Effects of microstructure on the oxidation behavior of multiphase Mo-Si-B alloys

The oxidation behavior of multiphase Mo-Mo₅SiB₂(T2)-Mo₃Si alloys was examined in terms of different starting morphologies and volume fractions of the phases. The multiphase alloys were cast and heat-treated at 1600 °C before performing the cydic oxidation tests at temperatures between 600 and 1300 °C. Alloys compositions were chosen near the Mo-T2 eutectic trough resulting in microstructures containing either primary Mo-dendrites or primary T2-dendrites. After cyclic oxidation tests at 800 °C, a Mo interfacial layer was found adjacent to the T2 phase along the oxide/matrix interface while a Mo₅Si₃ interfacial layer was found adjacent to the Mo₃Si phase. For specimens tested at 1300 °C, a Mo + glass interfacial region was found. The multiphase alloys have a poor oxidation resistance at 800 °C due to the competing oxide formation of the Mo oxides and a silicate glass. However, a protective glass scale can form on these alloys at 1300 °C. The presence of Mo₃Si was found to be an import source of Si in addition to the T2 phase in controlling the composition of the pretective glass scale. The near-eutectic alloys with a fine dispersion of T2, Mo, and Mo₃Si phases displayed the best overall oxidation resistance for the temperatures tested. © 2002 Elsevier Science B.V. All rights reserved.