[en] Highlights: • A new orientation relationship between the η-MgZn₂ hardening precipitate and the Al matrix has been established. • The precipiate interface was observed by HAADF-STEM in two projections, 90° to one-another. • Thickening ledges were observed to be anistropically stepped depending on directional misfit. • DFT relaxation of interface model fairly well explains the significant stepped thickneing ledges along the highest directional misfit. -- Abstract: Characterization of precipitates in Al-Zn-Mg alloys, using a combination of electron diffraction, bright field transmission electron microscopy and atomic scale scanning transmission electron microscopy imaging revealed the presence of an unreported η₁₃ orientation relationship between the η-MgZn₂ phase and the Al lattice with the following orientation relationship (0001)_η $\parallel$ (120)_Al and $\left(2\overline{1}\overline{1}0\right)$_η $\parallel$ (001)_Al, plate on (120)_Al. The precipitate interfaces were observed and analyzed along two projections 90° to one-another. The precipitate coarsening was through the common thickening ledge mechanism. The ledges were significantly stepped along one lateral direction. An interface relaxation model using density functional theory was carried out to explain the precipitate behavior.

[en] The microstructures of precipitates in Al–Zn–Mg alloys in peak-aged condition have been studied using scanning transmission electron microscope. The same thermo-mechanical treatment was applied in all alloys. Investigation of peak-aged samples revealed that the most commonly found phases were η#Prime# and η₁ with their respective habit planes on {111}_Al and {100}_Al. η#Prime# phases under [110]_Al were analyzed and compared with η#Prime# structure models. Furthermore, a close inspection of η₁ phase as the second most found precipitate revealed that it incorporates an anti-phase resembling boundary, not observed in other orientation relationships that precipitates create with Al matrix, in addition, differences in matrix-precipitate interfaces between η#Prime#/η₂ and η₁ phases were noticed. This paper addresses the first part to the analysis of η#Prime# phase. Next part is extended to the analysis of the η₁ phase.

[en] 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: (1) nearest to grain boundary (GB) about 70 nm in width (n-PFZ) and (2) 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. (author)