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 ??, which we have designated . Isomorphous with the well-known body-centred tetragonal Al2Cu phase that occurs in Al–Cu-based alloys, we observed the following novel orientation: , . A trace of Mg was associated with the 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 ? phase (Al5Cu6Mg2) in a new orientation analogous to that of , designated as and . These results are discussed in terms of microstructural design for age hardening in Al–Cu–Mg base alloys, much of which involves the ?? and ? precipitate phases.
