In the structures of all metastable precipitates in Al-Mg-Cu and Al-Mg-Si alloys, we find that column surrounding of an element column in the needle/lath direction order according to simple principles. Advanced transmission electron microscopy and DFT calculations support the principles originate with a line defect, which is a segment of a <100>Al column shifted to interstitial positions. We propose the defect aids solute decomposition by partitioning the FCC matrix locally into columns of fewer and higher number of nearest neighbours, which suit smaller and larger size solute atoms, respectively. The defect explains how <100> directionality of the precipitates can arise in a cluster. Ordering of a few defects leads naturally to GPB zones in Al-Mg-Cu and to β'' in Al-Mg-Si.
Scanning transmission electron microscopy (STEM) coupled with energy-dispersive X-ray spectroscopy (EDS) is a common technique for chemical mapping in thin samples. Obtaining high-resolution elemental maps in the STEM is jointly dependent on stepping the sharply focused electron probe in a precise raster, on collecting a significant number of characteristic X-rays over time, and on avoiding damage to the sample. In this work, 80 kV aberration-corrected STEM-EDS mapping was performed on ordered precipitates in aluminium alloys. Probe and sample instability problems are handled by acquiring series of annular dark-field (ADF) images and simultaneous EDS volumes, which are aligned and non-rigidly registered after acquisition. The summed EDS volumes yield elemental maps of Al, Mg, Si, and Cu, with sufficient resolution and signal-to-noise ratio to determine the elemental species of each atomic column in a periodic structure, and in some cases the species of single atomic columns. Within the uncertainty of the technique, S and β'' phases were found to have pure elemental atomic columns with compositions Al 2 CuMg and Al 2 Mg 5 Si 4 , respectively. The Q' phase showed some variation in chemistry across a single precipitate, although the majority of unit cells had a composition Al 6 Mg 6 Si 7.2 Cu 2 .
wt. %) alloy was examined in detail after aging at 170°C with different dwell times up to 96 h. Deformation by 3%stretching prior to aging was used to investigate the effect of dislocations on phase and hardness evolution and compared with the undeformed material. The small pre-deformation led to a slight decrease of peak strength due to reduced homogenous nucleation. Strong interaction between different phases caused the formation of hybrid structures both in the undeformed bulk areas and on dislocations. These phases consist of different fragments of the GP zones, θ"and θ'-phases from the Al-Cu system, GPB zones and S1-phase from the Al-Cu-Mg system, and β"-, β'-Cu, C-and Q'-phases from the Al-Mg-Si-Cu system. A new phase named C1, isostructural to the C-phase but having a different orientation with the Al matrix -(010) C //(010) Al , [001] C //[101] Al , has been found predominantly on dislocation lines, and to a lesser extent in hybrid precipitates in the bulk. Calculations of structural stability by density functional theory (DFT) were performed on experimentally found structures, consisting of a C-phase core in two different orientations, and having Cu segregations in GP-like structures at their interfaces with the matrix.
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