Journal of Materials Science

, Volume 48, Issue 10, pp 3638-3651

First online:

Atomic structure of hardening precipitates in an Al–Mg–Zn–Cu alloy determined by HAADF-STEM and first-principles calculations: relation to η-MgZn2

  • Calin D. MarioaraAffiliated withSINTEF Materials and Chemistry
  • , Williams LefebvreAffiliated withGroupe de Physique des Matériaux, UMR CNRS 6634, Institut des Matériaux de Rouen, Site Universitaire du Madrillet, Université de Rouen
  • , Sigmund J. AndersenAffiliated withSINTEF Materials and Chemistry Email author 
  • , Jesper FriisAffiliated withSINTEF Materials and Chemistry

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The structures of two nanoscale plate precipitates prevalent at maximum strength and over-aged conditions in a 7449 Al–Mg–Zn–Cu alloy were investigated. Models derived from images of high angle annular dark field scanning transmission electron microscopy were supported by first-principles calculations. Both structures are closely linked to the η-MgZn2 Laves phase through similar layers of a rhombohedral atomic subunit. The finest plate contains one such layer together with a layer of an orthorhombic unit. The second plate contains rhombohedral layers only, normally four, but rotated relatively to form different stacking variants, one of which may be likened to η. For both structures, the same atomic planes describe the main interface with Al. Both plates could be described in space group P3. The unit cells comprise interface and arbitrary numbers of {111}Al (habit) planes. Eight Al-planes were included in the first-principles calculations. The enthalpy indicates high layer/unit stability. The plate thickness can be understood by a simple mismatch formulation.