The effect of Cu-addition on age-hardening and precipitation have been investigated by hardness measurement, tensile test, high resolution transmission electron microscopy (HRTEM) and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) techniques. Higher hardness, strength, and lower elongation were caused by increasing amount of Zn + Mg because of increased number density of precipitates. Cu addition also provided even higher peak hardness, strength, and lower elongation. The alloy containing highest Cu content had fine precipitates of GPB-II zones or the second clusters, in the precipitate free zones (PFZs) and the matrix, together with η′/η in the matrix from the early stage of aging. Two regions have been confirmed as the PFZs in the peak aged alloy containing highest Cu: (i) nearest to grain boundary (GB) about 70 nm in width (n-PFZ) and (ii) conventional PFZ about 400 nm in width which can be confirmed by conventional TEM (con-PFZ). The con-PFZ contains fine precipitates consisting of GPB-II zones or the second clusters, even for 2 minutes of aging at 473 K which were not present in the n-PFZ. The fine precipitates, GPB-II zones or the second clusters in the con-PFZ and the matrix disappeared at overaged condition.

High angle annular dark field scanning transmission electron microscopy has been employed to observe precipitate structures in Al–Zn–Mg and Mg–Zn alloys. ${\eta }_1$ precipitate structures in Al–Zn–Mg are commonly formed by MgZn₂ Penrose bricks, but also frequently observed to incorporate Mg₆Zn₇ elongated hexagons via two different modes. Tilings of MgZn₂ and Mg₆Zn₇ building blocks in both ${\beta }_1^{\mathrm{\prime }}$ in Mg–Zn and ${\eta }_1$ in Al–Zn–Mg alloys, create overall patterns which deviate from the chemical and structural configuration of solely monoclinic Mg₄Zn₇ or MgZn₂ unit cells. Precipitate morphologies were found to be correlated to their internal sub-unit cell arrangements, especially to Mg₆Zn₇ elongated hexagons. Systematic or random arrangements of Mg₆Zn₇ elongated hexagons inside precipitates and therefore compositional and structural patterns, were found to be strongly related to the aspect ratio of the precipitates and altering of the precipitate/matrix interfaces. GRAPHICAL ABSTRACT