Structure and energetics of the coherent interface between the θ′ precipitate phase and aluminium in Al–Cu

The θ′ phase is a well-known intermediate phase that forms as part of the classic decomposition sequence of an aluminium–copper solid solution. It is also the archetypal precipitate phase of high-strength aluminium alloys. Here we use atomic-resolution electron microscopy imaging to reveal that the coherent interface of θ′ with aluminium can adopt a structure that differs from that previously assumed based on the bulk crystal structures. These observations, combined with first-principles energetics calculations for realistic models of embedded θ′ precipitates, show that this new interfacial structure constitutes an intermediate state in the atomistics of precipitate thickening. These findings will require revision of theoretical models of θ′ precipitate growth and nucleation.