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

Abstract

• A new ground-state T1 structure was proposed using the cluster expansion combined with convex hull analysis. • Atomic-resolution HAADF-STEM images verify the reliability of the new T1 structure. • The Al-Li corrugated layers at the phase boundary were revealed by atomic force and bonding calculations. Controversial experimental reports on the crystal structure of T 1 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 T 1 structure that maintains a coherent relationship with Al-matrix at the 〈 112 〉 Al orientation. The underlying phase transition between the variants of T 1 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 T 1 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 T 1 strengthening phase.