Coexistence of clusters, GPB zones, S″-, S′- and S-phases in an Al–0.9% Cu–1.4% Mg alloy Original Research Article

The precipitation phenomena in an Al–0.9 at.% Cu–1.4 at.% Mg alloy were studied by high resolution electron microscopy and calorimetry in order to clarify the transformation sequence. This paper shows for the first time that, at around 200°C, there is coexistence of four different phases: (i) partially ordered nanometer-sized Mg-rich clusters of ellipsoidal shape, (ii) ordered GPB zones of one monolayer thickness, (iii) monoclinic semi-coherent S″ phase, and (iv) orthorhombic semi-coherent S′ which evolves into the incoherent equilibrium S phase of Al2CuMg. At room temperature, Mg-rich clusters precipitate before Cu–rich GPB zones. Clusters are considered to be precursors of the S′ phase, whereas GPB zones may transform into S″. New lattice parameters for S″ and S′ are proposed.