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

Journal Article · 19 December 2017 · Acta Materialia

DOI: https://doi.org/10.1016/j.actamat.2017.12.013 · OSTI ID:1462028

Kim, Kyoungdoc ^[1]; Zhou, Bi-Cheng ^[1]; Wolverton, C. ^[1]

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1. Northwestern Univ., Evanston, IL (United States)

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 structuralinformation 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. Here, 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).

• Research Organization: Lawrence Berkeley National Laboratory, Berkeley, CA (United States). National Energy Research Scientific Computing Center (NERSC)
• Sponsoring Organization: USDOE Office of Science (SC), Advanced Scientific Computing Research (ASCR); USDOE Office of Science (SC), Basic Energy Sciences (BES). Scientific User Facilities (SUF); National Institute of Standards and Technology (NIST); USDOE
• Grant/Contract Number: EE0006082; AC02-05CH11231; 70NANB14H012
• OSTI ID: 1462028
• Alternate ID(s): OSTI ID: 1549005
• Journal Information: Acta Materialia, Vol. 145, Issue C; ISSN 1359-6454
• Publisher: ElsevierCopyright Statement
• Country of Publication: United States
• Language: English