Abstract
Transmission electron microscopy (TEM) of a peak-aged Al–Zn–Mg alloy with a low Zn/Mg ratio aged at 120 °C revealed a precipitate previously unreported with lath- or plate-like morphology. Their cross-sectional planes were aligned parallel to ⟨110⟩Al, and their longitudinal direction was parallel to ⟨100⟩Al. High-angle annular dark-field scanning TEM images acquired along the ⟨100⟩Al and ⟨110⟩Al projections were used to propose a structural model for these precipitates, supported by density functional theory calculations. Among the evaluated models, the Al₉Zn₂₄Mg₉ composition exhibited the lowest formation energy and the greatest thermodynamic stability. The previously unreported phase is based on the TCO unit that constitutes the building block of GP1 precipitates, laterally connecting along ⟨330⟩Al.