The effect of Zn on precipitation in Al–Mg–Si alloys

Abstract

Effects of addition of Zn (up to 1 wt%) on microstructure, precipitate structure and intergranular corrosion (IGC) in an Al–Mg–Si alloys were investigated. During ageing at 185 °C, the alloys showed modest increases in hardness as function of Zn content, corresponding to increased number densities of needle-shaped precipitates in the Al–Mg–Si alloy system. No precipitates of the Al–Zn–Mg alloy system were found. Using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), the Zn atoms were incorporated in the precipitate structures at different atomic sites with various atomic column occupancies. Zn atoms segregated along grain boundaries, forming continuous film. It correlates to high IGC susceptibility when Zn concentration is ~1wt% and the materials in peak-aged condition.

Keywords:

• Al–Mg–Si alloy
• precipitation
• intergranular corrosion (IGC)
• transmission electron microscopy (TEM)
• high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM)

Acknowledgements

The authors would like to thank Dr Olaf Engler, Hydro Bonn Germany, for composition measurement by inductively coupled plasma optical emission spectroscopy. One of the authors (T.S.) is grateful to Professor Kemal Nisancioglu, Norwegian University of Science and Technology (NTNU), for helpful suggestions for the corrosion testing. This research is supported by Hydro Aluminum and the Research Council of Norway though the bilateral KMB project: 193619 ‘The Norwegian-Japanese Al–Mg–Si Alloy Precipitation Project’.