-rolled alloy sheet with the reduction of 20%,and the stacking faults limit the transformation from β’ phase to β phase.YS and UTS of alloy sheet are increased by 24%and 23%,respectively.After cold rolling and annealing,the recrystallization grain size of ER10 alloy sheet becomes larger,and the deformation degree of 18R-LPSO structure is more serious with the former kink deformation turns into irregular bending and delamination.The texture type also changes from the former extrusion fiber texture to the rolling basal texture.After peak-aged treatment,YS,UTS and EL of alloy sheet are 383 MPa,420 MPa and 5.5%,respectively.(4)When of the homogenized sample is rolled by 70%accumulative deformation at 520℃,the microstructure is composed of fully dynamic recrystallized grains with strong basal texture,and plenty of β phase precipitate in the matrix.At the same temperature,the aggregation of 18R-LPSO structure gradually occurs with the increase of accumulative deformation.After peak-aged treatment,YS,UTS and EL of the sheet are 343 MPa,380 MPa and 7.5%.After hot rolling,the grain size of ER10 extruded sheet becomes coarser and a lot of β phase precipitates.The texture type of ER10 sheet changes from the former extrusion fiber texture to the strong rolled basal texture.Because of the morphology and distribution of 18R-LPSO structure,the sheet exhibits strong mechanical anisotropy.After peak aging treatment,the sheet with excellent properties was prepared,whose YS,UTS and EL are 391 MPa,462 MPa and 5.5%,respectively.