Revealing the atomic ordering in the Al-Cu sub-lattice of T1 phases in Al-Cu-Li alloys

The Al and Cu atom ordering within the Al-Cu sub-lattices of the T₁ phase have been unambiguously determined by a combination of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), atomic-resolution energy-dispersive X-ray spectroscopy (EDX), STEM simulations, cluster expansion (CE) methods and density functional theory (DFT) calculations. A new ordered arrangement of Al and Cu atoms within the Al-Cu sub-lattices has been proposed. These dual Al-Cu layers exhibit orthorhombic symmetry with an AlCu₂ stoichiometry, which plays a crucial role in defining the crystal structure of the T₁ phase. This configuration is different from disordered or ordered atomic arrangements of the Al-Cu sub-lattices reported in previous literature. Moreover, a new T₁ model with the composition of Al₆Cu₄Li₃ is proposed based on the new Al-Cu sub-lattices, which demonstrates thermodynamic feasibility.