HRTEM and HAADF-STEM tomography investigation of the heterogeneously formed S (Al2CuMg) precipitates in Al–Cu–Mg alloy

The distribution of variants and three-dimensional (3D) configurations of the heterogeneously formed S (Al2CuMg) precipitates at dislocations, grain boundaries and the Al20Cu2Mn3 dispersoid/Al interfaces were studied in this research. By means of high resolution transmission electron microscopy, we systematically investigated the orientation relationships (ORs) between these heterogeneously formed S precipitates and the Al matrix, and further unraveled that the preferred orientation of S variants at grain boundaries and at dispersoid/Al interfaces are respectively associated with the OR between the precipitate habit plane and the grain boundary plane, and the OR between the precipitate habit plane and the interface plane. The inherent characteristic of the crystal structure of the S phase, i.e. the symmetry of the pentagonal subunit, was considered to be the fundamental factor determining the preference of the variant pair. By using high angle annular dark field scanning transmission electron microscopy tomography, we successively obtained the 3D reconstruction of the S precipitates at these defects. Both the morphology of an individual S precipitate and the overall configuration of the S precipitates nucleated at these defects can be clearly observed without misunderstandings induced by the overlap and projection effects of the conventional two-dimensional methods.