Microstructure Evolution and Localized Corrosion Susceptibility of an Al-Zn-Mg-Cu-Zr 7xxx Alloy with Minor Cr Addition.

Microstructure optimization of Al-Zn-Mg-Cu-Zr aluminum alloys, particularly through recrystallization inhibition, for improved strength and corrosion resistance properties has been an important consideration in alloy development for aerospace applications. Addition of rare earth elements, sometimes combined with Cr, has been found to be beneficial in this regard. In this study, the role of a single addition of 0.1 wt.% Cr on microstructure evolution of an Al-Zn-Mg-Cu-Zr (7449) alloy during processing was systematically investigated by optical light microscopy, scanning electron microscopy, electron backscatter diffraction and scanning transmission electron microscopy. Susceptibility to localized corrosion after aging to T4, T6 and T76 conditions was evaluated by potentiodynamic polarization (PDP) measurements and an intergranular corrosion (IGC) test. A decrease in recrystallized fraction with 0.1 wt.% Cr addition was observed, which is attributed to the formation of Cu- and Zn-containing E (AlMgCr) dispersoids and the larger as-cast grain size. Moreover, the depletion of alloying elements from solid solution due to the formation of the Cu- and Zn-containing E (AlMgCr) dispersoids and Mg(Zn,Cu,Al) phase at its interface affects grain-boundary precipitation. The observed decrease in localized corrosion susceptibility with minor Cr addition is correlated with the microstructure and equally discussed.