PRECIPITATION HARDENING OPTIMIZATION BY PARTIAL SUBSTITUTION OF Si AND Mg WITH Ge AND Li IN LEAN 6XXX ALLOYS Eva A. Mørtsell 1,2* , Øyvind Paulsen 1 , Calin D. Marioara 3 , Sigmund J. Andersen 3 , Oddvin Reiso 4 , Jostein Røyset 4 , Yanjun Li 2 and Randi Holmestad 1 1 Department of Physics, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway 2 Department of Materials Science and Technology, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway ( * Corresponding Author: eva.mortsell@ntnu.no) 3 SINTEF Materials and Chemistry, N-7465 Trondheim, Norway 4 Hydro Aluminium R&D Sunndal, N-6600 Sunndalsøra, Norway ABSTRACT With the purpose of increasing the extrudability of lean alloys, we reduce levels of the alloying elements Mg and Si in a 6xxx extrusion alloy. To avoid sacrificing strength, we investigate the compensating effect of smaller amounts of lithium and germanium, alone and in combination. Recent studies by atomically resolved high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) together with density functional theory (DFT) calculations have demonstrated that while Li replaces Mg, Ge substitutes Si in the columns in the hardening β’’-phase. Several studies have shown that Ge replaces Si in other precipitates as well, and that both Li and Ge promote precipitation and can improve macroscopic properties, like strength and thermal stability. The scope of the current work has been to investigate and document the combined effect of the two alloying elements. Of the investigated alloys, the one containing both elements displayed highest hardness, corresponding with a significant increase in precipitate number density. We used HAADF-STEM to determine the extent of Li and Ge in the precipitate structures. KEYWORDS Al-Mg-Si, β’’, Li, Ge, HAADF-STEM, TEM Published in the Proceedings of the 16th International Aluminum Alloys Conference (ICAA16) 2018 ISBN: 978-1-926872-41-4 by the Canadian Institute of Mining, Metallurgy & Petroleum