Atomic structure of T1 precipitates in Al–Li–Cu alloys revisited with HAADF-STEM imaging and small-angle X-ray scattering

The structure of T1 precipitates which formed under ageing in Al–Li–Cu alloys has been studied using high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) imaging. Complementary information on the precipitate composition has been obtained on the same system by small-angle X-ray scattering (SAXS). According to HAADF-STEM imaging, the currently accepted structures based on the stacking of {1 1 1} type planes are ruled out. It is shown that the T1 precipitate structure is consistent with the one derived from X-ray diffraction study of the bulk T1–Al2LiCu phase. However, for this very thin plate shape precipitates (thickness less than one cell), deviations to the Al2LiCu stoichiometry may occur. A structural analysis based on local environment and packing rules classical of complex phases is proposed to make the connection between the atomic structure and precipitation features like nucleation and interface with the matrix.