Evolution of Ω phase in an Al-Cu-Mg-Ag alloy -a three-dimensional atom probe study

Evolution of plate-like precipitates of the Ω phase in an Al-1.9Cu-0.3Mg-0.2Ag (at.%) alloy has been investigated by three-dimensional atom probe (3DAP) and transmission electron microscope (TEM) techniques. In an as-quenched alloy, all solute atoms are uniformly dissolved. After 5 s aging at 180°C, co-clusters of Ag and Mg atoms are present. Initially, these co-clusters do not have well-defined shape, but after aging for 120 s, they are aligned on the {111} planes incorporating Cu atoms. Initially, they do not have the distinct structure consistent with the Ω phase, thus these are designated as {111}GP zones. The Ω platelet which appears after the {111}GP zones contain all solute elements within the precipitate having a composition of approximately 25 at.% Cu, 20 at.% Mg and 10 at.% Ag. In the well-defined Ω phase which appears after 2 and 10 h aging at 180°C, Ag and Mg atoms are strongly segregated to the broad face monolayer interfaces of α/Ω. No Mg and Ag are incorporated within the platelet, and its chemical composition is Al-33 at.% Cu. Based on these observations, it is concluded that the Ω phase evolves from co-clusters to plate-like precipitates continuously, rather than precipitating discretely by heterogeneous nucleation to their precursors. In a well-grown Ω plate, elemental mapping near a ledge was observed by 3DAP for the first time.