Type of presentation: Invited

MS-4-IN-2880 STEM studies of Ag-, Cu- and Zn-containing precipitates in Al-Mg-Si alloys

Saito T.¹, Wenner S.¹, Marioara C. D.², Andersen S. J.², Lefebvre W.³, Holmestad R.¹

¹Department of Physics, Norwegian University of Science and Technology (NTNU), 7491 Trondheim, Norway, ²SINTEF Materials and Technology, 7465 Trondheim, Norway, ³Université de Rouen, GPM, UMR CNRS 6634 BP 12, Avenue de I’Université, 76801 Saint Etienne du Rouvray, France

randi.holmestad@ntnu.no

The 6xxx series aluminium alloys, with magnesium and silicon as primary alloying elements, are widely used as structural materials, for example in the construction and automotive industry. The alloys are age-hardenable, as they acquire strength through the formation of nanoscale, needle-shaped, metastable precipitate phases during heat treatment. Our objective is to understand more of the fundamental physics going on at the atomic scale, which governs nucleation, phase stabilization and precipitation in these alloys. The morphology, structure and strengthening properties of age-hardening precipitates depend on the alloy composition and the thermo-mechanical history of the material. Being able to understand the atomic structure of the precipitates, how they affect each other and the material’s physical properties, composition and heat treatment can be optimized to tailor alloys with optimal properties. In recycled Al alloys, several heavier elements can be found, but they can also be added on purpose to obtain better properties. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) is an excellent technique to study the distribution of elements such as Ag, Cu and Zn in the precipitates [1], also electron energy loss spectroscopy (EELS) has been used to study the composition of individual atomic columns in the precipitates[2].

The main hardening precipitates form and grow as needles along <100>Al, and are observed in cross-section in high resolution HAADF-STEM. When Cu, Zn or Ag is added, the precipitate structures often become disordered, with no unit cell. All precipitates still contain an ordered network of Si atomic columns. We see preferred local atomic configurations which do not exist in the more common β’’ phase. Cu, Zn and Ag atomic columns are observed to locate either in-between the Si-network columns or (fully or partly) substituting a Si-network column. In both cases, they form the center in a three-fold rotational symmetry on the Si-network. Another observation is that the disordered precipitates consist of fragments of known phases in the Al-Mg-Si alloy system, connected through the common Si-network. All elements reside also in Al fcc positions at the precipitate/matrix interface. STEM image simulations and quantitative analysis are used to estimate the occupancy of heavier elements in the different columns. The presentation will show an overview of recent work done in the group.

References:

[1] T Saito, CD Marioara, SJ Andersen, W Lefebvre and R Holmestad, Phil. Mag., (2014) 94, 520.

[2] S Wenner, CD Marioara, QM Ramasse, DM Kepaptsoglou, FS. Hage, R Holmestad Scripta Mat. (2014) 74, 92.

The authors want to thank the Norwegian Research Council and Hydro Aluminium for financial support, and prof. Kenji Matsuda, Toyama University for good collaboration.

Fig. 1: HAADF-STEM images of disordered precipitates viewed in cross-section along <001> Al (a) in Cu-containing, (b) Zn-containing and (c) Ag-containing precipitates in Al-Mg-Si alloys at peak hardness. The images are filtered to reduce noise using a circular low-pass mask that removes all period shorter than 0.15 nm.