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.
Aluminium-copper alloys of the 2xxx type receive their excellent mechanical properties by the formation of copper-rich precipitates during hardening. Size, distribution and crystal structure of the formed precipitates determine the final strength of those alloys. Adding traces of certain elements, which bind to vacancies, significantly influences the decomposition behaviour, i.e. the diffusion of the copper atoms. For high-purity ternary alloys (Al-1.7 at.% Cu-X), we investigate the interaction of copper and trace element atoms (X=In, Sn, and Pb) with quenched-in vacancies by Positron Annihilation Lifetime Spectroscopy (PALS). By employing Vickers microhardness, Differential Scanning Calorimetry (DSC) and Small Angle X-Ray Scattering (SAXS) we obtain a comprehensive picture of the decomposition process: opposite to predicted binding energies to vacancies by ab-initio calculations we find during ageing at room and elevated temperature a more retarded clustering of copper in the presence of In rather than for Sn additions, while Pb, having the highest predicted binding to vacancies, shows nearly no retarding effect compared to pure Al-Cu. If the latter would be due to a limited solubility of lead, it had to be below 2 ppm. Transmission Electron Microscopy (TEM) as imaging method complements our findings. Annealing the quenched Al-1.7 at.% Cu-X-alloys containing 100 ppm In or Sn at $$150\,^\circ {\text {C}}$$
150
∘
C
leads to finely distributed $$\theta \, '$$
θ
′
-precipitates on the nanoscale, since due to the trace additions the formation temperature of $$\theta \, '$$
θ
′
is lowered by more than $$100\,^\circ {\text {C}}$$
100
∘
C
. According to TEM small agglomerates of trace elements (In, Sn) may support the early nucleation for the $$\theta \, '$$
θ
′
-precipitates.
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