The structural and compositional evolution of precipitates in Al-Mg-Si-Cu alloy

The structural and compositional evolution of precipitates in Al-Mg-Si-Cu alloys were systematically investigated by atomic resolution high-angle annular dark-field scanning transmission electron microscopy and atom probe tomography. In under-aged alloys, most of precipitates have a disordered structure, with a substructure of beta (LDC) and Cu sub-unit cluster or C unit cell. After aging to peak strength, disordered precipitates including beta, QP1 and QP2 phases are formed. The disordered QP1 and QP2 phases, which contain the unit cells of Q' and C phases, respectively, are the precursor phases of Q' phase in these alloys. The beta phase can transform into the disordered QP1 phase by incorporating Cu atoms, forming Cu sub-unit clusters and QP lattice. When the alloy is over-aged, the ordering and transformation of QP1 to Q'occurs by the formation of Cu sub-unit clusters, the ordering of QP lattice, and the ordering of QC lattice. In contrast, the transformation of the disordered QP2 phase are rather sluggish. After sufficient aging, Q', C and disordered QP2 transform into the Q phase. During the evolution of the precipitates in these alloys, a continuous incorporation of Mg, Si and Cu atoms and release of Al atoms occur. These findings provide new insights in understanding precipitation in Al-Mg-Si-Cu alloys. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.