GPB zones and composite GPB/GPBII zones in Al–Cu–Mg alloys

The needle-like, nanometer-scale metastable particles which are present after prolonged aging in an Al–1Cu–3Mg (wt.%) were investigated using high resolution scanning transmission electron microscopy. Two different structural forms are found to be associated with the particles. The structure designated as Guinier–Preston–Bagaryatsky (GPB) zones represent an agglomeration of sub-nanometer, rod-like units. The structure of these basic units was fully determined with the aid of ab initio calculations. As compared to the solid solution, the units show favorable enthalpy of formation. In many instances, the metastable particles comprise the structural units as a shell, and a crystallographically distinct core region (GPBII). The projected position of the atomic columns in the core was fully determined, although its full crystallographic description is not yet known. The relevance of these findings is discussed. Most importantly, it is shown that the GPB zones have the potential to explain the rapid hardening in Al–Cu–Mg alloys.