High-temperature oxidation behavior of Nbx(MoTaW)(1-x) (x = 0.25, 0.4, 0.55, and 0.7) refractory multicomponent alloys

A detailed microstructural and structural study of the high-temperature oxidation behavior of refractory multicomponent alloys (RMCAs) with composition Nbx(MoTaW)(1-x) (x = 0.25, 0.4, 0.55, and 0.7 at%, designated as Nb0.25, Nb0.4, Nb0.55, and Nb0.7 respectively) was carried out as a function of Nb content at temperatures of 873K, 973K, and 1073K for durations up to 9h. Before the oxidation test, the RMCAs mentioned above had the single-phase BCC structure. Thermogravimetric curves demonstrated a weight gain with increase in temperature, time, and Nb concentration, showing that Nb0.7 has low oxidation resistance. The weight gain curves were fitted using a power law equation and it was observed that the data show a good fit for the linear oxidation behavior for all samples. Quantification of the activation energy for oxide formation revealed that a higher Nb content results in a lower activation energy, suggesting poor oxidation resistance. XRD patterns show that in the above oxidized RMCAs, simple oxides such as Nb2O5, Ta2O5, MoO3, and WO3 form at 873K since these have the lowest free energy of formation. At 973K and 1073K, these simple oxides react to produce complex oxides such as Nb2W3O14, Nb14W3O44, and Ta8W9O47, with a fraction of the simple oxides continuing to be present. As the temperature and Nb concentration increased, the surface morphology of RMCAs, as studied by SEM, revealed the presence of a discontinuous non-protective oxide layer with pores, bursts, nano-sized rod-shaped particles and cracks. In this study, Nb0.25 exhibits superior oxidation resistance as compared to other RMCAs. •Oxidation behavior of Nbx RMCAs was examined at 873K, 973K, and 1073K temperatures.•The weight gain curves of RMCAs are in good fit with linear oxidation kinetics.•The oxidation resistance of Nbx RMCAs decreased with increasing Nb content.•The RMCA with high content of Nb is more prone to oxidation and oxide layer spallation.