Abstract: In addition to the three well-known Ag-related precipitates (Ω, X′ and Z) in the Al-Cu-Mg-Ag alloys, Ag can also be involved in the formation of the as-cast second phases. However, the effect of Ag addition in Al-Cu-Mg-Ag alloys has not been completely studied and even the structure of the as-cast Ag-containing phases is still controversial. By employing the focused ion beam (FIB) combined with transmission electron microscopy (TEM) techniques and density functional theory (DFT) calculations, the formation mechanisms of the Ag-containing phases in the as-cast Al-Cu-Mg-Ag alloys have been investigated. The Ag-containing phases are a series of hexagonal C14-type Laves phases with continuously varying Ag concentrations, described as (Al_xCu_yAg_1-x-y)₂Mg. Moreover, the specific occupancy sites of the atoms in (Al_xCu_yAg_1-x-y)₂Mg were determined. The formation of the (Al_xCu_yAg_1-x-y)₂Mg can be attributed to the stronger Ag-induced aggregation of solute atoms in the initial stage and the establishment of strong Ag-X (X = Al, Mg and Ag) bonding in the Ag-containing phases. Furthermore, our experiments have revealed the solidification sequence of Al-Cu-Mg-Ag alloys, and pointed out that (Al_xCu_yAg_1-x-y)₂Mg is formed at a lower temperature (493.9 °C) through the reaction L ⇌ Al₂CuMg + (Al_xCu_yAg_1-x-y)₂Mg. The study could have positive implications for refinement of the Al-Cu-Mg-Ag quaternary phase diagram and promote the composition-property design of novel aluminum alloys based on (Al_xCu_yAg_1-x-y)₂Mg in the future.

Key words: Al-Cu-Mg-Ag alloys, Laves phases, HAADF-STEM, First-principles calculation, Formation mechanisms