Effect of preaging temperature on microstructure evolution, mechanical and corrosion behavior of RRA-treated high-zinc 7068 alloy

摘要

Mechanical and corrosion behavior of high-strength, high-zinc (>7 wt) containing 7068 aluminum alloy is investigated after employing different retrogression and reaging (RRA) treatments. The effect of preaging conditions on the distribution of copper, zinc, and magnesium, the volume fraction of eta ' phase, and the width of precipitate free zones (PFZ) have been investigated. Microstructural and compositional features characterized by scanning electron microscope-energy dispersive spectroscopy, scanning transmission electron microscope, and differential scanning calorimetry are correlated with hardness and corrosion performance. The localized attack is manifested in the dissolution of second phase precipitates which occurs from selective leaching of magnesium and aluminum. A combination of two opposite effects, that is, the presence of nobler, high-copper grain boundary precipitates and microgalvanic effect of PFZ along with the distribution of alloying elements, that is, Cu, Zn, and Mg govern the electrochemical behavior of RRA-treated 7068 alloy. Optimum preaging and RRA conditions are identified for this high-zinc 7xxx series alloy.