Probing the defect state of individual precipitates grown in an Al-Mg-Si alloy

Precipitates forming in decomposable aluminum alloys such as Al-Mg-Si evolve toward the corresponding intermetallic phase, which is β (Mg2Si) in this case, depending on heat-treatment conditions. Individual β precipitates were produced in an Al-1.11 at. % Mg-0.77 at. % Si alloy and identified using optical as well as electron microscopy. The individual β precipitates could be investigated with regard to their intrinsic crystal defects using a finely focused positron microbeam provided by the Bonn Positron Microprobe. Comparison with theoretical calculations of the Doppler broadening of annihilation radiation reveals that β precipitates most likely do not contain vacancies in either sublattice and that 0.16 is the upper bound of the fraction of trapped positrons. The usage of different enhancement factors had only little influence on the calculations whereas the general gradient approximation affected the contribution of Si orbitals, in particular. Additional measurements of the Doppler broadening based on the radioactive source 68Ge, which emits high-energy positrons probing bulk regions of the sample, were carried out. These measurements show that β precipitates are sparsely distributed in the Al matrix.