Fine precipitation scenarios of AlZnMg(Cu) alloys revealed by advanced atomic-resolution electron microscopy study Part I: Structure determination of the precipitates in AlZnMg(Cu) alloys

Although they are among the most important precipitation-hardened materials for industry applications, the high-strength AlZnMg(Cu) alloys have thus far not yet been understood adequately about their underlying precipitation scenarios in relation with the properties. This is partly due to the fact that the structures of a number of different precipitates involved in electron microscopy in association with quantitative image simulations have to be employed; a systematic study of these hardening precipitates in different alloys is also necessary. In Part I of the present study, it is shown that there are five types of structurally different precipitates including the equilibrium η-phase precipitate. Using two state-of-the-art atomic-resolution imaging techniques in electron microscopy in association with quantitative image simulations, we have determined and clarified all the unknown precipitate structures. It is demonstrated that atomic-resolution imaging can directly suggest approximate structure models, whereas quantitative image analysis can refine the structure details that are much smaller than the resolution of the microscope. This combination is crucially important for solving the difficult structure problems of the strengthening precipitates in AlZnMg(Cu) alloys. •We determine and verify all the key precipitate structures in AlMgZn(Cu) alloys.•We employ aberration-corrected scanning transmission electron microscopy (STEM).•We use aberration-corrected high-resolution TEM (HRTEM) for the investigations.•We obtain atomic-resolution images of the precipitates and model their structures.•We refine all precipitate structures with quantitative image simulation analysis. •The hardening precipitates in AlZnMg alloys shall be classified into two groups.•Two precipitation scenarios coexist in the alloys.•The precipitation behavior of such an alloy depends on the alloy's composition.•Very detailed phase/structure transformations among the precipitates are revealed.