Improving the microstructure and properties of a high-alloyed Al-Zn-Mg-Cu alloy by optimizing the initial temperature of Pre-recovery multi-stage solution treatment (P-MST)

This study proposed a new strategy to improve the microstructure and properties of Al-Zn-Mg-Cu alloy via optimizing the initial temperature of pre-recovery multi-stage solution treatment (P-MST, T0°C/12h + 350 °C/12h + 450 °C/2h + 460 °C/2h + 470 °C/2h). This optimization facilitated the adjustment of dislocation density, element solubility, and grain size, resulting in a significant increase in mechanical properties and corrosion resistance. The research found that during the T0°C/12h + 350 °C/12h stage, as the T0 temperature increased, the alloy's dislocation density initially decreased before rising again. The minimum point was achieved at T0 of 300 °C, which led to the lowest driving force for grain growth in the subsequent MST, resulting in the smallest grain size and best corrosion resistance in the T6-aging state. Additionally, during the T0°C/12h and T0°C/12h + 350 °C/12h stages, with a rise in T0 temperature, the average size of the second phase gradually increased. The highest elemental solubility of the matrix after MST occurred at T0 of 300 °C, promoting the alloy to demonstrate the maximum aging precipitation strengthening. Nevertheless, at T0 of 250 °C, the alloy, after T6-aging treatment, presented the lowest dislocation density, thereby achieving the highest strain-hardening capacity and plasticity. At T0 of 250 °C and 300 °C, the alloy exhibited superior properties, with the yield strength of 774 MPa and 803 MPa, the ultimate tensile strength of 802 MPa and 823 MPa, the elongation of 10.1 % and 6.9 %, and the corrosion current density of 1.37 × 10⁻⁶ A cm⁻² and 2.44 × 10⁻⁶ A cm⁻² in 3.5 wt % NaCl solution, respectively.