Europhys. Lett.
Volume 73, Number 5, March 2006
Page(s) 719 - 725
Section Condensed matter: structural, mechanical and thermal properties
Published online 01 February 2006
Europhys. Lett., 73 (5), pp. 719-725 (2006)
DOI: 10.1209/epl/i2005-10462-x

Predicting metastable phase boundaries in $\chem{Al\tx{-}Cu}$ alloys from first-principles calculations of free energies: The role of atomic vibrations

C. Ravi1, 2, C. Wolverton1 and V. Ozolins3

1  Ford Research and Advanced Engineering, MD 3083/SRL Dearborn, MI 48121-2053, USA
2  Department of Materials Science and Engineering, The Pennsylvania State University - University Park, PA 16802-5006, USA
3  Department of Materials Science and Engineering, University of California Los Angeles, CA 90095-1595, USA

received 12 August 2005; accepted in final form 12 January 2006
published online 1 February 2006

Metastable precipitate phase boundaries are difficult to ascertain experimentally and yet are important for controlling the microstructure of precipitation-hardenable alloys. We demonstrate how first-principles calculations of configurational and vibrational free energies can be used to predict precipitate phase boundaries of stable and metastable phases in $\chem{Al\tx{-}Cu}$ alloys. Surprisingly, the formation entropy of a Cu impurity is found to be hugely positive (+2.7 kB/atom), leading to a dramatic enhancement in the solubility. The large entropy is dominated by the very low-frequency vibration of the small impurity atom (Cu) inside the large cage of the host (Al). The agreement between the GGA and experimental data is within 100$\un{K}$ for all phases, showing that accurate first-principles determination of metastable phase boundaries is now possible.

65.40.Gr - Entropy and other thermodynamical quantities.
63.20.Dj - Phonon states and bands, normal modes, and phonon dispersion.
81.30.Bx - Phase diagrams of metals and alloys.

© EDP Sciences 2006