The effect of native point defect thermodynamics on off-stoichiometry in β-Mg 17Al 12

Dongwon Shin, Christopher Wolverton*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

13 Scopus citations

Abstract

The mechanical strength of Mg-Al-Zn alloys can be affected by a fine spatial dispersion of β-Mg 17Al 12 precipitates in the Mg matrix. In an effort to understand the phase stability and the unusual asymmetric off-stoichiometry observed in β-Mg 17Al 12, we have performed a series of first-principles density functional theory (DFT) calculations of bulk and defect properties of Mg 17Al 12. Specifically, we consider native point defects (i.e. vacancies and anti-sites) in all four sublattices of Mg 17Al 12, i.e. 2a, 8c, 24g (Mg) and 24g (Al). The T = 0 K static energies of defect Mg 17Al 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 Mg and Mg 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 17Al 12. We also investigate the effect of atomic vibrations via DFT phonon calculations on native point defect free energies of Mg 17Al 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 Mg anti-site is not strongly stabilized by vibrational entropy. Thus, we conclude that the observed asymmetry in the off-stoichiometry of the β-Mg 17Al 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.

Original languageEnglish (US)
Pages (from-to)5135-5142
Number of pages8
JournalActa Materialia
Volume60
Issue number13-14
DOIs
StatePublished - Aug 1 2012

Keywords

  • First-principles calculations
  • Mg-Al
  • Native point defects
  • Vibrational entropy
  • β-Mg Al

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Ceramics and Composites
  • Polymers and Plastics
  • Metals and Alloys

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