Effect of Niobium Content Variation on the Hardness and High-Temperature Oxidation of SPS-Processed Ti–Al–Cr-xNb Alloy

This study investigated the effects of Nb additions on the microstructure, hardness, and high-temperature oxidation behaviour of mechanically alloyed and spark-plasma-sintered Ti–48Al–2Cr–xNb alloys containing 1–5 at.% Nb. The aim was to clarify the role of Nb in phase evolution, strengthening, and oxidation protection, and to identify factors that limit its effectiveness. Following mechanical alloying and consolidation at 950 °C, the alloys exhibited predominantly globular γ-TiAl/α2-Ti3Al microstructures with localized Nb enrichment. Relative densities remained below 92%, indicating that residual porosity was a significant processing-related factor. The alloy containing 5 at.% Nb exhibited the highest hardness, consistent with Nb-induced solid-solution strengthening and changes in phase constitution. Isothermal oxidation was assessed by measuring weight gain per unit area after exposure at 700 °C for 400 h and at 900 °C for 200 h. At 900 °C, the alloys formed stratified scales consisting primarily of an outer TiO2-rich layer and an underlying Al2O3-rich region. NbO2 was detected in selected samples; however, no continuous, protective Nb2O5-rich scale formed. Preferential TiO2 growth, together with cracking, exfoliation, and spallation, hindered the formation of a fully protective alumina barrier. At 700 °C, exposure for 400 h produced a substantially thinner, non-adherent oxide scale, consistent with the slower oxidation kinetics at lower temperatures. Consequently, increasing the Nb content did not result in a monotonic improvement in oxidation resistance. Overall, Nb enhanced hardness and influenced phase stability but did not, by itself, overcome the oxidation limitations of TiAl. Its effectiveness depends on densification, Al activity, microstructural homogeneity, and scale adhesion.