Non-Isothermal, Isothermal and Cyclic Oxidation Behaviour of Mo-Si-B and Mo-Si-B-Al alloys

Abstract

Non-isothermal, isothermal and cyclic oxidation behaviour of alloys, processed by reaction hot pressing of elemental powder mixtures with compositions as 76Mo14Si10B (MSB), 77Mo12Si8B3Al (MSB3AL), and 73.4Mo11.2Si8.1B7.3Al (MSB7.3AL), have been studied. Investigations by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy have shown α-Mo, Mo3Si, and Mo5SiB2 in microstructures of these alloys. Additionally, SiO2 and α-Al2O3 have been found in MSB and Al-containing alloys, respectively. Non-isothermal studies have shown transient mass gain at 700-860 oC, and loss at higher temperatures by vaporization of MoO3, with results confirming strong influence of heating rate (5-35 oC per minute). Exposures of MSB at 790 oC for 20 minutes and analyses of resultant scale have indicated that oxidation of α-Mo and Mo3Si precedes that of Mo5SiB2. Isothermal tests have been carried out in dry air between 600 and 1300 oC for either 24 or 170-300 h. While the MSB shows unabated mass loss at 700 oC, and a stable regime after transient loss at ≥800 oC; Al-containing alloys have shown pest-like disintegration at 700-900 oC. A stable regime is reached with the least mass loss at 1150 oC due to rapid formation of protective scale; and prior exposure at this temperature enhances the oxidation resistance at 700 oC. Analysis of oxide scales has shown presence of B2O3, SiO2 and Mo-oxides, with MoO3 as the major constituent for exposures at ≤700 oC, and formation of Mo/MoO2 phases at higher temperatures. Moreover, α-Al2O3 and mullite have been found in oxide scales of Al-containing alloys formed at ≤900 oC and above, respectively. Thermal cyclic tests involving exposure at 1150 oC for 1 h, followed by either air cooling to room temperature or furnace cooling to 700-900 oC, and oxide scale analyses, have confirmed that formation of B2O3-SiO2 scale provides complete and partial protection for MSB and Al-containing alloys, respectively. Residual stress in Mo and mullite phases of oxide scales measured by X-ray diffraction has been found as compressive, and calculations indicate that thermal shock and coefficient of thermal expansion mismatch between oxidation products contribute significantly to this stress. The results of this study indicate that Al additions to the extent attempted increases the susceptibility to oxidation.