Uncovering the influence of Cu on the thickening and strength of the δ′/θ′/δ′ nano-composite precipitate in Al–Cu–Li alloys

Nanoscale δ′/θ′/δ′ composite precipitate, as another important strengthening phase in the latest generation of Al–Li alloys, exhibits excellent resistance to coarsening. Here, we propose two thickening models for an anomalous δ′/θ′/δ′ that is discovered recently, by analyzing various influencing factors, including the static energy barrier, aging temperature-dependent nucleation conditions, and the elastic distortion suffered during growth. It indicates that the thickness of the δ′/θ′/δ′ composite precipitate seems to depend on the nucleation of the δ′. At elevated aging temperatures, θ′ precipitates can grow and thicken rapidly until the δ′ nucleates on them. This is strikingly different from the dislocation-induced growth mechanism. Subsequently, we propose a vacuum-added methodology to accurately extract the interfacial energy. The results show that this nano-composite precipitate can realize supra-nanostructure in thickness at low aging temperatures due to the spontaneous nucleation of the δ′ upon the pre-precipitate θ′. Cu atoms segregated at the interface suppress the nucleation of the δ′, but release the lattice distortion to some extent. Using Griffith fracture model-assisted ab-initio uniaxial tensile tests, the cohesion strength and fracture process of this composite precipitate with and without Cu segregation have been captured. It indicates that the Cu atoms induced interfacial expansion and the fracture of strong Al–Al covalent bond, resulting in a reduction of fracture strength of this nano-composite precipitate.