Structural analysis of 14 phase and its forming mechanism in Al-Zn-Mg-Cu alloy

The structural characteristic and formation mechanism of a latest reported eta 14 phase formed in Al-Zn-Mg-Cu alloy were systematically studied by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). By detail structural analysis, the orientation relationship (OR) of the eta 14 and Al matrix is modified as: (1120)eta 14//(112)Al, [1103]eta 14//[111]Al and [1100]eta 14//[311]Al. Structural analysis demonstrated that the eta 14 phase displays the C14 Laves stacking structure, however, the Zn1 and Zn2 columns composed the R'- units are only half filled by Zn. This phenomenon is mainly due to the Zn1-2 columns composed the R'-units and Zn3 column composed the R-units formed with different processes, leading to uneven intensity among these columns for the eta 14 phase. The formation mechanism of the eta 14 phase does not shows any relevance with the faulted layers reported in previous work. In contrast, The eta 14 phase is suggested to evolved from a metastable eta 14' phase with OR of (0001)eta 14'//(021)Al and [1120]eta 14'//[112]Al, formed by regularly replacements of Zn and Mg at the (021)Al habit planes The special habit plane of eta 14' phase contributes to the unusual OR of eta 14 phase with Al matrix. Our findings extend the understanding of the precipitates formation and evolution appearing in Al-Zn-Mg-Cu alloys.