Regulating Microstructure Evolution and Strengthening Mechanisms in Al-Zn-Mg-Cu Alloy via Pre-Aging Treatment

This study significantly enhances the mechanical properties of an Al-Zn-Mg-Cu alloy through the implementation of a pre-aging process. By optimizing the microstructure of the Al-Zn-Mg-Cu alloy with different pre-aging treatments, the evolution of the microstructure and mechanical properties of the alloy initially containing GP I, GP II, and η′ phases is systematically investigated during aging at 140 °C. The experimental results show that, under the three pre-aging processes, the peak tensile strengths are 590.8 MPa, 594.0 MPa, and 612 MPa, respectively, while the corresponding elongation rates are 8.2%, 8.4%, and 10.3%. When pre-aging produces an initial microstructure containing GP I and GP II, these GP zones rapidly coarsen within the grains during subsequent aging. This makes it difficult for solute atoms to diffuse to the grain boundaries, resulting in finer grain boundary precipitates and ultimately leading to a lower alloy strength. When the pre-aging temperature is 120 °C, the pre-aging process can reduce the vacancy concentration, thereby suppressing the phase transformation from η′ to η precipitates. For samples pre-aged to the η′ phase, solute atoms diffuse to the grain boundaries, resulting in grain boundary precipitates with a greater length during subsequent aging compared to the other two samples. These grain boundary precipitates exhibit a discontinuous distribution along the grain boundaries, which contributes to the improved elongation of the alloy. The present work provides a novel heat treatment strategy for producing high-strength Al alloys while effectively achieving a favorable balance between strength and ductility.