Microstructure And Properties Of Yb Microalloyed Ultra-high Strength Aluminum Alloy

Ultra-high strength Al-Zn-Mg-Cu alloy is an important structural material in aerospace and other fields.The alloys ensure the strength of alloy through high-density aging precipitates,but corrosion properties decreases.It has both strength,toughness and corrosion resistance,which is an unsolved problem in the field of aluminum in the world.Based on Al-（8.0-9.0）Zn-（2.3-3.0）Mg-（2.0-2.6）Cu alloy,the effects of microalloying of rare earth element Yb with transition elements Cr and Zr on the precipitation behavior of the second phase,microstructure,mechanics and corrosion properties in ultra-high strength Al-Zn-Mg-Cu alloy were studied by means of optical microscope（OM）,scanning electron microscope（SEM）,X-ray diffraction（XRD）and transmission electron microscope（TEM）,combined with room temperature tensile test,electrochemical test and intergranular corrosion test.The following conclusions were drawn:（1）During the preparation of rare earth Yb microalloyed ultra-high strength Al-Zn-Mg-Cu alloy,Yb is very easy to interact with the main alloy element Cu,which intensifies the dendritic segregation during solidification and crystallization,forming a coarse and insoluble compound Al₈Cu₄Yb phase,which is difficult to be removed by subsequent solid solution treatment.On the one hand,the formation of Al₈Cu₄Yb phase leads to the obvious decrease of Cu content in aluminum matrix,the decrease of matrix potential and the deterioration of corrosion resistance of the alloy;On the other hand,the content of Cu in low melting point eutectic phase（Sigma phase）is reduced,and the alloy is more prone to overburning;Moreover,the coarse Al₈Cu₄Yb phase is easy to induce matrix recrystallization and become a fracture source,which significantly reduces the mechanical properties of the alloy.（2）The addition of transition group element Cr in Al-Zn-Mg-Cu-Yb alloy can reduce the formation of Al₈Cu₄Yb phase by solidifying and crystallizing Al₂₀Cr₂Yb phase,improve the corrosion performance of solidified alloy,and the electrochemical corrosion current reduces from35580μA·cm^-2 to 26320μA·cm^-2,with a decrease of 26%,and the maximum depth of intergranular corrosion reduced from 195μm to 158μm.with a decrease of 19%.After hot extrusion,a large number of compound phases containing Cr(Al₂₀Cr₂Yb and E phases)precipitate,induce recrystallization,and improve the fracture and local corrosion tendency of the alloy.After hot extrusion and solution aging,compared with Al-Zn-Mg-Cu-Yb alloy,the mechanical properties and corrosion properties of the alloy decreased,the microhardness decreased from 165.5HV to 145.1 HV,the tensile strength decreased from 639.7 MPa to 579.3MPa,and the electrochemical corrosion current increased from 4136μA·cm^-2 to 4782μA·cm^-2,the maximum corrosion depth of intergranular corrosion increased from 142μm to 265μm.（3）Adding transition group element Zr to Al-Zn-Mg-Cu-Yb alloy can promote the solid solubility of Yb in the matrix and form Al₃（Yb,Zr）phase which is coherent with the matrix,so as to reduce the precipitation of Al₈Cu₄Yb phase,greatly improve the corrosion properties of solidified alloy,and the electrochemical corrosion current is reduced from 35580μA·cm^-2 to 11960μA·cm^-2,with a decrease of 66.4%,and the maximum depth of intergranular corrosion reduced from 195μm to 89μm,with a decrease of 54.4%.After hot extrusion and solid solution aging,only a small amount of S phase precipitates,and a large amount of Al₃（Yb,Zr）nano phase which is coherent with the matrix precipitates in the matrix.dislocation motion is effectively pinned,the alloy retains a large number of characteristic grain boundary structures dominated by small angle grain boundaries.Compared with Al-Zn-Mg-Cu-Yb alloy,the proportion of recrystallized grains is reduced from 66.7%to 3.2%.The mechanical and corrosion properties of the alloy were greatly improved,the microhardness and tensile strength were increased to 188.8 HV and 705.6MPa respectively,and the maximum corrosion depth of electrochemical corrosion current and intergranular corrosion were reduced to 961.4μA·cm^-2 and 139μm respectively.