| The age-hardening Al-Zn-Mg-Cu aluminum alloy is widely used in aeronautic industries due to its super-high strength compared with the other conventional aluminum alloys. However, a series of problems such as low fracture toughness and weak stress-corrosion resistance also occur because of its high alloying. By using tensile test, scanning electron microscopy, energy dispersive spectrometry, transmission electron microscopy and x-ray diffraction spectrometry, the effect of alloying elements and different heat treatments on the rnicrostructure and mechanical properties of high-zinc super-high strength aluminum alloys are discussed. The results show that (l)For the studied Al-10.4%Zn-2.2%Cu-2.4%Mg-0.1~0.15Zr-0.224%Ag alloy, the processing of strengthening solution heat treatment and ageing subsequently, of which the solution temperature is close to the melting point of non-equilibrium eutectic phases, can make the alloy obtain a good combination of strength and plasticity. Compared with the other two processes of single stage and low temperature promotive solution heat treatments, its tensile strength is obviously higher up to about 770MPa and the elongation ratio remains about 8% to 10%. (2) For the studied Al-10.4%Zn-2.2%Cu-2.4%Mg-0.1~0.15Zr-0.224%Ag alloy, it maintains high tensile strength of about 740-750MPa when pre-aged at slightly under-aged condition on 120癈. The RRA treatment can provide the rnicrostructure of T6 temper in the matrix and the grain boundary precipitate structure possessed of an over-aged condition. With the retrogression time prolonged, the tensile strength of the alloy gradually decreases while the elongation ratio a little increases. (3) For the studied Al-12.lZn-2.27Mg-2.4Cu-0.2Ag-0.15Zr alloy, its solution annealing temperature can be increased due to small addition of Ag. When the silver-bearing alloy is solutionized on a higher temperature and aged subsequently, the remain coarse particles in its microstructure can be solutionized into the matrix more sufficiently, and thus makes the alloy obtain a same level of the tensile strength as the silver-free one at peak-age hardening. During ageing, trace addition of Ag enhances theformation of metastable phases and stabilizes GP zones and metastable phases to a higher temperature. However, small addition of Ag prolongs the ageing time reaching the peak-age hardening for the studied alloy. (4) For silver-free high-Zn super-high strength aluminum alloy, undissolved Cu-rich phases M (AlZnMgCu) exist in its solid-solution microstructure. Zn content in M (AlZnMgCu) compounds gradually decreases during ageing, and Cu content become higher relatively. For silver-bearing high-Zn super-high aluminum alloy, undissolved Cu/Ag rich phases M(AlZnMgCuAg) exist in its solid-solution microstructure. At the early stage of ageing, the amount of Ag contained in the M(AlZnMgCuAg) compounds significantly lessens and the Zn content increases greatly. With the ageing time prolonged, the Zn content in the undissolved phases gradually decreases, while the Mg, Ag contents increase relatively. Small addition of Ag promotes a larger amount of undissolved phases within the microstructure of the as-quenched high-Zn aluminum alloy. These undissolved phases changes a little in their size and amount during ageing and become sites of stress concentration and micro-crack initiation, which reduces the elongation of alloy at the initial stage of ageing. (5) Zn and Mg are the main hardening alloying elements in 7XXX series aluminum alloys. Increasing the addition of Zn and Mg to the alloys can provide a good combination of high strength and well plasticity. However, when the Zn, Mg contents surpass a certain limit, their increasing addition promotes the formation of residual coarse phases within the microstructure of the alloys, which deteriorates the mechanical properties of the alloys. |
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