Oxidation Mechanism of Nb–Si-Based Ultra-High Temperature Materials

The oxidation behavior of Nb–Si-based alloy (Nb–20Ti–16Si–3Cr–3Al–2Hf, at%) at 1250 °C in air was investigated. Severe selective oxidation occurred in the alloy, which mainly took place in Nb solid solution and accompanied with large amount of precipitations of Ti-rich oxide. External oxidation of Nb solid solution was the selective oxidation, composed of two steps of solid-phase reaction among the oxides. The morphology of oxide-scale layers varied since the solid-phase reaction between the oxides, and it became porous and multilayer finally. TiNb2O7, Ti2Nb10O29, SiO2, and TiO2 were the main phase constitutions of the oxide layer which were rich in Ti2Nb10O29 in inner oxide-scale layer and TiNb2O7 in outer oxide-scale layer. The protection efficiency of the oxide-scale layer for the Nb–Si-based alloy at high temperature was weak. The weight gain during the beginning oxidation stage fitted the linear oxidation law with the rate of about 6 mg/(cm2 h).