The microstructural origin of a hardness double-peak in an age-hardened EN-AW 6082

In this study, the origin of a double-peak during artificial aging of an EN-AW 6082 is analyzed with (S)TEM. The acquired TEM data is used as an input into a strength model (Holmedal, 2015) to explore the origin of the double-peak, thus, a broader strength plateau. It is concluded that the first peak arises after about 160 min at 175 °C from a population of relatively wide β′′-type precipitates with a high number density and a narrow length spread. The second peak occurs after 10 h artificial aging and can be traced to a broader size distribution of precipitate lengths of roughly the same type of precipitates as found for the first peak. The longest precipitates, however, act more efficiently as obstacles for gliding dislocations and maintain therefore the high strength. Those long precipitates are of a hybrid type of different, but mainly overaged phases. It is found that this broad strength plateau with a double-peak can arise after a rapid heat-up to the artificial aging temperature. Due to the rapid heat-up, structures of overaged phases are already found within the β′′ precipitates of the first peak making them wider. Ultimately, however, this leads to the evolution of few but very large overaged hybrid-type precipitates co-existing with β′′-type precipitates at the second peak. The mixed morphology counteracts the loss of strength from Ostwald ripening and leads to the second peak. A slower heating rate results in fewer precipitate nucleation sites and in the formation of longer and purer but slimmer βperfect′′ phases.