High temperature oxidation behavior of niobium-molybdenum-silicon-boron-chromium alloy

Materials for high temperature applications, such as jet engines, gas turbines and turbine blades, require a balanced combination of physical and chemical properties to withstand the aggressive environments in which they are utilized. Some of the properties required are high strength, low density, high melting point and good oxidation resistance at elevated temperatures [1-3]. For this reason, the microstructure and oxidation behavior of the Nb-20Mo-15Si-5B-20Cr alloy (at. %) has been studied as a potential candidate to replace nickel-based alloys currently used in the aerospace industry. Short term oxidation (STO) and long term oxidation (LTO) studies have been performed in air for this alloy over a temperature range from 700°C to 1400°C, for 24 and 168 hours respectively. Oxidation curves for this alloy were obtained by plotting the mass gain per unit area as a function of temperature and time to determine its oxidation resistance. STO was temperature dependent and oxidation curve shows a minimum mass gain per unit area between 900 and 1000°C. LTO was time dependent and curves show a minimum mass gain at 1200 and 1300°C. Parabolic behavior was observed at those temperatures. Microstructure of the alloy and oxidized products were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). According to results, the microstructure is composed of body center cubic (BCC) solid solution (α), tetragonal Nb 5Si3, Nb3Si, hexagonal Laves phase NbCr2 and a eutectic like microconstituent α, Nb3Si and NbCr2. The oxidation products vi at 700 to 1300°C show the presence of Nb2O5, SiO2, CrNbO4, among others. The parabolic and linear oxidation rate constants values indicate that the studied alloy is competitive when compared to the values for Nb-based alloys reported by different authors