Isothermal oxidation of bulk dense Fe₂AlB₂ and Mn₂AlB₂ phases in 700–1000 °C temperature range

Herein, the isothermal oxidation of Mn₂AlB₂ and Fe₂AlB₂ MAB phases in air was studied. When oxidized at 700 °C, the weight gain kinetics for Mn₂AlB₂ are sub-cubic up to 96 h as a result of the formation of a Mn_2-xAl_xO₃ protective layer. Severe spallation of the oxides formed was observed at 800 °C and 900 °C, limiting its service temperature. The oxidation kinetics at 900 °C, for Fe₂AlB₂ become sub-parabolic beyond 60 h due to the formation of a passivating Αl₄Β₂Ο₉ with excellent corner adherence. At both 800 °C and 1000 °C, however, the oxidation is rapid and severe. Therefore, both Mn₂AlB₂ and Fe₂AlB₂ may be used in practical application up to 700 °C and 900 °C, respectively. As far as we are aware, Fe₂AlB₂ is the second transition metal boride that is oxidation resistant at 900 °C; the first, MoAlB, is also a MAB phase.