Precipitates in an Al–Mg–Ge alloy studied by aberration-corrected scanning transmission electron microscopy R. Bjørge a , P.N.H. Nakashima b,c , C.D. Marioara d , S.J. Andersen d , B.C. Muddle b,c , J. Etheridge e,c , R. Holmestad a,⇑ a Department of Physics, Norwegian University of Science and Technology (NTNU), 7491 Trondheim, Norway b ARC Centre of Excellence for Design in Light Metals, Monash University, Victoria 3800, Australia c Department of Materials Engineering, Monash University, Victoria 3800, Australia d Department of Synthesis and Properties, SINTEF Materials and Chemistry, 7465 Trondheim, Norway e Monash Centre for Electron Microscopy, Monash University, Victoria 3800, Australia Received 17 February 2011; received in revised form 9 June 2011; accepted 13 June 2011 Available online 19 July 2011 Abstract The precipitates present in an Al–0.59Mg–0.71Ge (at.%) alloy have been studied using aberration-corrected high-angle annular dark- field scanning transmission electron microscopy. Two types of needle-shaped precipitates growing along h001i Al were found: a phase isostructural to the trigonal U1 phase found in Al–Mg–Si alloys, and finer precipitates with a hexagonal arrangement of Ge columns. The study revealed the presence of a complex interface structure surrounding the U1-like precipitates, and an explanation based on inter- atomic distances is proposed. Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Al alloys; Precipitation; STEM HAADF; Interface structure; Al–Mg–Si alloys 1. Introduction Al–Mg–Ge alloys have been shown to share similarities with the industrially important Al–Mg–Si(–Cu) alloys [1–3]. Some of the precipitate phases known from the Al– Mg–Si system have isostructural counterparts in Al–Mg– Ge, specifically the trigonal U1 phase (a = b = 0.405 nm, c = 0.674 nm, c = 120°) [4] (also known as type-A or b 0 A [5]) and the hexagonal b 0 phase (a = b = 0.715 nm, c = 0.405 nm, c = 120°) [6]. These two phases play a more important role in precipitation-hardening in the Al–Mg– Ge alloys than in the Al–Mg–Si system [1]. An isostructural counterpart to the b 00 phase, the most important hardening phase in the Al–Mg–Si alloy system, was not observed in Al–Mg–Ge, yet these alloys reach hardnesses similar to those of comparable Al–Mg–Si alloys. In the present work, precipitates in the Ge-rich alloy in [1] are studied using aberration-corrected high-angle annular dark-field scan- ning transmission electron microscopy (HAADF STEM). In Ref. [1], it was shown that a U1-like phase forms in this alloy (therefore called U1-Ge), as well as precipitates of an indeterminate phase. Both precipitate types are nee- dle-shaped, grow in the h001i Al direction, and are coher- ent with the matrix in this direction. The needle cross- section is typically elongated along one h100i Al direction, with almost planar interfaces between precipitate and matrix parallel to this elongation direction. The U1 precip- itates are finer in the Al–Mg–Ge alloy than in the corre- sponding Al–Mg–Si alloy. The c-axis of the U1-Ge unit cell in most of the precipitates was found to lie close to h210i Al . This is different from U1 in Al–Mg–Si, where the c-axis is parallel to h310i Al and where the cross-section is not elongated [4].A h110i U1 direction is parallel with the needle direction in both the Al–Mg–Si and Al–Mg– Ge systems. 1359-6454/$36.00 Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2011.06.021 ⇑ Corresponding author. Tel.: +47 73593173, fax: +47 73597710. E-mail address: randi.holmestad@ntnu.no (R. Holmestad). www.elsevier.com/locate/actamat Available online at www.sciencedirect.com Acta Materialia 59 (2011) 6103–6109