The precipitation process during aging at 573K of an Al-0.25mass%Sc alloy has been investigated by means of electrical resistivity and hardness measurements, transmission electron microscopy (TEM) and atom-probe field ion microscopy (AP-FIM). It was found that spherical particles of the Al3Sc phase having the L12 structure precipitate homogeneously from beginning. The coarsening of the particles follows the Lifshitz-Wagner rule after the completion of the nucleation-and-growth stage, and the

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... ticles have the coherency with the matrix even in the over-aged state. The composition of the precipitates was measured using the AP-FIM. The Sc concentration of the precipitates was less than the equilibrium value (25at.%) at the early stage of aging and reached to 25at.% upon over-aging.