Determination of interfacial atomic structure, misfits and energetics of Ω phase in Al–Cu–Mg–Ag alloy

The interfacial atomic structure and misfits of Ω precipitates formed in the face-centered cubic Al in the Al–Cu–Mg–Ag alloy have been determined by combining scanning transmission electron microscopy (STEM) Z-contrast imaging with chemical analysis and ab initio density functional theory (DFT) calculations. Precipitates of several thicknesses formed after heat treatment at 250 °C, starting from 0 to 2 unit cells of Ω phase, were examined by STEM in four different projections. The results show that a remarkably stable double-layered interface is formed at all observed thicknesses, which separates the Ω phase from the Al matrix. The outermost interfacial layer next to Al is composed of Ag atoms in a hexagonal structure and Mg or Cu atoms below the center of the hexagon. Structural models constructed based on the experimental data were relaxed using DFT-based molecular dynamics calculations. The results show that interfacial Mg atoms, together with Ag atoms, greatly stabilize the interface structure and consequently the Ω phase on the Al {1 1 1} habit planes. Comparison between the measured and calculated precipitate misfit along the thickness direction suggests that atomic substitutions of light atoms, Al and Mg, at the interface mediate the misfit strain and free energy, which is further supported by experimental evidence obtained from STEM. Thus, we have identified here: (i) the driving force for the Ag and Mg segregation in the formation of the Ω phase; (ii) the precipitation sequence characterized by a stable interfacial double-layer; and (iii) an interfacial substitution mechanism for misfit accommodation.