Oxidation behavior of Nb–24Ti–18Si–2Al–2Hf–4Cr and Nb–24Ti–18Si–2Al–2Hf–8Cr hypereutectic alloys at 1250 °C

Nb–24Ti–18Si–2Al–2Hf–4Cr and Nb–24Ti–18Si–2Al–2Hf–8Cr alloys were prepared by arc melting in a water-cooled crucible under argon atmosphere. Microstructural characteristics and oxidation resistance of the alloys at 1250 °C were investigated. The results show that, when the Cr content is 4 at%, the microstructures consist of (Nb,Ti)_SS and Nb₅Si₃; as Cr content increases to 8 at%, C14 Laves phase Cr₂Nb is formed. The isothermal oxidation tests show that the oxidation kinetics of the two alloys follow similar features. The weight gains of the two alloys after oxidation at 1250 °C for 100 h are 235.61 and 198.50 mg·cm⁻², respectively. During oxidation, SiO₂, TiO₂, Nb₂O₅ and CrNbO₄ are formed at first. Then, Ti₂Nb₁₀O₂₉ is formed after oxidation for 20 min and begins to change into TiNb₂O₇ as the oxidation proceeds. SiO₂ is formed as solid state at first but later evolves into glassy state to improve the cohesion of the scale. After oxidation for 100 h, oxidation products consist of SiO₂, TiNb₂O₇, Nb₂O₅ and CrNbO₄.