Abstract
We have performed a quantum-mechanical study of off-stoichiometric Al2Ge2Mg τ-phase intermetallic compounds. Our calculations focused on evaluating the formation energy of static lattices, their elastic properties as well as free energy of formation including phonon contributions. While the stoichiometric Al2Ge2Mg τ phase was found thermodynamically, mechanically and dynamically stable, we have identified an off-stoichiometric composition Al7Ge8Mg5 which exhibits the free energy of formation lower than the stoichiometric compound. The Al7Ge8Mg5 was found both mechanically and dynamically stable as well. This off-stoichiometric compound has Mg atoms located also at the Al sublattice, i.e. these defects exhibit a stabilizing effect. Phonon contributions into the free energy of formation have turned out to be crucial for a proper description of the thermodynamic stability. In contrast, several other off-stoichiometric states are significantly thermodynamically destabilized by anti-site atoms at the Ge sublattice. The formation energy and the homogenized polycrystal Young's modulus are found anti-correlated indicating a connection between the thermodynamic and mechanical stability. Regarding the theoretical values of homogenized polycrystal Young's moduli, our measurements resulted in values quite close to the predicted ones.
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