In-situ monitoring of artificial aging and solution heat treatment of a commercial Al-Mg-Si alloy with a high intensity positron beam.

Microstructural changes of a commercial Al?Mg?Si alloy were studied during artificial aging by in situ Doppler broadening spectroscopy using a high-intensity positron beam. The in situ positron annihilation characteristics at high temperatures differ considerably from the conventionally applied ex situ measurements at low temperatures. Therefore, a more comprehensive view of precipitation processes in Al?Mg?Si alloys is obtained. Further, in situ positron?electron annihilation techniques allow for an investigation of aging processes with increased sensitivity. For the artificial aging temperatures of 180 C and 210 C pronounced variations of the Doppler broadening S-parameter reveal (i) the evolution of clusters into larger precipitates and (ii) the time of the formation of ? precipitates and the role of vacancies in connection to this. Towards higher aging times, the transformation from coherent ? to semi-coherent ? precipitates could be verified. Additional insights are gained by in situ measurements of the S-parameter during the solution heat treatment of the previously overaged sample. Here, the S-parameter reveals both the dissolution of precipitates starting from temperatures of 364 C and the thermal generation of vacancies.