Atomic structure of GPB-II zone and its evolution in an Al-Zn-Mg-Cu-Si crossover alloy during aging

Abstract

The GPB-II zone is the main strengthening precipitate in the Al-4.5Zn-1.5Mg-1.0Cu-0.35Si (wt.%) crossover alloy, resulting in increased alloy strength and ductility. The atomic structure of the GPB-II zone and its evolution process during aging have been studied by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) in the present work. It is shown that the GPB-II zone is a hybrid precipitate consisting of the L phase (a disordered phase with a QP lattice and is normally present in Al-Mg-Si-Cu alloy) on the inner part and Guinier-Preston-Bagaryatsky (GPB) zones (an important strengthening phase in Al-Cu-Mg alloy) on the outer part. The GPB-II zone is formed by the GPB zone nucleating at the Cu site (in-between and on the Si-network) of the L phase. The unique combination of the GPB zone and L phase was investigated, and the “Si parallelogram embedding with Cu” was proposed. The GPB zone provides the atoms for the L phase’s growth and results in the coarsening of the L phase with few GPB zones after a long aging time. The role of the Zn element in the formation of the GPB-II zone was discussed. It is thought to have a similar effect as Cu and can also promote the formation of GPB zones on the L phase.