Abstract
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}} leads to finely distributed theta , '-precipitates on the nanoscale, since due to the trace additions the formation temperature of theta , ' is lowered by more than 100,^circ {text {C}}. According to TEM small agglomerates of trace elements (In, Sn) may support the early nucleation for the theta , '-precipitates.
Highlights
In early works on aluminum alloys, after quenching, an enhanced diffusion at room temperature was noticed [1]
The time of several hours to take one single Positron Annihilation Lifetime Spectroscopy (PALS) spectrum could not shed light on these complicated processes immediately starting after quenching when storing the samples at room temperature
In the first case the solubility has to be smaller than 2 ppm, since this is the lower sensitivity limit for detecting vacancies with PALS
Summary
In early works on aluminum alloys, after quenching, an enhanced diffusion at room temperature was noticed [1]. Later the method of Positron Annihilation Lifetime Spectroscopy (PALS) has been applied [5, 6] These early works were hampered from a poor understanding of the complex interaction of positron trapping to quenched-in vacancies, vacancysolute-atom complexes and clusters of solute atoms forming immediately after quenching [7]. The time of several hours to take one single PALS spectrum could not shed light on these complicated processes immediately starting after quenching when storing the samples at room temperature. This problem is evident for all methods where data acquisition takes more than a few minutes
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