Microstructure, strength and corrosion resistance in an Al-Zn-Mg-Cu alloy after artificial aging and subsequent long-term natural aging

Natural aging after artificial aging is inevitable in the long-term service of Al-Zn-Mg-Cu alloy. However, the mechanism of microstructural evolution and its effect on the properties remains largely unexplored. Aberrationcorrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray spectroscopy (EDS) were employed to analyze the precipitation behavior, especially at the grain boundary, and the impact on the properties of the alloy after long-term natural aging. After natural aging, the precipitate free zone (PFZ) widens substantially, and a new Zn-rich grain boundary phase with MgZn3 structure is observed. The phase increases the hardness in conjunction with other grain boundary precipitation phases, which promotes grain boundary strengthening and subsequently enhances the mechanical properties of the alloy. In addition, the formation of Zn-rich grain boundary phase creates multiple electrochemical microcouples with other phases, the widened PFZ and the Al matrix, which increase electrochemical activity and consequently
affect the corrosion properties. These results reveal the grain boundary precipitation behavior and the influence
mechanism on the properties, providing valuable insights for the development of high-strength and corrosionresistant
alloys.