The direct observation of copper segregation at the broad faces of η’ and η precipitates in AA7010 aluminium alloy

High strength Al-Zn-Mg-Cu (7xxx) aluminium alloys obtain the required balance of mechanical and corrosion properties through complex heat treatments developed through empirical experiment. Understanding the influence of these heat treatments on the composition of the strengthening precipitates (η’ and η-phase) is critical to obtain a better mechanistic understanding of their effect. In this work, the composition of strengthening precipitates has been studied at the atomic scale in microstructures of commercially processed AA7010 alloy in the standard overaged temper state. Copper enrichment in the precipitates on over-aging is confirmed for both small (η’) and large (η) particles. It is demonstrated that in addition to entering the precipitates, a copper enriched layer forms at the particle/matrix interface on the broad faces of the precipitate plates. The implications of this structure for the mechanical and environmentally assisted cracking performance of the alloy are discussed.