Interface structures and thickening mechanisms of the Q′ and QP2 phases for Al-Mg-Si-Cu alloys

Abstract

Precipitate/matrix interfacial structure is a key issue for controlling the nucleation and growth of precipitates, affecting their morphologies, sizes and thus determining the hardening potential and thermal stability of the alloy. The interfacial structures and thickening mechanisms of two typical precipitates (Q′ and QP2 phases) in the Al-Mg-Si-Cu alloys were systematically studied by atomic-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) in this article. It was revealed that the different thickening rates of the precipitates are mainly determined by their interfacial structures. Two different interfaces are detected for both the two precipitates: the interface I constituted by a transition layer arranged of Mg-Si-Mg-Si( Al) chain, while the interface II comprised by a row of modified Cu sub-unit clusters embedded in the Al matrix. The thickening of the two phases is supposed to be achieved by alternate transformation between the two interfaces. The thickening of the Q′ phase is controlled by the Cu replacing certain site at the habit plane and its evolution to Cu sub-unit clusters, while the thickening of the QP2 phase is controlled by the nucleation of the ledges at its habit plane. The limited supply of ledges at the habit plane of the QP2 phase is a main reason for its high resistance of thickening. • Different thickening rates of precipitates are determined by their interfacial structures. • Two different interfaces are detected for both the Q' and QP2 precipitates. • The limited supply of ledges in the QP2 phase is responsible for its high thickening resistance.