First-principles study of crystal structure and stability of T1 precipitates in Al-Li-Cu alloys

• Published in: Acta materialia Vol. 145; no. C; pp. 337 - 346
• Main Authors: Kim, Kyoungdoc, Zhou, Bi-Cheng, Wolverton, C.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Ltd 15.02.2018; Elsevier
• Subjects: MATERIALS SCIENCE
• ISSN: 1359-6454; 1873-2453
• DOI: 10.1016/j.actamat.2017.12.013

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) T1 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 T1 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 T1 phase by comparing the energetic stability of five previously-proposed models of the crystal structure of T1. The DFT formation energy of these proposed T1 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 T1 phase. We search for a new, lower-energy structure of T1 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 (ΔE  = 0.013 eV/atom), which could be affected by finite temperature entropic effects (i.e., vibrational entropic stabilization). [Display omitted]