Pre-precipitation stages of Al-0.70Mg-0.33Si and Al-0.65Mg-0.70Si (at.%) alloys have been investigated by atom probe field ion microscopy (APFIM) and high resolution transmission electron microscopy (HREM). Atom probe results show that clusters of Mg atoms are present in the as-quenched state. After a prolonged aging at room temperature, clusters of Mg, Si and their co-clusters are detected, although no contrasts suggesting the presence of precipitates are observed in HREM images. In the specimens aged at 175°C for 30 min, small equiaxed Mg-Si precipitates are observed by TEM. APFIM results show that the ratio of Mg to Si atoms in the precipitates is close to 1, rather than 2, which is expected from the equilibrium concentration of Mg2Si.
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