Understanding the Role of Short-Range Order in the Nucleation and Transformation of the B′/Q′ Precipitates in Al-Mg-Si(-Cu) Alloys

The understanding of metastable hardening precipitate evolution in Al-Mg-Si(-Cu) is still quite confused and elusive due to the disordered nature of several precipitates and the complex processes involved. Atomic-resolution scanning transmission electron microscopy is used here to study the nucleation and transformation mechanisms of the B′/Q′ precipitates in the Al-Mg-Si(-Cu) alloys. It is emphasized that understanding the formation and evolution of short-range order (SRO) in the precipitates is a core to revealing their transformation mechanisms. Although the B′ and Q′ phases have the same crystal structure, and both originate from the β″ precipitate, the formation mechanisms of these two precipitates are different. For B′, formed in the Al-Mg-Si alloy, the β″ phase initially transforms to the U2 phase by the transition of the “low density cylinder” (LDC) to Mg hexagons. Subsequently, the U2 phase transforms to the B′ phase by the rotation of Al-Si columns, and the formation and ordering of triangle B′ sub-units. For Q′, formed in the Al-Mg-Si-Cu alloy, the Cu atoms incorporate initially the interior of the β″ phase and form the substructure of Cu sub-unit clusters. In most cases, these Cu sub-unit clusters are randomly distributed, and the QM and QP lattices are formed throughout the precipitates. In some precipitates, the arrangement of Cu sub-unit clusters however constitutes a hexagonal lattice and leads to the formation of the QC phase. During subsequent aging, the formation and ordering of the triangle Q′ sub-units can lead to the formation of the Q′ phase. Clarifying the role of the SRO in these processes provides a new insight in the understanding of transformation mechanisms of precipitates in Al-Mg-Si(-Cu) alloys.