Nanoscale perspective on the stress-corrosion cracking behavior of a peak-aged 7XXX-Al alloy

Abstract

High strength 7xxx Al-alloys are currently commonly used in aerospace and are expected to be increasingly employed in the automotive sector for weight reduction purposes. These alloys can however be sensitive to stress-corrosion cracking (SCC) depending on temper and loading conditions. Both the alloy's grain structure and composition are believed to play a key role in determining sensitivity to SCC. Here, we study at the nanometer scale the evolution of the microstructure near stress corrosion cracks on two different model variants of the 7140 aluminum alloy. We performed double cantilever beam (DCB) crack growth tests in hot (70{\deg}C) humid air, on samples extracted at quarter-thickness (T/4) and mid-thickness (T/2) and heat treated to a non-industrial, SCC sensitive T6 condition. The sample at T/4 shows a lower KISCC along with flatter grains and a higher solute content, whereas both samples exhibit similar crack growth rates at higher stress intensities. We report on precipitate dissolution and matrix solute enrichment near the crack tips, with the T/4 position presenting the higher increase in solute levels. The near grain boundary microstructure ahead of the crack is modified, with evidence of precipitate dissolution and transport of solutes towards the stress-corrosion crack tip. These results agree with a recent report on another 7xxx Al-alloy after SCC in Cl-solution, supporting the possibility that these mechanisms are generally occurring. We relate our findings with the measured SCC behavior and provide an array of possible mechanisms that could be widely applicable in SCC of high strength Al-alloys.