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Published online by Cambridge University Press: 02 July 2020
The mechanical properties of Al-Cu-Mg alloys depend on a dispersion of Al2CuMg phase precipitates which form as laths along <100>A1, with {012}A1 as the habit plane. Despite extensive study, the crystal structure, morphology and interface structure between the Al2CuMg intermetallic phase and the Al matrix are still not well understood. Since the mechanical behavior of Al alloys strengthened by the S-phase cannot be controlled without a clear understanding of its crystallography and interface structure, the goal of this investigation was to characterize the crystal structure, evolution and interface structure of the S-phase using quantitative electron beam characterization techniques. Several models have been proposed for the crystal structure of Al2CuMg. A widely accepted model of the S-phase was given by Perlitz and Westgren (PW) [1], shown in Fig. 1. The unit cell of the PW model 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.