High-resolution characterization of the precipitation behavior of an Al-Zn-Mg-Cu alloy

The metastable particles in an Al-Zn-Mg-Cu alloy have been examined at atomic-resolution using high-angle annular dark field (HAADF) imaging. In under-aged conditions, thin eta' plates were formed with a thickness of seven atomic planes parallel to the {111}(Al) planes. The five inner planes of the eta' phase appear to be alternatively enriched in Mg and Zn, with two outer planes forming distinct Zn-rich interfacial planes. Similar Zn-rich interfacial enrichment has also been identified for the eta phase, which is a minimum 11-plane thick structure. In rare instances, particles less than seven planes were found indicating a very early preference for seven-layer particle formation. Throughout the aging, the plate thickness appears constant, while the plate radius increases and no particles between 7 and 11 planes were observed. Based on the HAADF contrast, our observations do not support the eta' models previously set forth by other authors. Clear structural similarities between eta' and eta were observed, suggesting that drawing distinctions between eta' and eta phases may not be necessary or useful.