The aging strengthening effect, stress corrosion cracking (SCC) behaviour and microstructure of 7075 aluminium alloys under double peaks aging treatment have been investigated in this paper. The results show that the hardness and strength of alloys under the second aging peak are equal or slightly higher than those under the first peak. The microstructure analysis demonstrates that the aging strengthening effect of the first peak and second peak mainly depend on fine and intensive GP zones (2-6 nm in size with an inter-particle distance of 4-10 nm) and η’ phase (4-8 nm in size), respectively. Meanwhile, the SCC resistance of specimens under the second aging peak is better than the first peak aging treated ones. Lower Mg and higher Cu segregation on grainboundary induced by second peak aging treatment are considered to be the reason for better SCC behaviour according to high-resolution transmission electron microscopy and energy dispersive spectrometer results.
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