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

Abstract 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 } θ II ′ / / { 1 1 0 } α , 〈 1 0 0 〉 θ II ′ / / 〈 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 } σ II and 〈 1 1 0 〉 α / / 〈 0 1 0 〉 σ II . 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.