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 eta 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]eta of eta 1 and eta 12, [112]Al//[2110]eta of eta 2 and [100]Al//[2110]eta of eta 13, atomic projections of (2110)eta have been investigated. In those types of eta, elongated hexagonal lattice defects (labelled as Type I defects) can be found; they are apparently related to local disorder in atomic stackings. Furthermore, in eta 12, 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 eta 1 and eta 12, 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, eta 14, consequently bring about a new orientation relationship: (513)Al//(0001)eta 14 and [112]Al//[2110]eta 14. Moreover, in an atomic STEM image of eta 14, the significant Z-contrast gradient adjacent to the transformation front of eta 14 elucidates the Zn/Cu diffusion from the matrix to the precipitate along {111}Al planes at the interface.

Automated summary

High-resolution HAADF-STEM analysis revealed atomic lattice defects in various types of eta precipitates in an Al-Zn-Mg-Cu alloy undergoing creep-age forming treatment. These defects, including elongated hexagonal lattice defects labeled as Type I and Type II, are associated with lattice accommodation during the forming process. The presence of complex defects like Penrose tiling defect and entirely-passed faulted layers also influences the morphology and growth direction of the precipitates. Furthermore, Zn/Cu diffusion from the matrix to the precipitate along specific planes was observed in the atomic STEM image.