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

Abstract

A detailed microstructural and structural study of the high-temperature oxidation behavior of refractory multicomponent alloys (RMCAs) with composition Nb_x(MoTaW)_(1-x) (x = 0.25, 0.4, 0.55, and 0.7 at%, designated as Nb_0.25, Nb_0.4, Nb_0.55, and Nb_0.7 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 Nb_0.7 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, Nb_0.25 exhibits superior oxidation resistance as compared to other RMCAs.