Atomic model for GP-zones in a 6082 Al-Mg-Si system

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 beta "). To determine the structure of the phase formed before the beta " phase, a detailed high resolution electron microscopy (HREM) study was performed. The pre-beta " phase is needle like, as is the beta " 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 beta " 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 beta " 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 beta " (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 beta ". (C) 2001 Published by Elsevier Science Ltd on behalf of Acta Materialia Inc.