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 cyclic 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 protective 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.