Microstructure Control And Strengthening Mechanism Research On Novel Al-Mg-Sc-Zr Alloy Based On Severe Plastic Deformation Modified By Friction Stir Processing

High-performance Al-alloys have been widely used in the field of food handling and chemical processing industries due to their superior comprehensive properties such as low density,medium strength,high ductility and excellent corrosion resistance.In order to broaden the application in automotive and aerospace industries,however,alloys with higher mechanical properties are required.For this purpose,friction stir processing?FSP?,as an effective method of severe plastic deformation,has been verified to significant improve the mechanical properties of the Al-alloys.In this thesis,Al-Mg-Sc-?Zr?alloys containing different contents of Sc and Zr were prepared using ingot metallurgy processing.The effect of Sc,Zr and Sc/Zr on the microstructures and mechanical properties of the alloys were investigated based on optical microscopy?OM?,electron back-scatter diffraction?EBSD?,transmission electron microscopy?TEM?and mechanical properties measurement.The morphologies,inner substructure and spatial distribution of primary particles formed in as-cast Al-Mg-Sc-Zr alloys had also been investigated.And,the microsturctural evolution of the different regions in the studied alloy after FSP were investigated and the corresponding mechanism of dynamic recrystallization were discussed.Furthermore,the existing forms and acting mechanisms of Sc and/or Zr in Al-5Mg alloy were also analysed.The Al-5Mg alloy is characterized by coaren dendritic microstructure in as cast condition.A significant grain-refining effect is observed for the alloy with Sc content greater than 0.6 wt.%Sc,which leading to the change of the microstructure from the typical dendritic into fine equiaxed grains,and the corresponding average grain size decreases from 259.1?m to 50.2?m.While,simultaneous addition of 0.2%Sc and 0.1%Zr to Al-Mg alloy can not only result in the same grain-refining but significant improve the strength and the ductility of the alloy as well.Primary Al₃Sc/Al₃?Sc,Zr?particles will be formed when the addition contents are high than the eutectic composition.These primary particles have a similar crystal structure(?_Al=0.4050nm,?_Al3Sc=0.4106nm,?_Al3?Sc,Zr?=0.410nm)and the same in orientation with the matrix,which will locate at grain boundaries and inside grains,hindering the undesirable diffusion and subsequent crystal growth and behaving as heterogeneous nucleation cites for?-Al matrix,leading to the reduce of the grain size and refine the microstructure.The tensile strength,yield strength and elongation of the as-cast Al-5Mg-0.2Sc-0.1Zr alloy is 315.6 MPa,170.1MPa and 21.1%,respectively.Primary particles formed in Al-Mg-Sc-Zr alloy are believed to be the eutecticum consisting of Al₃?Sc,Zr?and?-Al phases,exhibiting an‘Al₃(Sc_x,Zr_1-x)+?-Al+Al₃(Sc_x,Zr_1-x)'layered structure.It is found that the three habit planes of the squa_-re on th_-e_-intersection plane c_-lose to_-?100?and triangle particles on?111?are close to3?21?,?11?1,?144?and?32?1_-,_-2?3?0_-,_-?114?respectively._-By transf_-ormtion,the coordinates of habit planes are3?2?1,1??11,?144?and1?33?,?011?,?301?respectively,indicating that the primary particles in as-cast Al-Mg-Sc-Zr alloy can be randomly distributed in the matrix.After FSP,the mircostructure of the alloys are characterised by four typical regions consisting of Nugget Zone?NZ?,the mechanical affected zone?TMAZ?,heat affected zone?HAZ?and base material?BM?.Owing to the effect of thermal cycle,the microstrucutre of HAZ shows the deflecting coarsen grains along the direction of rotation.Meanwhile,some small-size grains is observed in the coarsen grains.The microstrucutre of TMAZ is made up of elongated grains consisting of small-size grains due to the simultaneously effect of stirring and thermal cycle,the NZ possesses the small-size equiaxed grains due to the occurance of dynamic perfect recrystallization.The dynamic recrystallization mechanism of the HAZ and TMAZ regions is a continuous dynamic recrystallization?CDRX?whereas that of NZ region is geometric dynamic recrystallization?GDRX?.The average grain sizes of the Al-5Mg,Al-5Mg-0.2Sc and Al-5Mg-0.2Sc-0.1Zr alloys after FSP are calculated to be about 10,5.35,2.34?m,respectively.Compared to the mechanical properties of as-cast alloys,the tensile strength of these three FSPed-alloys increased by 54.3,85.9 and 53.5%respectively;the yield strength increased by 113.8,161.5 and 89.8%,respectively;and the elongation decreased by57.6,114.3 and 54.1%,respectively.The tensile strength,yield strength and elongation of FSPed Al-5Mg-0.2Sc-0.1Zr alloy is 484.3 MPa,322.8 MPa and 32.5%,respectively.Compared to the FSPed Al-5Mg alloy,both solo addition of 0.2 wt.%Sc and simultaneous addition of 0.2 wt.%Sc and 0.1 wt.%Zr have a remarkable influence on their mechanical properties.The formation of the nanoscaled coherent Al₃Sc/Al₃?Sc,Zr?dispersoids precipitated from supersaturated matrix during the friction stir processing can effectively pin the grains and subgrain boundaries,inhibiting recrystallization grain growth,and thus resulting in an intense substructure strengthening and dispervise strengthening.