Unique hybrid precipitate structures forming in an Al–Cu–Mg–Si alloy

Crystal structures of hybrid precipitates have been analysed by atomic-resolution scanning transmission electron microscopy in an aged Al–Cu–Mg–Si alloy. A new unique hybrid precipitate (UHP) has been found as rod-shaped discrete particles along ⟨100⟩Al in bulk aluminium matrix. The UHP consists of a core with local 4-fold symmetry surrounded by GPB zone unit pillars often combined with monoatomic Cu layers in fcc-Al matrix. A certain atomic configuration fragment of the UHP structure is often seen to inter-grow with other hybrid precipitate complexes consisting of diverse phases belonging to the Al–Cu–Mg–Si and Al–Cu alloy systems. Density functional theory (DFT) calculations performed on a discrete UHP showed that the most energetically favorable core configuration resembles a β′′-eye from the Al–Mg–Si system rotated 45 ∘C in respect to its usual orientation with the fcc-Al matrix. •Unique hybrid precipitate (UHP) structure was identified in an Al–Cu–Mg–Si alloy.•UHP are found as rod-like discrete particles aligned along ⟨100⟩Al directions.•UHP fragments interact with other AlCu/AlMgSiCu-type precipitates.•UHP fragments appear in different commercial alloys containing Cu, Mg and Si.