Atomic structure and evolution of a precursor phase of Ω precipitate in an Al-Cu-Mg-Ag alloy

Precipitates in an aged Al-Cu-Mg-Ag alloy have been characterised using atomic-resolution energy-dispersive X-ray spectroscopy (EDXS) scanning transmission electron microscopy (STEM) and high-angle annular dark-field (HAADF) STEM. A metastable precursor phase, designated Ω′, which forms as thin plates on {111}α planes of the α-Al matrix phase, is found for Ω precipitate which is the key strengthening constituent in the Al-Cu-Mg-Ag alloy. The Ω′ phase has a hexagonal structure that is distinguishably different from that of the Ω precipitate, and this hexagonal structure is isomorphous to that of the single unit-cell thick T1 precipitate plates in Al-Cu-Li alloys. Our proposed hexagonal structure is also supported by density functional theory (DFT) calculations. One interesting feature of Ω′ phase is that its thickness is invariably one unit-cell height, ∼0.915 nm thick. In contrast to the commonly accepted view that shear is not involved in diffusional phase transformations, the formation of Ω′ phase involves an appreciably large shear component, which is equivalent to two 30° Shockley partial dislocations, and the magnitude of this shear component is significantly larger than the dilatational strain associated with the α-Al → Ω′ structural transformation. With continued ageing, the metastable Ω′ phase transforms in-situ into the Ω phase. Compared to the direct α-Al → Ω structural transformation, the existence of Ω′ considerably lowers the energy barrier to the nucleation of Ω via the reduction of the shear strain.