The structure and the properties of S-phase in AlCuMg alloys

The S-phase is one of the key strengthening precipitate phases in high-strength 2xxx (AlCuMg) aluminum alloys. Nonetheless, the crystal structure of the S-phase remains controversial, and therefore its physical properties are almost unknown. Using atomic-resolution imaging in electron microscopy and first-principles energy calculations, it was confirmed at last that the structure model proposed by Perlitz and Westgren in 1943 is the only right model for well-developed S-phase precipitates. The structure model proposed by Perlitz and Westgren in 1943 is the only right model. Detailed calculations analysis of the S-phase structure reveals that the characteristics of Cu-Mg bonding and Cu-Al bonding in the structure are basically metallic, but with an ionic character due to electron charge transfer between atoms, leading to a large elastic modulus for the S-phase precipitates to strengthen AlCuMg alloys.