Al-Cu-Mg based alloys are of significant interest for aerospace and other applications, due to their low weight, mechanical strength and corrosion resistance. Their mechanical properties are based on a dispersion of S-phase precipitates, which have the composition Al2CuMg. The crystal structure of this inter-metallic phase has been studied using different diffraction techniques for more than five decades. While several models have been proposed for the structure of S-phase[l], only one of the previously proposed models were found to give a reasonable fit to our experimental data. This model, shown in Fig. 1 and given by Perlitz and Westgren (PW) [2] based on X-ray diffraction data, is orthorhombic with unit cell dimensions a = 0:4 nm, b = 0.923 nm, and c = 0.714 nm, space group Cmcm, containing 16 atoms in the ratio Al:Cu:Mg = 2:1:1.

Several HREM images of S-phase precipitates located near the edge of the foil, Fig. 2, recorded along the [100]s and [010]s directions, were digitized from film and used for analysis.