Natural ageing of Al–Cu–Mg revisited from a local perspective

Although Al alloys based on the Al–Cu–(Mg) system have been investigated for decades, information about the evolution of microstructure on the atomistic level during natural ageing is scarce. Therefore, the early stages of natural ageing in laboratory Al–Cu–(Mg) alloys and in AA2024 were investigated using positron annihilation and X-ray absorption spectroscopy. This complementary approach allows for accessing both essential components of Al alloys, namely vacancies and alloying elements. It is found, that during natural ageing the chemical environment of vacancies is formed both of Cu and Mg atoms and that the rearrangement of vacancy surroundings persists the hardness increase of Al–Cu–Mg alloys. During natural ageing two different regimes of vacancy environment are detected and interpreted in terms of cluster growth and vacancy capture. Features of the near edge structure of the X-ray absorption are interpreted using theoretical calculations obtained by the FEFF8.4 code. Thus the agglomeration of Cu and Mg, which is accompanied by lattice distortions around Cu atoms, can be probed. A minimal size of Cu–Mg co-clusters is proposed for later stages of natural ageing.

Research highlights▶ Vacancies are surrounded by Mg and Cu atoms in Al–Cu–Mg alloys. ▶ Natural ageing of Al–Cu–Mg alloys consists of two distinct regimes on an atomic scale. ▶ Clusters formed during natural ageing contain more than 2–8 atoms.