The roles of Nb2O5 phase evolution and solute interactions in temperature-dependent oxidation degradation mechanisms of Nb-based alloys

Catastrophic oxidation remains a major barrier to the application of Nb-based refractory alloys at elevated temperatures. In this study, Oxidation kinetics, oxide scale morphology, phase evolution, and solute interactions at 750 °C and 1100 °C were investigated to elucidate the mechanistic origin of catastrophic oxidation of Nb-based alloys. Precipitation-strengthened Nb-Si alloys, Nb-Si-(Al, Zr)-Sn and Nb-Si-(Al, Zr), were employed as model systems, and oxides were characterized using XRD, SEM/EDS, and Raman spectroscopy. At 750 °C, both Al- and Zr-containing alloys underwent pest oxidation; however, Al-containing alloys showed lower mass gain due to the formation of a continuous amorphous SiO 2 layer, whereas Zr-containing alloys underwent rapid fragmentation. At 1100 °C, oxidation proceeded by rapid external scale growth with mixed linear-parabolic behavior. Under these conditions, ternary alloys developed extensive cracks in oxide scales, whereas the Sn-modified alloys exhibited markedly improved scale continuity and adhesion. Detailed analysis revealed that the formation and evolution of Nb 2 O 5 polymorphs were strongly dependent on both alloy composition and temperature, which were closely associated with oxidation kinetics and scale morphology. In the Sn-modified alloys, inward diffusion of Sn and segregation at the interface between oxide scale and an underlying diffuse oxygen-enriched subsurface region indicate the formation of a Sn-rich diffusion barrier, which contributed to reduced oxidation and improved scale adhesion. These results suggest that oxidation resistance in Nb-based alloys is governed by the coupled effects of temperature-dependent Nb 2 O 5 phase evolution, solute interactions, and Sn-assisted diffusion-barrier formation, providing mechanistic guidance for the design of oxidation-resistant refractory alloys.