First-principles investigation of oxidation mechanism of Al-doped Mo5Si3 silicide

Abstract

The hydrogen and oxygen are harmful elements to influence the mechanical properties of high-temperature materials. Although the Mo–Si–Al silicide is a promising high-temperature material, the oxidation mechanism of Mo–Si–Al silicide is unclear. To solve the key problem, in this work, the oxidation mechanism of Mo–Si–Al ternary silicide is studied by the first-principles calculations. In particular, the oxidation behavior of the prefect Mo5Si3 is revealed. The calculated result shows that the O atom prefers to occupy the octahedral interstice (Oct2) position because the calculated oxygen impurity formation energy of Oct2 model is smaller than the tetrahedral interstices (Tet1 and Tet2) and octahedral interstice(Oct1). In particular, it is found that the Mo5Si2Al, Mo5SiAl2, Mo5Al3 and Mo4AlSi3 are stability based on the analysis of thermodynamic model and phonon dispersion. The O atom prefers to occupy the Mo5SiAl2-Oct2 and Mo5Al3-Oct2 models because the calculated oxygen impurity formation energy of Mo5SiAl2-Oct2 and Mo5Al3-Oct2 models is smaller than the other Mo–Si–Al silicides and the perfect Mo5Si3. Naturally, the improvement of oxidation resistance is that the additive Al can enhance the localized hybridization between Al and O. Therefore, the formation of Al–O bond improves the oxidation resistance of Mo5Si3.