Abstract
The ability to correlate atomic-scale solute distribution with changes in precipitation behavior and consequently mechanical properties is increasingly important for process and composition optimization of age-hardenable alloys. Minor solute additions, such as through recycling, can significantly impact the final performance of an alloy.
This study investigates how a minor Zn addition and pre-deformation influence both precipitation behavior and solute distribution in an artificially aged Al-Mg-Si-Cu alloy. Small amounts of Zn have been shown to increase strength in 6xxx-alloys, but the underlying mechanisms are yet not fully explained. Zn has a high solubility in aluminum, and recent work has shown that, although most of it remains in solid solution, a small fraction of Zn enters the precipitates [1]. An additional question is whether deformation, which increases vacancy density, changes the behavior of Zn.
To investigate these effects, two different alloys were compared. A standard 6061 alloy and a 6061 + 0.1wt.% Zn alloy were extruded, deformed by cold-rolling and subsequently age hardened to peak strength. Tensile testing confirmed that both Zn and pre-deformation increased strength, but only pre-deformation was shown to reduce elongation. Microstructural characterization using transmission electron microscopy (TEM) showed that pre-deformation increases density of precipitates, but at the cost of a reduced size. The Zn-addition, on the other hand, increased density without reducing size, resulting in an increased precipitate volume fraction.
High-angle annular dark field scanning TEM (HAADF-STEM) was used to determine precipitate structures and to identify the position of the Al, Mg, Si and Cu columns based on the construction rules of precipitates in Al-Mg-Si(-Cu) alloy systems [2]. However, the placement of Zn cannot be inferred from these rules or from atomic number contrast in HAADF-STEM images, although there are indications that it partially occupies other elements columns [3, 4]. Atom probe tomography (APT) confirms that only some Zn enters the precipitates and atomic-resolution EDS and EELS are used to determine the distribution of Zn. These results clarify how Zn is distributed in Al-Mg-Si-Cu alloys, and how it contributes to changes in precipitation behavior and ultimately mechanical properties.
[1] Marioara, C.D., Holmestad, J., Arbo, S.M. et al. The Effect of Increased Fe, Cu and Zn on the Microstructure and Properties of Recycling Friendly 6082 and 6005 Aluminium Alloys. Metall Mater Trans A (2026). https://doi.org/10.1007/s11661-026-08165-4
[2] Andersen, S.J., Marioara, C.D., Friis, J. et al. Precipitates in aluminium alloys, Advances in Physics: X, 3:1, 1479984 (2018). https://doi.ord/10.1080/23746149.2018.1479984
[3] Saito, T., Flemming, J.H., Lefebvre, W. et al. HAADF-STEM and DFT investigations of the Zn-containing β″ phase in Al–Mg–Si alloys. Acta Mat (2014). https://doi.org/10.1016/j.actamat.2014.06.055.
[4] Bartawi, E.H., Marioara, C.D., Shaban, G. et al Effects of grain boundary chemistry and precipitate structure on intergranular corrosion in Al-Mg-Si alloys doped with Cu and Zn. Corrosion Science (2024). https://doi.org/10.1016/j.corsci.2024.112227.