Abstract:Backward extrusion 7A99 ultrahigh-strength aluminium alloy was treated by T6 peak aging treatment and peak cold-heat cycle aging treatment (T6-DCT). TEM, HRTEM and 3DAP were employed to study the influences of T6-DCT treatment on the types, distribution, size and density of the precipitated phases. The result shows that the types of the precipitated phases increase, that η" phase turn to η" phase and η phase coexistence together. The avearge equivalent radius of the precipitated phases change from 1.2 nm to 1.0 nm after T6-DCT. The density of the precipitated phases increase from 4.53×1024/m3 to 7.55×1024/m3, which lead to dispersion-strengthening. After T6-DCT, the segretion of Zn and Mg decreases, and the precipitated phases distribute more homogeneous than that of T6 treatment.