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.

Aberration corrected scanning transmission electron microscopy combined with electron energy loss spectroscopy has been used to determine the distribution of Cu and Ag atomic columns of precipitates in an Al-Mg-Si-Cu-Ag alloy. Cu columns were commonly part of C and Q ′ phases, with the atomic columns having large projected separations. Columns containing Ag were more tightly spaced, in areas lacking repeating unit cells and at incoherent precipitate-host lattice interfaces. Cu-rich and Ag-rich areas were not found to intermix.Keywords: Aluminium alloys, Precipitation, Scanning transmission electron microscopy, Electron energy loss spectroscopy Al-Mg-Si alloys are heat-treatable and gain a significant strength increase upon nucleation and growth of hardening nano-sized metastable phases. Detailed investigations of the precipitation sequence have been performed over the years, and crystal structures of most metastable phases have been solved by means of quantitative transmission electron microscopy (TEM) combined with first-principles calculations [1,2]. When Cu is addded to the alloys, the precipitate phases of the Al-Mg-Si system are suppressed [3], and new, Cucontaining phases such as C [4] and Q' [5,6,7] form. Additionally, areas with no repeating unit cell become more common in the structure of the precipitates. Characteristic for all metastable precipitates in the Al-Mg-Si(-Cu)Email address: sigurd.wenner@ntnu.no (Sigurd Wenner)

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