Effect of Moist Air and Minor Zr Addition on Oxidation Behavior of Arc-Melted Multiphase Mo-Si-B Alloys in the Temperature Range of 1000 degrees C-1300 degrees C

摘要

Arc-melted multiphase intermetallic alloys, Mo76Si14B10, Mo74Si14B10Zr2, Mo79.5Si12B8.5 and Mo77.5Si12B8.5Zr2 with microstructures containing Mo-ss, Mo3Si and Mo5SiB2 were isothermally exposed for 1 h or 24 h at 1000 degrees C, 1150 degrees C and 1300 degrees C in dry or moist air. The residual area fraction was found to be the highest on exposure at 1150 degrees C in dry air, but the least at this temperature in moist air. Examination of dry air formed oxide scales by X-ray diffraction, and scanning electron microscopy with energy-dispersive spectroscopy showed the presence of Mo, and MoO2 with a continuous, protective glassy B2O3-SiO2 top layer. Formation of Zr(MoO4)(2) in the Zr-containing alloys had lowered mass loss by arresting MoO3 vaporization, and had enhanced the B2O3-SiO2 scale formation kinetics. Further, Si(OH)(4), cristobalite, H3BO3 and MoO3 center dot(H2O)(2) appeared to be constituting the oxidation products formed in moist air. Alloying with Zr enhanced the kinetics of protective scale formation by lowering the Mo-ss content, and microstructural refinement. The deterioration of oxidation resistance in moist air was conjectured to be due to longer duration required for protective scale to form, formation of beta-cristobalite permitting grain boundary diffusion of oxygen anions, as well as volatile hydrated oxidation products as mentioned above. Formation of ZrSiO4 and dissolved ZrO2 in B2O3-SiO2 scale of the Mo-Si-B-Zr alloys in both dry air and moist air is considered to have contributed to the enhanced protective character of oxide scale by stabilizing the glassy scale.