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.