Distribution of Ag and Mg in T1 precipitate plates in an Al-Cu-Li-Mg-Ag alloy

T₁ precipitate plates of one, two and three unit-cell thicknesses in a peak-aged Al-Cu-Li-Mg-Ag alloy are examined using atomic-resolution high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) and near atomic-resolution energy-dispersive X-ray spectroscopy (EDX)-STEM. In contrast to previous conflicting reports regarding the positions of Ag and Mg atoms, STEM observations reveal that Ag tends to segregate to the broad interfaces of T₁, irrespective of the thickness of the plates. Conversely, Mg consistently stays at both the T₁/α-Al broad interfaces and the interior region of the T₁ plates, acting as a constituent element of the T₁ phase in the Al-Cu-Li-Mg-Ag alloys. The observed distribution of Ag and Mg in the T₁ plates is supported by density functional theory (DFT) calculations. The DFT results further suggest that Mg atoms tend to replace Li atoms originally located in the T₁ plates. Moreover, the near absence of Ag in the interior region of the T₁ plate can be attributed to the destabilisation of Ag-Cu bonds through significant filling of antibonding states if Ag atoms were present within the T₁ plate. Regarding Mg, its ability to replace Li atoms in the T₁ plate is driven by the formation of enhanced chemical bonding.