Effect Of Alloy Elements On Microstructure Evolution And Corrosion Resistance Of 7xxx Aluminum Alloys

7xxx series aluminum alloys are widely used in the aerospace field due to their high strength,high modulus and low density,but their corrosion resistance is not ideal,which seriously restricts the safe use of this series of alloys.Microalloying technology is an important means to improve the corrosion resistance of 7xxx series aluminum alloys.Among them,Sc has the most obvious effect,but it is expensive and difficult to be widely used in industrial production.Er is inexpensive,and its mechanism of action in aluminum alloys is similar to that of Sc,so it is regarded as an ideal substitute for Sc.At present,the research on Er element mainly focuses on the precipitation of primary or secondary strengthening phase particles,such as Al₃Er phase and Al₃（Er,Sc）composite phase,in 7xxx series alloys.However,Er also forms an insoluble Al₈Cu₄Er phase with Cu elements,and the morphology,quantity and distribution of this phase have a great influence on the microalloying effect of Er.The fine Al₈Cu₄Er phase can pin dislocations,inhibit recrystallization,destroy the continuity of the grain boundary phase,and improve the corrosion resistance of the alloy.However,the hard and brittle Al₈Cu₄Er phase is easily broken during processing deformation,which leads to crack initiation and deteriorates the mechanical properties of the alloy.Therefore,it is very important to prove the formation and growth mechanism of Al₈Cu₄Er phase to improve the comprehensive properties of 7xxx series alloys.To this end,this paper takes the self-made Al-Zn-Mg-Cu-Er-Sc-Zr alloy as the research object,adopts the performance testing methods such as hardness,electrical conductivity,intergranular corrosion,electrochemical corrosion,etc.,using metallographic and electronic probes,X-ray diffraction,transmission electron microscopy and other characterization methods,The interaction between rare earth element Er and main element Cu in the alloy and the effect of microstructure evolution on the corrosion properties of the alloy were studied and analyzed by changing the alloy composition（individual addition of Er element,Cu element;changing Zn/Mg,and adjusting the content of Zn+Mg）.The research results are as follows:（1）Er and Cu interact to form Al₈Cu₄Er phase in the Al-Zn-Mg-Cu-Er-Sc-Zr alloy.With the increase of Er,the primary Al₃（Er,Sc）phase was formed in the center of the Al₈Cu₄Er phase,and there is a symbiotic and competitive growth relationship with the Al₈Cu₄Er phase.The addition of Er element makes the grain boundary phase change from network-shaped to block,rod-shaped,which can significantly improve the corrosion resistance of the alloy.（2）The Al₈Cu₄Er phase mainly relies on the formation and growth of the Al-Fe phase,and the two belong to an associated relationship.With the addition of Cu content,the Al₈Cu₄Er phase gradually formed and got rid of the attachment of the Al-Fe phase.Compared with the Al-Fe phase,the Al₈Cu₄Er phase has better corrosion resistance,but the coarsening of this phase and the mixed state of the two based on the accompanying growth seriously reduce the corrosion resistance of the alloy.（3）The change of the Zn/Mg ratio can significantly change the type,distribution and growth characteristics of the second phase of the alloy.The high Zn/Mg ratio is beneficial to the independent growth of the Al₈Cu₄Er phase and to get rid of the dependence on the Al-Fe phase.When the ratio is 4.2,both the Al₈Cu₄Er phase and the Al-Fe phase are significantly refined,which greatly improves the corrosion resistance of the alloy.（4）The change of Zn+Mg content will affect the formation and distribution of Al₈Cu₄Er phase.The increase of Zn+Mg content can increase the formation tendency of Al₈Cu₄Er phase,making it agglomerate from small particles to coarse phase,which deteriorates the intergranular corrosion performance of the alloy to a certain extent.