Atomistic building blocks of one-dimensional Guinier–Preston–Bagaryatsky zones in Al-Cu-Mg alloys

Knowledge of the decomposition process of the microstructure during the precipitation reaction is a requisite step in many cases for exploiting their unique properties. While the earlier work demonstrated that the formation of one-dimensional (1D) Guinier–Preston–Bagaryatsky (GPB) zones is critical for the initial- and peak-hardening of the Al-Cu-Mg alloys, the essential questions remain: what are the basic structural features and the precursors of GPB zones, and how do these precursors transform to the GPB zones? Here we employ atomic-resolution scanning transmission electron microscopy (STEM) and first-principles calculations to address these problems. In mostly GPB zones-strengthened alloys, we identify two atomistic building blocks of 1D GPB zones, which merge together maintaining a crystallographic orientation of [014]Al. These two particular structures are formed from initial copper double (Cu2) column-skeleton, associated with charge transfer between adjacent Cu and Mg as well as Cu and Al inside the structural units and at the interface between two building block units. Existence of initial and transitional phases is further suggested by ab initio calculations based on STEM observations and image simulations. These results demonstrate the atomistic mechanisms governing Cu2 columns-guided nucleation, growth, maturation and coarsening of the atomistic building blocks of GPB zones.