On the interaction of solute atoms with vacancies in diluted Al-alloys: A paradigmatic experimental and ab-initio study on indium and tin

To study the vacancy-solute-atom interaction, diluted Al-In and Al-Sn alloys with 0.005 and 0.025 at.% indium/tin cast from very high purity elements were investigated by positron annihilation spectroscopy (PAS). Therefore, the alloys have been solution heat treated at temperatures ranging from 320 to 620 °C and then rapidly quenched into ice water freezing-in most thermal vacancies. Positron annihilation lifetime (PALS) and coincidence Doppler broadening spectroscopies (CDBS) were combined to unambiguously identify vacancy-solute-atom complexes. For enabling a direct comparison to experiment, we did employ ab-initio DFT calculations of vacancy-solute-atom complexes providing relaxed atomic coordinates, which are used to calculate PAS annihilation parameters. In the as-quenched state vacancy-solute-atom pairs as well as vacancy clusters were observed in both alloys for all concentrations. During isochronal annealing vacancy clusters, formed during quenching, dissolved at about 130 °C, leaving vacancy-solute-atoms complexes as the only remaining defects. Thus, we could unambiguously identify those by a combination of PALS and CDBS with ab-initio calculations. Employing isothermal annealing the binding energy EB of vacancies to In and Sn solute atoms was determined experimentally by PALS as well as by ab-initio calculations. We find from our experiments EB = (0.20 ± 0.03) and (0.32 ± 0.10) eV for In and Sn, respectively, which is in very good agreement with our ab-initio calculations giving 0.23 and 0.26 eV, respectively. Our results clearly identified vacancy-In/Sn complexes as responsible for the retardation of vacancy migration after quenching. The vacancies bound to In and Sn solute-atoms are released around 150 °C shifting, when added to AlCu alloys as trace elements, the transport of copper atoms to higher temperatures necessary for the formation of finely distributed ″precipitates″, which efficiently strengthen this kind of alloys.