Investigation on precipitate behavior and mechanical properties of Al-Zn-Mg-Cu alloys with various Zn/Mg ratios

The effect of Zn/Mg ratio on the microstructure and properties of Al-Zn-Mg-Cu alloys under single-step and double-step aging processes were investigated by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), as well as hardness, room temperature tensile, and fracture toughness testing. During single-step aging, as the Zn/Mg ratio increases, the aging response of the alloys accelerates. However, the peak hardness and strength gradually decreased while the average size of precipitates increased. During double-step aging, as the secondary aging time increases, the strength and hardness of the alloys gradually decrease. Additionally, the higher the Zn/Mg ratio, the faster the strength and hardness decline. Under the T74 temper, the alloy with higher Zn/Mg ratios exhibits lower hardness and strength but better fracture toughness and larger precipitate sizes. A simple method was proposed to statistically determine the critical diameter size of GPII zone and eta ' phase, which was approximately 4.47 nm. A strength model was developed to predict the yield strength of alloys, considering factors such as solid solution strengthening, grain boundary strengthening, and precipitation strengthening. The model demonstrates good predictive accuracy for both peak-aged and T74 temper alloys, providing valuable guidance for alloy design and microstructure regulation.