A Direct Observation of Ordered Structures Induced by Cu Segregation at Grain Boundaries of Al 7075 Alloys

The experimental investigation of the atomic-scale structure and chemistry of the grain boundaries by resolving the sites and chemical identities of the atoms comprising the interface is crucial to understand the implication of preferential atomic segregation at grain boundaries. Here, the authors report the ordered-structures induced by substitutional Cu segregation at the Al alloy 7075 coincidence site lattice grain boundaries, namely, ∑13b and ∑7, using Cs-aberration corrected scanning transmission electron microscopy. Unlike the single-atom interstitial hollow site segregation behavior at the boundary region (predicted in the theoretical studies and considered on analysis of segregation effects), the authors demonstrate segregation of Cu atoms on two columns opposite to each other across the interface forming an ordered parallel array of segregating atoms in substitutional core sites. This is the first time that this type of parallel array segregation behavior is reported. Furthermore, based on intensity profiles and scanning transmission electron microscopy Z-contrast principle, non-uniformly (high and low) segregated mixed atomic columns are predicted at the grain boundaries.