The main mechanism for the strengthening of aluminium‐copper alloys of the 2xxx type is hardening by copper‐rich precipitates. However, their size, distribution, and crystal structure determine the final mechanical properties of the material. It has been shown that alloying additionally small amounts of cadmium, indium, or tin influences the precipitation behavior as well as the final strength of Al‐Cu alloys. The binding energy of quenched‐in vacancies to trace elements in the aluminium matrix is recognized as an influence on the diffusion behavior of the copper atoms and thus the preferred type of precipitate changes. A precondition for this influence is the transition of trace elements into solid solution during the solution heat treatment. In the present work, solubility and interaction with quenched‐in vacancies is analyzed for the elements In, Sn, Sb, Bi, and Pb in high‐purity binary alloys using differential scanning calorimetry (DSC), positron annihilation lifetime spectroscopy (PALS) as well as scanning and transmission electron microscopy (SEM, TEM). The results confirm on one hand literature data and deliver on the other hand new structural details. A subsequent anneal at moderate temperature leads to finely distributed precipitations on the nanoscale.

Although binary aluminium alloys seem to be uninteresting and well known, some aspects of their precipitation sequence -especially the early stages immediately after quenching -are still not well understood. Since the Al-Cu system is the basis for many ternary and quaternary high-strength alloys with application in the aviation sector, it is important to understand this binary system in detail. This problem is here tackled by a unique combination of differential scanning calorimetry and X-ray absorption fine structure measurements, where relaxed atomic coordinates for simulation of the spectra have been obtained by ab initio calculations. Thereby, it is possible to attribute any exo-or endothermal peak to a certain type of precipitate, even though formation and dissolution regions have a large overlap in this system. This unique combination of experimental and numerical methods allows one to determine the local atomic environment around Cu atoms, thus following the formation and growth of Guinier-Preston zones, i.e. Cu platelets on {100} planes, during the precipitation process. research papers 1340 Danny Petschke et al. Time-resolved XAFS on Al-Cu alloys

Adding trace elements (Cd, In, Sn) to Al‐Cu‐based alloys can significantly improve their strength by the growth of small and finely distributed θ′ precipitates. However, the underlying atomic mechanisms of their nucleation are so far only superficially understood. We follow the precipitation process, that is changes in the microstructure, by different methods: differential scanning calorimetry (DSC), giving information on formation and dissolution of precipitates, 3D atom probe tomography (3DAP), giving information on size and density of precipitates and finally, positron annihilation lifetime spectroscopy (PALS), being sensitive especially to quenched‐in vacancies and their interaction with alloying elements. By the use of these complementary methods we obtain information on vacancy binding to the alloying elements and also on structure, kind and distribution of precipitates while correlating this with hardness measurements.

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Even though, the crystal structure of the intermediate (S') and the equilibrium S (Al2CuMg) phase were subject of many investigations by using mostly imaging or diffraction techniques, the results remain still controversial. In this study, we used X-ray absorption spectroscopy (XAFS) to verify the correct crystal structure considering the well-known models reported by Perlitz & Westgren (PW), Mondolfo, Radmilovic & Kilaas and Yan et al. The S phase structure was confirmed by direct comparison to simulated XAFS spectra using FDMNES. Our results support the widely accepted PW model as the correct structure while other models do not match our observations.