A considerable part of the available literature on scandium in aluminium alloys is reviewed. Experimental data and assessments of the binary Al-Sc phase diagram, a wide range of ternary Al-Sc-X phase diagrams and a few higher order phase diagrams are accounted for, with emphasis on the aluminium rich part of the diagrams. The phase which is in equilibrium with Al, Al 3 Sc, can form by several different mechanisms, all of which are described. The precipitation kinetics of Al 3 Sc in binary Al-Sc alloys are discussed, and an overview of the reported influences of ternary alloying elements on the precipitation of Al 3 Sc is given. The Al 3 Sc phase particles can serve as a grain refiner in the Al melt, a dispersoid for controlling the grain structure of the alloy and a strengthening precipitate. Several examples of these three effects are mentioned, both in binary Al-Sc alloys, and in more complex alloys. The reported effects of Sc on the precipitation behaviour in Al-Cu, Al-Mg-Si, Al-Zn-Mg and Al-Li alloys are also revised. A brief account of the effects of Sc additions on the corrosion behaviour of Al and Al-alloys is given. Finally, some views on the current and future use of Sc-containing Al alloys are given.

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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.