Effect Of Sc Microalloying On Microstructure Evolution And Mechanical Properties Of Al-Zn-Mg-Cu-Zr Alloy

The rapid development of aerospace,transportation,automotive and military industries,as well as the implementation of the“dual carbon strategy”,has increased the demand for comprehensive performance of lightweight materials.Aluminum alloys with the advantage of high strength play an important role in the field of lightweight materials.Among them,Al-Zn-Mg-Cu series alloys are internationally recognized as the mainstay of aviation material due to their advantages of high specific strength,low density,high ductility and good corrosion resistance.At present,the service temperatures of conventional aluminum alloys are below 150℃.The sub-stableη′-Mg Zn₂ precipitates,as the main strengthening precipitates of the aged Al-Zn-Mg-Cu series alloys,have poor high-temperature resistance to coarsening.In order to meet the fulfill the demands of current aerospace material development for the application of high-strength and heat-resistant Al-Zn-Mg-Cu series alloys,there is an urgent need to conduct relevant research to achieve the synergistic optimization of strength,plasticity and thermal stability.In this context,a novel Al-Zn-Mg-Cu series alloys with high strength,high plasticity and excellent thermal stability is expected through microalloying,plastic deformation and suitable heat treatment processes.Therefore,in this work,Al-6Zn-2Mg-1.5Cu-0.15Zr alloy was studied in perspectives of Sc microalloying and tertiary homogenization treatment,which not only refined theα-Al matrix grains,but also induced the re-solution of coarse Mg（Zn,Cu,Al）₂ eutectic phases.The presence of Sc element in the form of Al-Zn-Mg-Cu-Zr alloy and their influence mechanism on microstructure evolution and mechanical properties were investigated systematically through high-temperature hot rolling,solid solution and aging treatments to provide a reference for the development of new high-performance Al-Zn-Mg-Cu-Zr-Sc alloys.The main conclusions are as follows:（1）Sc microalloying can significantly refine the cast alloy grains.With the increase of Sc content,the generated primary Al₃Sc and Al₃（Sc,Zr）particles acting as heterogeneous nucleation sites contribute to the nucleation ofα-Al matrix,and the grain morphology shown a trend of transformation from dendritic to equiaxed.Coarse granular,continuous long non-equilibrium eutectic phases and severe grain boundary segregation were present in the as-cast alloy.After three-stage homogenization treatment,the grain morphology was completely changed from dendritic to equiaxed,and the grain boundary segregation disappeared.The best room-temperature tensile mechanical properties were obtained for the three-stage homogenized Al-6Zn-2Mg-1.5Cu-0.15Zr-0.2Sc alloy with yield strength of～228 MPa,tensile strength of～397MPa,and elongation at fracture of～19.6%.（2）The primary Al₃（Sc,Zr）particles inheriting from the as-cast state resulte in inhomogeneous deformation and trigger the particle simulation nucleation（PSN）mechanism during hot rolling.The dynamic recrystallization（DRX）grains preferentially nucleate near the primary Al₃（Sc,Zr）particles,what’s more,Mg Zn₂phases dynamically precipitated during hot rolling prevent the DRX grains from growing,which synergistically manipulate the microstructure characteristics of the rolled alloy.In addition,the secondary Al₃（Sc,Zr）particles exert enhanced pinning force on the dislocations during the deformation process,which increases the local dislocation density.The promotion of dislocation substructure evolution and local DRX has contributed to the formation of high angle grain boundaries（HAGBs）.The best room-temperature tensile mechanical properties were obtained for the rolled Al-6Zn-2Mg-1.5Cu-0.15Zr-0.2Sc alloy with yield strength of～372 MPa,tensile strength of～461 MPa,and elongation at fracture of～13.1%.（3）After solid solution treatment at 470℃for 1 h,the rolled alloys undergo different degrees of recrystallization,and their microstructure evolution mainly depends on the volume fraction and distribution of secondary Al₃（Sc,Zr）particles.Complete static recrystallization（SRX）occurred in the alloy without the addition of Sc.In contrast,a large number of low angle grain boundaries（LAGBs）are still present in0.1Sc and 0.2Sc alloys.This is related to the large number of dispersed secondary Al₃（Sc,Zr）particles,which exert enhanced pinning force on the dislocations and prevent the sub-grain boundaries from absorbing dislocations to evolve into HAGBs,effectively inhibiting the SRX process.（4）Low temperature aging at 120℃can effectively improve the hardness of Al-6Zn-2Mg-1.5Cu-0.15Zr-x Sc alloys and reach the peak hardness at about 24 h.The recrystallization ratio of 120°C peak-aged alloys is increased,the dislocation density is slightly decreased,the room-temperature strength and ductility is simultaneously increased compared with that of the hot-rolled alloy.This is due to the precipitation of a large amount ofη′-Mg Zn₂ nano-precipitates during aging,which improves the age strengthening and work hardening ability of the alloy.The best room-temperature tensile mechanical properties were obtained for the 120℃peak-aged Al-6Zn-2Mg-1.5Cu-0.15Zr-0.2Sc alloy with yield strength of～568 MPa,tensile strength of～607MPa,and elongation at fracture of～15.9%.The contribution of the strengthening mechanism was discussed based on a simplified model of strengthening theory.The increase in yield strength compared to the 0Sc alloy is mainly due to the Orowan strengthening of the Al₃（Sc,Zr）particles and the increased contribution of grain boundary strengthening and dislocation strengthening by the addition of Sc element.（5）Sc microalloying can significantly improve the high-temperature strength of the alloy.During high temperature tensile process at 300℃,the recrystallization ratio of 0Sc and 0.1Sc alloys gradually increased and the dislocation density gradually decreased as the tensile deformation proceeded,resulting in lower high temperature strength.In contrast,the high-temperature tensile strength increased from～106 MPa（0Sc）to～131 MPa（0.2Sc）after the alloying 0.2 wt.%Sc.This is attributed to the significant suppression of DRX by Al₃（Sc,Zr）particles,which significantly improves the thermal stability of the microstructure.