The effect of native point defect thermodynamics on off-stoichiometry in beta-Mg17Al12

Journal Article · Sun Jan 01 04:00:00 EST 2012 · Acta Materialia

OSTI ID:1049182

Wolverton, Christopher ^[1]

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1. Northwestern University, Evanston

The mechanical strength of Mg-Al-Zn alloys can be affected by a fine spatial dispersion of {beta}-Mg{sub 17}Al{sub 12} precipitates in the Mg matrix. In an effort to understand the phase stability and the unusual asymmetric off-stoichiometry observed in {beta}-Mg{sub 17}Al{sub 12}, we have performed a series of first-principles density functional theory (DFT) calculations of bulk and defect properties of Mg{sub 17}Al{sub 12}. Specifically, we consider native point defects (i.e. vacancies and anti-sites) in all four sublattices of Mg{sub 17}Al{sub 12}, i.e. 2a, 8c, 24g (Mg) and 24g (Al). The T = 0 K static energies of defect Mg{sub 17}Al{sub 12} supercells indicate that anti-site defects are energetically favored over vacancies, and the lowest anti-site defect formation energies are in 24g sites for both Al{sub Mg} and Mg{sub Al}. These Al-rich and Mg-rich anti-site defect formation energies are similar in magnitude, and thus do not explain the asymmetric off-stoichiometry of Mg{sub 17}Al{sub 12}. We also investigate the effect of atomic vibrations via DFT phonon calculations on native point defect free energies of Mg{sub 17}Al{sub 12} and combine these entropic contributions with the point defect formation energies to evaluate the thermodynamics of off-stoichiometry in this phase. We find that the formation of the Al{sub Mg} anti-site is not strongly stabilized by vibrational entropy. Thus, we conclude that the observed asymmetry in the off-stoichiometry of the {beta}-Mg{sub 17}Al{sub 12} phase in the Mg-Al phase diagram is not explained by simple native point defect thermodynamics, and must involve a more complicated defect formation mechanism, such as multi-defect clustering.

• Research Organization: Oak Ridge National Laboratory (ORNL)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-00OR22725
• OSTI ID: 1049182
• Journal Information: Acta Materialia, Journal Name: Acta Materialia Journal Issue: 13-14 Vol. 60; ISSN 1359-6454; ISSN ACMAFD
• Country of Publication: United States
• Language: English

Related Subjects

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
ALLOYS
ASYMMETRY
DEFECTS
ENTROPY
FUNCTIONALS
PHASE DIAGRAMS
PHASE STABILITY
PHONONS
POINT DEFECTS
THERMODYNAMICS
VACANCIES