Transmission electron microscopy investigation of separated nucleation and in-situ nucleation in AA7050 aluminium alloy

High resolution transmission electron microscopy (HRTEM) with nanometer-scaled energy-dispersive X- ray (EDX) was employed to investigate the transformation mechanisms of the GP zone -> eta' -> eta precipitation sequence of AA7050, an Al-Zn-Mg-Cu alloy. Serial in-situ HRTEM frames revealed that separated nucleation of an eta' precipitate occurred elsewhere as the adjacent GPII zone dissolved. Evidence from HRTEM coupled with EDX showed that in-situ nucleation of a new eta(2) precipitate (one form of 11) took place, wherein it gradually developed from the original eta' precipitate via a similar hexagonal structure with different compositions. The in-situ transition product was composed of two distinctive regions; one was identified as eta', and the other, as eta . (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.