Nonisothermal and Cyclic Oxidation Behavior of Mo-Si-B and Mo-Si-B-Al Alloys

Abstract

A study on nonisothermal and cyclic oxidation behavior of the reaction-hot-pressed 76Mo-14Si-10B, 77Mo-12Si-8B-3Al, and 73.4Mo-11.2Si-8.1B-7.3Al alloys has been carried out in dry air, and the results have been compared with those of isothermal tests. Nonisothermal studies by thermogravimetric (TG) analysis up to 1300 °C have shown a transient mass gain between 700 °C and 860 °C, followed by a sharp mass loss with increased temperature, with the amount of mass change dependent on the heating rate (5 °C/min to 35 °C/min). The X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies of oxide scales formed on the alloys held at 790 °C and 820 °C for 10 or 20 minutes suggest that the oxidation of α-Mo and Mo3Si precedes that of Mo5SiB2. Thermal cyclic tests involving exposure at 1150 °C for 1 hour, followed by either air cooling to room temperature (RT) or furnace cooling to 700 °C, 800 °C, or 900 °C, and the subsequent examination of oxidation products, have confirmed that the formation of B2O3-SiO2 scale provides complete and partial protection for the Mo-Si-B and Mo-Si-B-Al alloys, respectively. The results of this study show that oxidation resistance is deteriorated upon Al addition. Residual stress measured by XRD is found to be largely compressive in Mo and in mullite phases of oxide scales. Thermal shock and the mismatch in the coefficients of thermal expansion (CTEs) between the constituent phases of the oxide scale appear to be the main causes of damage.