Relating the Early Evolution of Microstructure with the Electrochemical Response and Mechanical Performance of a Cu-Rich and Cu-Lean 7xxx Aluminum Alloy

In this work AA7150 and AA7079 were studied as examples of 7xxx aluminum alloys with a high and low Cu content. The alloys were carefully examined to determine the evolution (incl. density and size) of precipitates during heat-treatment using small angle X-ray scattering (SAXS). Such characterization made it possible to relate the evolution of microstructure to specific properties such as electrochemical response (including metastable pitting analysis), hardness and crack growth rate (from SCC testing). The results revealed several important findings, indicating that there is a critical precipitate size above which a transition in electrochemical response occurs; in addition it is seen that Cu has an important role in both metastable pitting and crack propagation. The corrosion initiation and propagation phases are distinct in their mechanisms, and this is a function of the nature and state of the precipitates. Increased aging times increased the metastable pitting rate and SCC velocity in the Cu-lean alloy, whereas increased aging times decreased the metastable pitting rate the Cu-rich alloy. A correlation between metastable pitting rate and stable pitting is also presented.