First-principles study of crystal structure and stability of T-1 precipitates in Al-Li-Cu alloys

Aluminum-lithium-copper alloys have a low density, high elastic modulus and high specific strength. Due to this combination of properties, alloys strengthened with the ternary (Al-Li-Cu) T-1 phase have attracted a great deal of interest especially in aerospace applications. Determining the atomic structural information of the precipitate is a fundamental step in developing a basis for advanced alloy design; however, even though many experimental studies have addressed the T-1 crystal structure, it remains the subject of some controversy. Here, we use density functional theory (DFT) calculations to investigate the structure and composition of the T-1 phase by comparing the energetic stability of five previously-proposed models of the crystal structure of T-1. The DFT formation energy of these proposed T-1 crystal structures was calculated using a special quasi-random structure (SQS) approach to describe a disordered Al-Cu sub-lattice. In conflict with the experimental phase diagram, DFT calculations of all five proposed models result in an energetically unstable T-1 phase. We search for a new, lower-energy structure of T-1 using a cluster expansion approach, and find a new structural model with DFT energy that is stable (at T = 0 K), i.e., on the calculated convex hull of the Al-Li-Cu ternary system. However, this new predicted phase does not have a tie-line with Al, but the formation energy of the phase is very close to the energy required to make a tie-line with Al (Delta E = 0.013 eV/atom), which could be affected by finite temperature entropic effects (i.e., vibrational entropic stabilization). (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.