HR-STEM investigation of atomic lattice defects in different types of η precipitates in creep-age forming Al–Zn–Mg–Cu aluminium alloy
Journal
Materials Science and Engineering A
Journal Volume
815
Date Issued
2021
Author(s)
Chung T.-F
Abstract
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. ? 2021 Elsevier B.V.
Subjects
Aspect ratio; Atoms; Copper alloys; Crystal lattices; Defects; High resolution transmission electron microscopy; Scanning electron microscopy; Al-Zn-Mg-Cu aluminium alloys; Atomic lattice; Atomic lattice defect; Creep age forming; Elongated hexagonal lattice defect; Hexagonal lattice; High resolution; High resolution scanning transmission electron microscopies; High-angle annular dark fields; Scanning transmission electron microscopy; Creep
SDGs
Type
journal article