Precipitation strengthening in naturally aged Al–Zn–Mg–Cu alloy

We investigated the precipitation behavior and mechanical properties of naturally aged Al–7.6Zn–2.7Mg–2.0Cu–0.1Zr–0.07Ti (wt.%) alloy using various experimental methods including electron backscatter diffraction, microhardness, electrical resistivity, differential scanning calorimetry, X-ray diffraction, transmission electron microscopy (TEM), atom-probe tomography (APT), and tensile tests. GPI zones commenced forming after 0.5 h of natural aging and increased in volume fraction during further aging. GPII zones were observed in alloys that were naturally aged longer than 1500 h at 25 °C. The APT and TEM analyses identified blocky GPI zones and elongated GPII zones on the {111}_Al planes in the alloy that were naturally aged for 1500 h. The compressive lattice strains of the GPI and GPII zones were ~2.6 and ~7.8%, respectively, at the centers of the zones. For the GP zones, the Zn/Mg atomic ratio was ~1.2, the mean radius 0.76 nm, the number density 3.22 × 10²⁴ m⁻³, and volume fraction 0.58%. Yield strength modeling, strain hardening behavior, and fractured morphology analyses indicated that the improved strength from natural aging was due to the coherency plus modulus mismatch strengthening of the GP zones, which had an average misfit strain of ~4%.