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

Abstract

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 5 0]β ′, and [5 4 1 0]β ′ for β ′ precipitates, and [0 0 0 1]Q ′, [1 4 5 0]Q ′, and [3 2 1 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.