Precipitate characteristics and selected area diffraction patterns of the β′ and Q′ precipitates in Al-Mg-Si-Cu alloys

In this study, the precipitate characteristics and selected area diffraction patterns (SADP) of the β′ and Q′ precipitates formed at the over-aged state of Al-Mg-Si-Cu alloys are systematically investigated by means of high-resolution transmission electron microscopy and transition matrix. The β′ precipitates have two cross-sections, rectangle-shaped and round-shaped aligned with [0 0 1]_Al direction, but only rectangle-shaped cross-section exists for Q′ precipitates. And, both of them have 12 variants and orientations with Al matrix. However, there are only three different zone axes, [0 0 0 1]_β′, [1 4 5̄ 0] _β′, and [5 4̄ 1̄ 0] _β′ for β′ precipitates, and [0 0 0 1] _Q′, [1 4 5̄ 0] _Q′, and [3 2̄ 1̄ 0] _Q′ for Q′ precipitates, parallel to the [0 0 1] _Al direction when they are precipitated from the Al matrix, respectively. Then, a new [0 0 1]Al SADP model, which superposes diffraction patterns of the β′ and Q′ precipitates, is established. Furthermore, some "cross-shaped" diffraction streaks appeared at over-aged state can be explained reasonably by this model, which is good in agreement with the experimental data.