Identifying the crystal structure of T₁ precipitates in Al-Li-Cu alloys by ab initio calculations and HAADF-STEM imaging

Controversial experimental reports on the crystal structure of T₁ precipitates in Al-Li-Cu alloys have existed for a long time, and all of them can be classified into five models. To clarify its ground-state atomic structure, herein, we have combined high-throughput first-principles calculations and CALPHAD, as well as aberration-corrected HAADF-STEM experiments. Employing the special quasi-random structure (SQS) and supercell approximation (SPA) methods to simulate the local disorder on Al-Cu sub-lattices, we find that none of the present models can satisfy the phase stability in Al-Li-Cu ternary system based on temperature-dependent convex hull analysis. Using the cluster expansion (CE) formulas, structural predictions derived from the five-frame models were performed. Subsequently, by introducing the vibrational contribution to the free energy at aging temperatures, we proposed a novel ground-state T₁ structure that maintains a coherent relationship with Al-matrix at the 〈112〉_Al orientation. The underlying phase transition between the variants of T₁ precipitates was further discussed. By means of ab initio molecular dynamics (AIMD) simulations, we resolved the controversy regarding the number of atomic layers constituting the T₁ phase and acknowledged the existence of Al-Li corrugated layers. The root cause of this structural distortion is triggered by atomic forces and bondings. Our work can have an positive impact on the novel fourth generation of Al-Cu-Li alloy designs by engineering the T₁ strengthening phase.