Abstract:Indirect extrusion of 7A99 ultrahigh-strength aluminium alloy was heat treated by T6 peak aging treatment and peak cold-heat cycle aging treatment of -180℃ (T6-DCT). XRD, TEM, HRTEM and 3DAP were applied to study the influences of T6-DCT treatment on the strength and toughness properties, precipitation behavior. The result shows that The lattice constant of Al matrix increases from 0.40551 nm to 0.40626 nm after T6-DCT, which plays a solid solution strengthening role. A large number of η phase generate in the grains, and the η phases at the grain boundaries are discontinuous, meanwhile the precipitation free zone is formed at the grain boundaries, which reduces the tensile strength. After T6-DCT treatment, the segreation degree of Zn and Mg increases, the precipitation ratio and distribution uniformity of precipitated phases decrease. The avearge equivalent radius of the precipitated phases changes from 1.2 nm to 1.14 nm after T6-DCT treatment, the density of the precipitated phases decrease from 4.53×1024/m3 to 3.87×1024/m3, which weaken the precipitation-strengthening. The strength and toughness improve significantly after T6-DCT treatment, which shows that the strength is slightly reduced and the toughness is improved remarkably.