Achieving Excellent High‐Temperature Oxidation Resistance in a Novel L1₂‐Reinforced Co‐Rich Medium‐Entropy Alloy

Excellent high‐temperature oxidation resistance for traditional high‐entropy alloys/medium‐entropy alloys (MEAs) is still a huge challenge. Herein, a novel Co_51.3Ni_19.5Cr₁₉Al_6.4Ti_3.4Nb_0.4 MEA with superior high‐temperature oxidation resistance is designed. The specific high‐temperature oxidation behaviors of the studied MEA are systematically investigated at 800, 900, and 1000 °C for 200 h. Specifically, the studied MEA remains a single face‐centered cubic (FCC) phase after solution treatment under vacuum at 1200 °C for 15 h, while high‐density L1₂ nanoprecipitates form inside the FCC matrix after aging treatment under vacuum at 750 °C for 12 h, revealing the consistency between the experimental results and the theoretical prediction. The results of oxidation tests indicate that the studied MEA with FCC matrix and L1₂ nanoprecipitates exhibits a superior oxidation resistance. The oxidation kinetics of the oxide layer follow a parabolic law at oxidation temperature of 800, 900, and 1000 °C and the corresponding oxidation rate constants (k_p) are 0.001, 0.011, and 0.047 mg² cm⁻⁴ h⁻¹, respectively. The diffusion activation energy of the studied MEA is determined to be 258 kJ mol⁻¹ and the oxidation process is controlled by outward diffusion of metallic elements and inward diffusion of oxygen. The research results provide guidance for the development of superalloys.