HR-STEM investigation of atomic lattice defects in different types of η precipitates in creep-age forming Al–Zn–Mg–Cu aluminium alloy

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//[21‾1‾0]η of η₁ and η₁₂, [112]_Al//[21‾1‾0]η of η₂ and [100]_Al//[21‾1‾0]η of η₁₃, atomic projections of (21‾1‾0)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//[21‾1‾0]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 {11‾1‾}_Al planes at the interface.