| Five kinds of Al-8.1Zn-2.3Cu-2.05Mg-0.12Zr containing different quantity of Sc were prepared using water chilling copper mould ingot metallurgy processing which was protected by active flux. These ingots were homogenized, hot rolled, intermediate annealed, warm rolled, annealed and cold rolled into plates with thickness of 2.3 mm. The investigation of effects of minor Sc addition on the microstructure and mechanical properties of the above alloys has revealed that 0.21% addition of Sc corresponds the optimum mechanical properties of Al-8.1Zn-2.3Cu-2.05Mg-0.12Zr alloy. On the basis of this alloy, the homogenization of the alloy, the flow stress behavior of the alloy during hot compression deformation, the microstructure and mechanical properties of the alloy under single solution, two-stage solution, single-aging, two-stage aging, retrogression and re-aging(RRA), the intergranular corrosion, exfoliation corrosion and stress corrosion behaviour of the alloy have been investigated. The existing forms and acting mechanisms of Sc in Al-8.1Zn-2.3Cu-2.05Mg-0.12Zr alloy, as well as the corrosion process and its mechanism have been discussed. The evolution of electrochemical impedance spectroscopy (EIS) features and the relation with exfoliation corrosion under different aging regimes of the alloy in EXCO solution have been also analyzed. The main results are listed as follows:(1)The additions of 0.11～0.49% Sc in Al-8.1Zn-2.3Cu-2.05Mg-0.12Zr alloy obviously improve the ultimate strength and yield strength of the alloy. The alloy containing 0.21% Sc under peak aging condition exhibits a ultimate strength of 696 MPa, a yield strength of 654 MPa and high elongation of 11.1% which increases 94 MPa and 110 MPa compared with the alloy without Sc.(2) The minor Sc in Al-8.1Zn-2.3Cu-2.05Mg-0.12Zr alloy can form Al3Sc or Al3(Sc,Zr) particles with aluminum. The Al3Sc or Al3(Sc,Zr) particles in the alloy will results in obvious fine grain and the disappearing of dendritic regions. During homogenization and subsequent heat treatment process, the secondary Al3Sc or Al3(Sc,Zr) particles can effectively pin the dislocations and subgrain boundaries, at the same time inhibit the recrystallization of the alloy, producing precipitation strengthening and substructure strengthening.(3) The suitable homogenization regime of the alloy is 470℃/24 h which is consistent with the theoretical result of 470℃/22 h in analysis of homogenization kinetics. In the cast alloy with such homogenization, most of the nonequilibrium eutectic disappeared, the grain boundary tend thin and the alloying elements distribute homogeniously.(4) When Al-8.1Zn-2.3Cu-2.05Mg-0.21Sc-0.12Zr alloy is compressed at high temperature, lnεand ln[sinh(aa)], ln[sinh(ασ)] and the 1/T satisfy linear relationship, indicating that the deformation process of the alloy is similar to a thermal activation process of high temperature creep process. The following formula is the softening stress equation under hot compressed deformation:(5) The main softening mechanisms of the alloy during hot deformation are dynamic recovery and dynamic recrystallization. At the same strain rate and under the temperature of 410℃, the hot deformation structure is mainly characterized as dynamic recovery structure. The predominant dynamic recovery mechanism is found to be cross-slip of screw dislocations. It is showed that when the temperature is higher than 440℃, a part of dynamic recrystallization can only be activated during hot compression. The main nucleation mechanism during dynamic recrystallization is grain boundary protruding and subgrain coalescence.(6) The favorable solution treatment and single aging treatment of the alloy is 475℃/40 min+quenching+120℃/24 h aging. Under such condition, theσh,σ0.2 and 8 are 676 MPa,623 MPa and 8.1% respectively. The strengthening phase of the alloy isη'(MgZn2),Al3Sc,Al3(Sc,Zr) and S'(Al2CuMg) phase. Following double solution(465℃/40 min+490℃/30 min), the strength of alloy increases and the precipitatη' phase distributes finely and dispersively. (7) The proper two-stage aging treatment is 120℃/8 h+150℃/16 h. The correspondingσh,σ0.2,δ, HV hardness and conductivity are 679 MPa,642 MPa,7.4%,195 HV and 30.2% IACS respectively. The two-stage aging can improve elongation, conductivity and the resistance of stress corrosion cracking. The microstructures within the grain coarsen obviously and the precipitates on the grainboundary distribute discontinuously.(8) The optimal RRA(120℃/24 h+180℃/30 min+120℃/24 h) treatment has been concluded. The correspondingσh,σ0.2,δand conductivity are 687 MPa,648 MPa,7.4% and 30.2% IACS respectively. After RRA treatment, the strength declines slightly, but the conductivity increases greatly which reflects a high resistance of stress corrosion cracking of the alloy. The precipitates whthin grain consist of fine and dispersiveη'phase which is similar with those under peak aging condition. The coarsened precipitates on grain boundary tend to be discontinuous.(9) The 77'phase whthin grain, the coarsed 7 phase on grain boundary and the precipitate free zone(PFZ) are the main factors of corrosion susceptibility of the alloy. While elevating the aging temperatures and increasing the aging time, the resistance of intergranular corrosion and exfoliation corrosion of the alloy rises. The order of corrosion susceptibility rank as follows:100℃>120℃140℃>160℃, Natural aging>Under aging>Peak aging>Over aging. The peak-aged alloy displays high strength and a poor performance of stress corrosion cracking (SCC). The over aging or two-stage aging obviously improve the resistance of stress corrosion cracking and decrease the strength apparently. The alloy undertaking RRA treatment shows better resistance of stress corrosion cracking and remains similar high strength of the peak-aged condition.(10)At the beginning of immersion in EXCO solution, the EIS of the alloy under various aging conditions is comprised of a capacitive arc at high-mediate frequency and an inductive arc at mediate-low frequency, and the inductive component is decreased or disappeared with time. Once exfoliation occurs on the surfaces of peak-aged, under-aged and natural-aged alloys, the EIS patterns are comprised of two overlapping capacitive arcs. However, on the over-aged alloy, two capacitive arcs appear in the EIS patterns due to the occurrence of grievous pitting corrosion. The equivalent circuit is designed according to the structure of corrosion and the mechanism of electrochemical corrosion, and all EIS patterns are simulated after the appearance of two capacitive arcs, and the good agreement between the experiment results and the simulated results has been obtained.(11)In peak-aged, under-aged and nature-aged alloys, with prolonging the immersion time in EXCO solution, exfoliation is deteriorated, leading to increased exposed area, and the corresponding capacitance (C2) is increased. The high increasing rate of C2 value in C2vs time curve is related to more sensitive exfoliation susceptibility of the corresponding alloy. In over-aged alloy, C2 is decreased with immersion time, and no exfoliation has been observed.
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