The effect of native point defect thermodynamics on off-stoichiometry in β-Mg17Al12 Original Research Article

The mechanical strength of Mg–Al–Zn alloys can be affected by a fine spatial dispersion of β-Mg17Al12 precipitates in the Mg matrix. In an effort to understand the phase stability and the unusual asymmetric off-stoichiometry observed in β-Mg17Al12, we have performed a series of first-principles density functional theory (DFT) calculations of bulk and defect properties of Mg17Al12. Specifically, we consider native point defects (i.e. vacancies and anti-sites) in all four sublattices of Mg17Al12, i.e. 2a, 8c, 24g (Mg) and 24g (Al). The T = 0 K static energies of defect Mg17Al12 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 AlMg and MgAl. 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 Mg17Al12. We also investigate the effect of atomic vibrations via DFT phonon calculations on native point defect free energies of Mg17Al12 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 AlMg anti-site is not strongly stabilized by vibrational entropy. Thus, we conclude that the observed asymmetry in the off-stoichiometry of the β-Mg17Al12 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.