Atom-Probe Tomographic and Electron Microscopic Analyses of a High Strength 7075-T4 Aluminum Alloy

The 7000 series aluminum alloys are widely used in the aerospace and automotive industries due to their high strength and low weight. Those advantages of the 7000 series aluminum alloys, however, are hampered by an occurrence of the hydrogen embrittlement and/or stress corrosion cracking under certain heat treatment conditions such as natural or artificial aging treatment. Herein, we utilize an atom-probe tomography (APT) and a transmission electron microscopy (TEM) to study the microstructural evolution of a naturally aged commercial 7075-T4 grade aluminum alloy. Both techniques reveal the formation of secondary phases such as ή'-Mg(Zn, Cu)₂, η-Mg(Zn, Cu)₂, and E-phase (Al₁₈Mg₃Cr₂) precipitates. Based upon the three-dimensional reconstructed images obtained using APT, we quantitatively analyze each phase composition and segregation of the interface such as Mg, Zn, and Cu alloying elements. In addition, the hydrogen atoms are substantially segregated in an interface between E-phase and aluminum matrix. Importantly, these findings demonstrate that correlative APT and TEM characterization are able to provide a fundamental idea as well as a roughly quantitative analysis in the three dimensions of the precipitate morphologies. At the same time, our observation could correlate mechanical failure caused by hydrogen embrittlement and stress corrosion cracking performances in the 7075-T4 grade aluminum alloy.