Studies of Orientations of β″ Precipitates in Al-Mg-Si-(Cu) Alloys by Electron Diffraction and Transition Matrix Analysis

Transition matrix, combine with high-resolution transmission electron microscopy (HRTEM) and stereographic projection, was used in this article to predict and characterize the orientations and electron diffraction patterns of β″ precipitates in Al-Mg-Si-(Cu) alloys. It was found that the β″ phases as the main strengthening precipitates had 12 variants with Al matrix, and the orientation relationships could be expressed as (010) _β″//{100}_Al, [001] _β″//〈310〉_Al, and [100] _β″//〈230〉_Al. Further, based on the C-centered monoclinic structure of β″ precipitate, a new diffraction patterns model under the [001]_Al zone axis could be established, which was in good agreement with the experiment date. Further, it could be used to explain reasonably some issues (for example, the “cross-shaped” diffraction streaks phenomenon). Simultaneously, we also found that the β″ precipitates had only three different zone axes, [010] _β″, [304] _β″, and \( \left[ {\bar{1}06} \right]_{{\beta^{\prime \prime } }} , \) parallel to the [001]_Al direction when they were precipitated from the Al matrix, and the β″ precipitate might deviate 1.6 deg from its original orientation because of the lattice relaxation.