Effect of Over-Aging Degree on Microstructures, Precipitation Kinetics, and Mechanical Properties of an Ultra-High-Strength Al-Zn-Mg-Cu Alloy

The effect of over-aging on the precipitation behavior and mechanical properties of an ultra-high-strength Al-Zn-Mg-Cu alloy was investigated using various over-aging treatment regimes. To reveal the influence of over-aging on matrix precipitation, nucleation and coarsening mechanisms were analyzed based on thermodynamic models and calculated precipitation data. Precipitation kinetics at different over-aging degrees were determined through differential scanning calorimetry analysis and the Johnson–Mehl–Avrami–Kolmogorov equations, revealing the activation energy required to precipitate the η’ phase precipitates ranging from 166.08 to 343.28 kJ/mol, and the activation energy required to precipitate the η phase precipitates ranging from 802.03 to 288.42 kJ/mol from the T6 to T73 conditions. In conjunction with a quantitative microstructure analysis, a highly accurate model was developed by systematically calculating the strengthening components of the ultra-high-strength Al-Zn-Mg-Cu alloy under various aging conditions.