Effects of Mo/Si Ratio Inversion on the Oxidation of Nb-Cr-Mo-Si-B Alloys

The effect of inverting the Mo/Si ratio on the oxidation of two boron-containing alloys has been studied in air for temperatures 700–1400°C. The compositions of the alloys in atomic percent are Nb-25Cr-20Mo-15Si-15B, Nb-25Cr-20Mo-15Si-10B, Nb-25Cr-15Mo-20Si-10B, and Nb-25Cr-15Mo-20Si-15B hereafter referred to as 10B, 15B, 2010, and 2015 alloys, respectively. The as-cast microstructure of the alloys contains a mixture of NbCr2 Laves phase, Nb5Si3 silicide, and additionally Nb3Si silicide for the Mo/Si modified alloys. Primary oxides developed for all alloys are CrNbO4, Nb2O5, and SiO2. Improvements in high-temperature oxidation have been seen for the Mo/Si-modified 15B containing alloy under cyclic testing. Samples have shown similar oxidation responses in both long-term static and cyclic oxidation for 168-h exposures up to 1300°C. Characterization of oxide products was done by x-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy.