Abstract
Scanning transmission electron microscopy imaging of Al–Zn–Mg alloys has provided new information on the atomic structures of solute rich clusters forming from a supersaturated solid solution at low temperatures. A unique unit of high Zn/Mg ratio is the fundamental cluster building block. The unit is essentially a partial substitution by Mg and Zn on the cubic aluminium cell and its surrounding truncated cube octahedral shell. A simple set of principles based on Frank–Kasper structures describes how the basic units arrange with respect to each other to form larger clusters. Density functional theory calculations, atom probe tomography and simulated diffraction patterns support the proposed atomic models. The results provide new insight into the very early stages of age-hardening in aluminium alloys.