Crystal structure of the orthorhombic U2-Al4Mg4Si4 precipitate in the Al-Mg-Si alloy system and its relation to the β′ and β″ phases

The atomic structure of a common precipitate in the Al-Mg-Si system has been determined. It is isotypic with TiNiSi (space group Pnma) and contains four units of MgAlSi in a unit cell of size a = 0.675 nm, b = 0.405 nm, c = 0.794 nm. EDS analyses support the composition. A model was based on the atomic structure of the β′ precipitate, electron diffraction and high-resolution transmission electron microscopy (HRTEM) images. A quantum mechanical refinement of the model removed discrepancies between simulated and experimental diffraction intensities. Finally, a multi-slice least square refinement confirmed the structure. The structural relation with β″ is investigated. A similar Mg-Si plane also existing in β″ and β′, can explain most coherency relations between the precipitate phases and with matrix.