Effect Of Minor Sc Addition On Microstructure And Property Of Al-Zn-Mg-Cu-Zr Alloy

Al-Zn-Mg-Cu series aluminum alloys have good comprehensive mechanical properties and corrosion resistance,which are widely used in aerospace and aircraft industries,transportations and vehicles.Microalloying can effectively improve the properties of aluminum alloys.Adding scandium into aluminum alloys can refine grains,inhibit recrystallization and improve alloy performance.Three kinds of Al-Zn-Mg-Cu-Zr alloy with different Sc contents was studied by using scanning electron microscope(SEM),transmission electron microscope(TEM),three-dimensional atomic probe(3DAP),Vickers microscopy hardness tester and conductivity tester and other equipment.The microstructure and mechanical properties of alloys with different compositions and states,as well as the existing forms and acting mechanisms of minor Sc in Al-Zn-Mg-Cu-Zr alloy were investigated.The influence of minor Sc addition on quenching sensitivity and optimization of the heat treatment processes were studied.The results shows that adding 0.1wt.%Sc can refine the as-cast grains,and the grain size decreased from 131?m to 96?m.The segregation at the grain boundary in the as-cast alloy is severe.After homogenization,the grain boundary becomes fine,and the Mg(Zn,Cu,Al)₂ phase is basically dissolved back into the matrix.The suitable homogenization system is 250 ^oC/20h+450 ^oC/24h.Al-Zn-Mg-Cu-Zr alloys with different Sc content will precipitate secondary Al₃Zr and Al₃(Sc,Zr)precipitation in the homogenization heat treatment.3DAP will reveal the distinct segregation behaviors of Zn,Mg and Cu in and around Al₃(Sc,Zr)precipitate in the Al-Zn-Mg-Cu-Sc-Zr alloy.The Al₃(Sc,Zr)precipitate have a core-shell structure.The segregation of Zn atoms was found in the secondary Al₃Zr particles,as well as in the Zr-rich shells of the secondary Al₃(Sc,Zr)precipitate.Secondary Al₃(Sc,Zr)particles that precipitated during the homogenization heat treatment can strongly pin dislocations and subgrain boundaries,hinder the migration of grain boundaries,which can effectively inhibit recrystallization of Al-Zn-Mg-Cu-Zr alloy during solution treatment.The addition of 0.1wt.%Sc did not significantly increase the hardness of the Al-Zn-Mg-Cu-Zr alloy in aging conditions.In the artificial aging conditions,the conductivity increases with the extension of the holding time.However,in natural aging,the conductivity decreases with the increasing of time.