Oxidation behavior of a three-phase eutectic MoNbSiB alloy from 600 °C to 1100 °C

In this study, microstructure and intermediate-to-high-temperature oxidation behavior of a three-phase eutectic 43.2Mo-32.6Nb-19.5Si-4.7B (at.%) alloy were systematically investigated. The quaternary alloy exhibits homogeneous distribution of a Mo-based BCC solid solution along with two highly ordered intermetallic compounds (T 1 and T 2 ) of fine-scaled microstructure. The oxidation behavior of this alloy is more analogous to that of the well-known NbSi-based alloys rather than the MoSiB-based ones. Fortunately, catastrophic oxidation, from which the NbSi-based alloys usually suffer, never occurs in intermediate temperature range for this alloy. When oxidized at 600 °C and 700 °C, the alloy demonstrates only a slight mass gain and the total specific mass change within 48 h was approximately 4.4 mg/cm 2 . The three-phase eutectic microstructure has been largely inherited in the oxidation scale. The homogeneous distribution of three phases in the substrate alloy and the comparable Nb content in each phase played positive roles to prevent the localized accumulation of volume expansion and internal stress concentration in the oxide scale caused by the formation of the non-protective Nb 2 O 5 . However, when the oxidation temperature was elevated to 800 °C and 1100 °C, the alloy exhibits a drastic mass loss and the oxide scale becomes extremely porous. Significant swelling and spallation of the oxides occur at the early stage of oxidation, after which the alloy undergoes rapid depletion.