Effects Of Multistage Homogenization On Microstructural Evolution And Precipitation Behavior Of Al₃Zr Dispersoids In 2196 Al-Li Alloy

Al-Li alloys are emerging as the most promising lightweight materials for aerospace and space applications due to their excellent comprehensive properties,such as low density,high strength,high stiffness,and corrosion resistance.As a typical third-generation Al-Li alloy,2196Al-Cu-Li alloy has been widely used in manufacturing aircraft floor beams,fuselage beams,trusses and pillars,exhibiting greatly advantages compared with conventional 2xxx and 7xxx aluminum alloys.However,the formation of coarse grain layer during thermal machining of industrial 2196 aluminum alloy often deteriorates its properties and hinders it from meeting application requirements.The addition of Zr to the aluminum alloy as a recrystallization inhibitor results in the formation of Al₃Zr（β’）during homogenization process,which effectively inhibits recrystallization through grain boundary pinning mechanism.In the present work,a commercial 2196 alloy with a low Cu/Li ratio of 1.67 was selected and a series of double-step homogenization processes with different first-stage temperatures were designed.Scanning electron microscopy（SEM）and transmission electron microscopy（TEM）were employed to investigate the microstructure evolution and precipitation behavior of Al3Zr dispersion phase during homogenization.The regulation of dispersion phase of Al₃Zr was found to be effective in controlling recrystallization and formation of coarse-grained layer;The grain coarsening phenomenon of alloys treated with slowly-heated and rapidly-heated homogenization after extrusion and solid solution were analyzed;The mechanical properties and microstructure of the peak-aged alloys were characterized.The main conclusions are as follows:（1）The original microstructure of 2196 alloy is typical as cast eutectic structure,characterized by a multitude of dendrites.The primary phase with large size exists at the grain boundary of the alloy,while the second phase with small size precipitates in the grain,which mainly consists of Al₂Cu,Al₂CuMg and Al₇Cu₂Fe.The main solute elements Cu,Mg and Fe are evidently segregated at grain boundaries.（2）During the first-stage homogenization,the dendritic network begins to dissolve,resulting in the high-densityθ’phase were nucleated inuniformly within the grain.After the double-step homogenization,all the micro-segregation is eliminated,and most of the coarse second phases are dissolved in the matrix,leaving only the insoluble Al₇Cu₂Fe.The distribution of Al₃Zr dispersion phase varies with different first-stage homogenization temperatures in two-stage homogenization processes,and the optimal process parameters are 400℃/10h+520℃/24 h.Precipitation models for Al₃Zr dispersion phase and dissolution kinetics for primary phases have been established,which agree well with experimental results.（3）Although adjusting the heating rate alone cannot completely eliminate the issue of uneven precipitation in Al₃Zr dispersion phase,implementing a slow heating homogenization process with extended holding time results in a more uniform distribution of Al₃Zr dispersion phase and larger average particle size.As such,it provides stronger grain boundary pinning effect.（4）The reasons for the different recrystallization phenomena of alloys with different heating rates are as follows:high extrusion ratio,larger deformation degree of surface than center,and larger dislocation density,which provide nucleation points for recrystallization;A large number of heterogeneous particles larger than 1μm exist in the rapidly-heated specimen matrix,which will trigger the particle excitation nucleation（PSN）mechanism and become the new grain nucleation points.The recovery process of slow-heated specimen consumes a large amount of deformation distortion energy,so the recrystallization can be inhibited to a certain extent in the subsequent solution process;The particle size and numerical density of Al₃Zr dispersoids controlled by slow-heated specimen are much larger than those of the rapidly-heated sample,which can produce the strong pinning effect on the grain boundaries.（5）Under the peak-aged condition,the yield strength,tensile strength,and elongation of the slowly-heated specimen are 500.6 MPa,524.2 MPa,and 4.6%respectively.The yield strength,tensile strength,and elongation of the rapidly-heated sample are 558.9 MPa,577.9MPa,and 3.9%respectively.At this time,a large amount of needle like T₁phases precipitated in the alloy.This is because pre-deformation and high extrusion ratio greatly increase the number of dislocations in the alloy,which promotes the precipitation of T₁phase.