Study On Microstructure And Mechanical Properties Of High-Zn-Content Al-Zn-Mg-Sc Alloys

High-zinc-content Al-Zn binary alloys have gained attention due to their simple chemical composition,unique dynamic precipitation behavior and excellent plasticity.In this study,three high-zinc-content Al-Zn-based alloys containing magnesium and scandium,namely Al-15Zn-0.5Mg-0.5Sc,Al-20Zn-0.5Mg-0.5Sc and Al-15Zn-1Mg-0.5Sc,were prepared by casting process.Their solid solution state（ST）was cold-rolled（CR）and subsequently treated with natural（NA）or artificial aging（AA）.The precipitation behavior,microstructure evolution and their effects on mechanical properties of the aforementioned alloys were studied during this process,and a process combining cold deformation and heat treatment was proposed to obtain high-zinc-content Al-Zn-Mg-Sc alloys with excellent mechanical properties.The research results show that Sc elements in high-zinc-content Al-Zn-Mg-Sc alloys form primary Al₃Sc phases during alloy solidification,significantly refining as-cast grains.During homogenization treatment,nano-Al₃Sc precipitates are formed that effectively pin grain boundaries,greatly improving the alloy’s thermal stability.Hot rolling refines the grains of high-zinc-content Al-Zn-Mg-Sc alloys while also causing dynamic precipitation of Zn andηphases.Increasing Zn and Mg content lowers the recrystallization temperature of the alloys,causing recrystallization to occur after solution treatment and resulting in deformed grains with incomplete recrystallization in the alloys.Cold rolling further refines the grains of ST alloys and causes dynamic precipitation of Zn phases in high-zinc-content Al-Zn-Mg-Sc alloys.This phase not only strengthens the alloys but also improves their plasticity by forming Al/Zn interfaces.Cold rolling also increases the dislocation density of the alloy and forms dislocation entanglements,thereby enhancing their strength through dislocation strengthening effect.Furthermore,high-density dislocations further promote the precipitation of nano-phases during subsequent aging.For Al-15Zn-0.5Mg-0.5Sc alloy,cold rolling accelerates the transformation of theη’phases to theηphases during aging.NA can preserve high-density dislocations while precipitating phases,so the ST+CR+NA sample exhibits high strength with strengthening mechanisms including grain refinement strengthening,solid solution strengthening,dislocation strengthening and precipitation strengthening.However,raising the aging temperature causes excessive decomposition of the solid solution and accelerates the dislocation annihilation,which reduces the strength of the alloy.For Al-20Zn-0.5Mg-0.5Sc alloy,due to its high Zn content,the ST sample can precipitate nano-Zn andη’phases after aging treatment and increase the alloy’s strength through precipitation strengthening effect.Cold rolling accelerates the precipitation of Zn phases in solid solution alloy and optimizes the alloy’s plasticity by forming more Al/Zn interfaces.The ST+CR+NA sample retains a large number of solute atoms and dislocations while obtaining high density nano-Zn andη’precipitates,resulting in the highest strength.For Al-15Zn-1Mg-0.5Sc alloy,due to its high Mg content,the precipitation of Zn phases is difficult,and the alloy tends to precipitate Zn₂Mg phases during deformation and heat treatment.Raising the aging temperature accelerates the reaction between Mg and Zn atoms,and defects promote the transformation of precipitates in the ST sample from Zn phases toηphases.Increasing Mg content prolongs the time for the alloy to reach peak strength and significantly increases the yield strength of the peak strength sample by increasing theη’phase density.Cold rolling further increases the alloy’s strength,but due to the suppression of Zn phases precipitation during aging,aging parameters have little effect on the mechanical properties of Al-15Zn-1Mg-0.5Sc alloy.Cold rolling after solution treatment can significantly optimize the mechanical properties of high-zinc-content Al-Zn-Mg-Sc alloys.By combining dynamic precipitation during cold deformation with static precipitation during aging treatment,the comprehensive mechanical properties of high-zinc-content Al-Zn-Mg-Sc alloys can be more effectively improved.