Morphological evolution of GP zones and nanometer-sized precipitates in the AA2050 aluminium alloy

Cs-corrected high-angle-annular-dark-field scanning-transmission-electron microscopy (Cs-corrected HAADF-STEM) was employed to examine the phases in Al[sbnd]Cu[sbnd]Li alloy (AA2050), including GP(T₁), GP(θ″) and GPB zones with their subsequent nanometer-sized products, T₁ (Al₂CuLi), θ′ (Al₂Cu), and S (Al₂CuMg) precipitates, respectively. Under the peak-aging condition, some solute-atom enriched clusters could still be found, and the newly-formed nucleus of GP(θ″) with a mono-layer {100} plane of Cu atoms occurred at the adjacent area of the joint between θ′ and S precipitates or the edge of an individual S precipitate. The transition of a single Cu-layer GP(θ″) → θ′ was presumed to be transformation via in-situ nucleation. The developing GP(θ″) zones and θ′ precipitates were easily subjected to soft impingement. However, hard impingement between two variants of θ′ presumably occurred, wherein one θ′ variant precipitate was blocked out by the other θ′ variant. As for the creep-ageing forming (CAF) treated sample, some precipitates of T₁ and θ′ were found to have the cutting characteristic on specific ledges.