Precipitation reactions in Al–4.0Cu–0.3Mg (wt.%) alloy Original Research Article

Analytical transmission electron microscopy was used to characterize precipitation processes in a model Al–4.0Cu–0.3Mg (wt.%) alloy aged at 200 °C. The evolution of microstructure was more complex than previously reported and involved processes common both to the binary Al–Cu and ternary Al–Cu–Mg systems. We report precipitation of a novel orientation of the common intermediate phase θ′, which we have designated image. Isomorphous with the well-known body-centred tetragonal Al2Cu phase that occurs in Al–Cu-based alloys, we observed the following novel orientation: image, image. A trace of Mg was associated with the image orientation and we suggest that this is a consequence of the role that Mg atom clusters play during nucleation. For the first time, we also report the occurrence of the σ phase (Al5Cu6Mg2) in a new orientation analogous to that of image, designated as image and image. These results are discussed in terms of microstructural design for age hardening in Al–Cu–Mg base alloys, much of which involves the θ′ and σ precipitate phases.