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 θ_II^′. Isomorphous with the well-known body-centred tetragonal Al₂Cu phase that occurs in Al-Cu-based alloys, we observed the following novel orientation: {0 0 1}_θII′ / / {1 1 0}_α, 〈 1 0 0 〉_θII′ / / 〈 0 0 1 〉_α. A trace of Mg was associated with the θ_II^′ 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 θ_II^′, designated as σ_II: {0 0 1}_α / / {0 0 1}_σII and 〈 1 1 0 〉_α / / 〈 0 1 0 〉_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.