Heat treatments of 6082 Al-alloy with 0.6 wt% Mg, 0.9 wt% Si, 0.5 wt% Mn and 0.2 wt% Fe can lead to a considerable increase in hardness. This increase is due to the presence of several metastable phases (in particular β″). To determine the structure of the phase formed before the β″ phase, a detailed high resolution electron microscopy (HREM) study was performed. The pre-β″ phase is needle like, as is the β″ phase. Based on reconstructed exit waves, two models were possible, one of which could be rejected because of the interatomic distances. The model resembles that of the β″ but with different positions for some of the Mg atoms along the needle direction. The structure is more similar to the Al matrix than that of the β″ phase. The Mg sites, and to a lesser extent also the Si sites are partly occupied by Al atoms. The composition is therefore less Mg-rich than β″ (Mg5Si6). The content of Al in the structure of the precipitates increases with the degree of coherency in the Al matrix. The space group of the new phase is C2/m, as for β″.
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