Oxidation behavior of multiphase Nb-Mo-Si-B intermetallics

Abstract

The mixed substitution of Nb and Mo in the ternary systems Mo-Si-B and Nb-Si-B was studied with the goal of balancing oxidation resistance with mechanical behavior. The microstructure and oxidation behavior of six compositions in the Nb-Mo-Si-B system were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy, electron probe microanalysis (EPMA), and thermogravimetric analysis. Proper selection of the total metal content and the Nb/Mo ratio results in the co-existence of a T1 phase, as (Nb, Mo)5Si3Bx, and a solid solution (Nb,Mo) metal phase. At 800 °C, all compositions exhibited catastrophic oxidation, while changing to a quasi-steady-state mass gain at 1200 °C. The high rate constants at 1200 °C indicate that the scales formed were not passivating. A complex scale consisting of four layers formed that was about 350-to 450-µm thick after oxidation for 50 hours at 1200 °C. Borosilicate glass did form within the scale, but the significant prevalence of Nb2O5 within the glass, and the resulting inability of the glass to seal pores formed by the evaporation of MoO3, contributed to the overall poor oxidation resistance compared to the ternary Mo-Si-B system. The Nb and Mo content of the alloy must be further studied and optimized before these alloys may be considered for further development for hightemperature applications.