Precipitation in an Al-Zn-Mg-Cu alloy during isothermal aging: Atomic-scale HAADF-STEM investigation

The present study, using advanced Cs-corrected high-angle annular dark field - scanning transmission electron microscopy (HAADF-STEM), reports on a comprehensive investigation into the precipitate structures in an Al-Zn-Mg-Cu alloy aged at 150 ℃, including GP zones, η' and η precipitates. In the nucleation stage, Zn atoms enrich on the {111}Al-planes abutting spherical Mg clusters that are approximately 3-6 nm in diameter. In the subsequent growth, the as-nucleated structures extend with an increasing diameter and a constant width along [111]Al and grow into platelet precipitates. η' is proved to be a group of metastable structures existing in the transition from FCC Al to HCP MgZn2 (η). Some metastable structures are assembled by local-ordered rhombohedral units and orthorhombic units as building blocks. Subsequently, the precipitates evolve into η phases with stacking faults.