High-resolution (HR) high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) has
revealed the atomic lattice defects in different types of η precipitates in the Al–Zn–Mg–Cu aluminium alloy subjected
to creep-age forming treatment (with a constant stress lower than its room-temperature yield strength during
ageing). Along the zone axes of [110]Al//[2110]η of η1 and η12, [112]Al//[2110]η of η2 and [100]Al//[2110]η of
η13, atomic projections of (2110)η have been investigated. In those types of η, elongated hexagonal lattice defects
(labelled as Type I defects) can be found; they are apparently related to local disorder in atomic stackings.
Furthermore, in η12, elongated hexagonal lattice defects with a much higher aspect ratio (labelled as Type II defects)
are uniquely observed. These atomic lattice defects are presumably pertinent to the lattice accommodation in the
course of creep-age forming. Additionally, in η1 and η12, the features of a Penrose tiling defect connecting with Type
I defects are observed, and these complex defects obviously affect the growth direction of the precipitate, resulting in
a nearly spherical morphology. Alternatively, several entirely-passed faulted layers in a new type of precipitate, η14,
consequently bring about a new orientation relationship: (513)Al//(0001)η14 and [112]Al//[2110]η14. Moreover, in
an atomic STEM image of η14, the significant Z-contrast gradient adjacent to the transformation front of η14 elucidates the Zn/Cu diffusion from the matrix to the precipitate along {111}Al planes at the interface.