Transmission electron microscopy study of a precipitate which forms in the Al-Mg-Si system

In this work, a detailed study of precipitates formed after ageing for 600 h at 185°C of an alloy containing 1·1 wt%Si, 1 wt% Mg, 0·01wt% Cu and 0·7 wt% Mn is presented. The precipitates have a lath shape with their longer axis aligned with [001] A1 and have average dimensions of about 300 nm x Al 12 nm x 5 nm. The unit cell is determined to be hexagonal with a= 1·03nm and c= 0·4505 nm. The orientation relationship is (0001) p //(001) A1 and [21¯1¯0]p is almost parallel to [510] A1 . The crystallographic information of these precipitates are in agreement with that of the known quaternary stable phase Q (approximate stoichiometry, Al 5 Cu 2 Mg 8 Si 6 ). Since the Cu is not present in the precipitate, it may be replaced by another atom.