The oxidation of six NbTi-i refractory medium- and high-entropy alloys (NbTi + Ta, NbTi + CrTa, NbTi + AlTa, NbTi + AlMo, NbTi + AlMoTa and NbTi + AlCrMo) were investigated at 1000 °C for 20 h. According to our observation, increased Cr content promoted the formation of protective CrNbO₄ and Cr₂O₃ oxides in NbTi + CrTa and NbTi + AlCrMo, enhancing oxidation resistance. The addition of Al resulted in the formation of AlTi-rich oxide in NbTi + AlTa. Ta addition resulted in the formation of complex oxides (MoTiTa₈O₂₅ and TiTaO₄) and decreased oxidation resistance. Meanwhile, Mo’s low oxygen solubility could be beneficial for oxidation resistance while protective Cr₂O₃/CrNbO₄ layers were formed. In NbTi + Ta, NbTi + AlTa and NbTi + CrTa, a considerable quantity of Ti-rich oxide was observed at the interdendritic region. In NbTi + AlCrMo, the enrichment of Cr and Ti at the interdendritic region could fasten the rate of oxidation. Compared to the recent research, NbTi + AlCrMo alloy is a light-weight oxidation-resistant alloy (weight gain of 1.29 mg/cm² at 1000 °C for 20 h and low density (7.2 g/cm³)).

This study investigates the effect of Re addition on the microstructure evolution and oxidation resistance of Co-Cr-Ta-Ti-C alloys. The alloys tested in this study consist of a Co-rich matrix with mixed HCP and FCC phases, together with TaTiC2 and Ti-based carbides. The addition of Re promotes the precipitation and growth of a CoCr-rich σ phase. Differential thermal analysis (DTA) showed that the melting temperatures did not change drastically up to 10 at% Re addition, whereas 15 at% Re addition significantly increased the melting temperature of the alloys. Oxidation tests in air at 1200°C showed that excessive Re content could lead to deterioration in oxidation resistance, mainly due to the accelerated formation of volatile Re oxides.

This study investigated the effect of Re addition on the microstructural evolution and oxidation resistance of Co-Cr-Ta-Ti-C alloys. The alloys tested in this study consisted of a Co-rich matrix with mixed HCP and FCC phases, together with TaTiC₂- and Ti-based carbides. The addition of Re promotes the precipitation and growth of a CoCr-rich σ-phase. Differential thermal analysis (DTA) showed that the melting temperatures did not drastically change up to 10 at% Re addition, whereas 15 at% Re addition significantly increased the melting temperature of the alloys. Oxidation tests in air at 1200 °C showed that an excessive Re content could lead to a deterioration in oxidation resistance, mainly due to the accelerated formation of volatile Re oxides.