STEM-HAADF tomography investigation of grain boundary precipitates in Al–Cu–Mg alloy

The high angle annular dark field scanning transmission electron microscopy (STEM-HAADF) tomography technique was applied to understand the three dimensional (3D) morphology and distribution characteristic of grain boundary precipitate (GBP) in peak-aged Al–Cu–Mg alloy. The results indicate that GBPs show both spherical and lenticular shapes and triangularly distribute along grain boundary (GB). Based on 3D observations from various directions, the values of GBP relevant parameters such as GBP size, center to center distance, number of GBPs per unit GB area and area fraction of GB covered by GBPs are further determined. The 3D method for GBP relevant parameter determination seems an effective way to avoid misunderstandings in the conventional two dimensional (2D) methods induced by GBP overlapping and projection effect as well as curved GB surface.

Research highlights
► Grain boundary precipitate (GBP) is reconstructed by STEM-HAADF tomography.
► Both spherical and lenticular GBPs are observed.
► The 3D method for GBP relevant parameters determination is introduced.
► Misunderstandings induced by conventional 2D methods can be effectively avoided.