Search

The microstructures and mechanical properties in the peak ageing condition, warm forming process, and paint baking process of two Al–Zn–Mg–Cu aluminum alloys, which possessed the same base compositions but different levels of Mn (0 and 0.24 wt%), have been investigated. The results show that Mn addition can hardly refine the grain size (around 20 μm) in AA7075 aluminum alloy in warm forming and paint baking processes. Mn-free and Mn-containing alloys subjected to all three processes possess nano-precipitates of nearly the same size, indicating that Mn addition does not influence the nano-precipitate evolution in the aluminum matrix. Energy-dispersive X-ray spectroscopy (EDS) mapping results of dispersoids show that dispersoids in Mn-bearing specimens are enriched in Mn and depleted in Zn and Mg. The Zn and Mg expelled from dispersoids in Mn-containing specimens into the aluminum matrix presumably formed Zn- and Mg-rich zones around the dispersoids, which improved the precipitate transformation reation from GPII to η′ phase at the boundaries between aluminum matrix and dispersoids. Although the solid solution strengthening effect triggered by Mn addition might be less effective because of the formation of dispersoids, the slight enhancement of ultimate tensile strength (UTS) in Mn-containing specimens rather than in Mn-free specimens in all states can be attributed to the formation of nano-precipitates around the dispersoids.

Cs-corrected high-angle-annular-dark-field scanning-transmission-electron microscopy (Cs-corrected HAADF-STEM) was employed to examine the phases in AlCu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. Keywords: Cs-corrected HAADF-STEM, AlCuLi (AA2050) aluminium alloy, GP(θ″) zones, T1 precipitates, θ′ Precipitates, S precipitates