High temperature oxidation of Nb-Cr-Mo-Si-B alloys
It has been the trend to use nickel based superalloys in high temperature applications, however higher temperatures are required in the effort to enhance the efficiency of such components as jet turbine engines. The melting temperature of the nickel based superalloy limits the extent to which temperature can be raised as these materials are typically used at temperatures within 200 degrees of their melting point. Refractory metal based systems are attractive due to their high melting points, low densities, and good high temperature strength. However, in such cases as niobium, poor oxidation resistance limits the widespread use of such alloys. The development of new alloy systems that retain the desirable characteristics of the refractory metal while reducing the oxidation of the system is one of the goals of several studies. For this study the compositions of the alloys in atomic percent are Nb-25Cr-20Mo-15Si-10B, Nb-25Cr-20Mo-15Si-15B, Nb-25Cr-15Mo-15Si-15B, Nb-25Cr-15Mo-20Si-15B, and Nb-25Cr-15Mo-20Si-10B. The oxidation behavior and microstructural characteristics of these alloys is studied in the temperature range of 700-1400°C. Oxidation is performed in air from 24 hours up to two weeks (336 hours) of exposure time. The kinetics of oxidation will be compared through the weight gain per area method. Oxidation products are a mixture of CrNbO4, Nb2O5, and SiO2, the amounts of which are dependent on the phase content. An intermediate oxidation layer which appears to be protective has been found to form in this system with high boron content. Increasing the amount of boron content within the alloy has proven to enhance high temperature oxidation resistance after exposure of a period of two weeks. Examination of oxidation products and microstructural evolution with increased temperature was carried out utilizing x-ray diffraction and scanning electron microscopy in secondary, backscatter imaging modes, energy dispersive spectroscopy, and x-ray mapping.
Thomas, Kathryn Suzanne, "High temperature oxidation of Nb-Cr-Mo-Si-B alloys" (2015). ETD Collection for University of Texas, El Paso. AAI3708571.