Precipitation reactions in Al-4.0Cu-0.3Mg (wt.%) alloy

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 θII′. Isomorphous with the well-known body-centred tetragonal Al2Cu phase that occurs in Al-Cu-based alloys, we observed the following novel orientation: {001}θII′//{110}α, 〈100〉θII′//〈001〉α. A trace of Mg was associated with the θII′ 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 θII′, designated as σII:{001}α//{001}σII and 〈110〉α//〈010〉σII. 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.