  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.
