Oxidation behavior of extruded Mo5Si3Bx–MoSi2–MoB intermetallics from 600°–1600 °C

The oxidation behavior of Mo5Si3Bx (T1)–MoSi2–MoB composites fabricated by green extrusion processing was investigated over the temperature range of 600–1600 °C in air. Low temperature experiments from 600 to 1000 °C revealed that unoxidized material initially forms a mixed molybdenum and silicon oxide scale. The unoxidized material undergoes rapid mass loss over a narrow temperature range of 660–760 °C, with complete disintegration of the material occurring at about 730 °C. At temperatures above about 760 °C, a coherent glassy scale forms to protect the alloy from further oxidation. High temperature oxidation tests from 1150 to 1600 °C show that the material forms a continuous glassy scale, 10–20 μm in thickness, which protects the underlying alloy. Steady state oxidation kinetics were not achieved until exposure at 1600 °C, due to the effect of contamination introduced during green extrusion processing. The calculated parabolic rate constant at 1600 °C is 9.7×10−3 mg2/cm4/h, which compares favorably to that of MoSi2-based materials. The observed oxidation behavior and kinetics over the entire temperature range were not significantly influenced by sample porosity (79% versus 96% of theoretical density). Boron depletion of the near-surface scale was observed at 1150 °C, leading to the formation of pure silica in the outer portion of the scale. The extent and rate of boron evaporation increased significantly as temperature increased to 1600 °C. Formation of pure silica in the scale implies that oxygen transport through silica must be the rate-limiting step in the high temperature oxidation of these alloys.