Effect of Ni/Co ratio on the mechanical properties and high-temperature oxidation behaviors of Ni-Co-Cr-Al-Ti high-entropy alloys

Developing L1₂-strengthened high-entropy alloys (HEAs) with an excellent combination of mechanical properties and robust oxidation resistance is crucial for their structural applications at elevated temperatures. However, the influence of major elements like Ni and Co on these critical properties has not yet been clearly identified. In this study, we showcased that optimizing the Ni/Co ratio in Ni-Co-Cr-Al-Ti HEAs yields an excellent synergy of strength and oxidation resistance, circumventing the common trade-off observed in many HEAs. Specifically, increasing the Ni content enhances mechanical strength by promoting a higher volume fraction of L1₂ precipitates, with the alloy achieving a yield strength of approximately 1028 MPa and ductility of around 17%. Furthermore, detailed oxidation kinetic analyses revealed that a higher Ni concentration improves oxidation resistance at 800 ∼ 1000 ℃, 900 ℃ in particular. This improvement is attributed to the impeded outward diffusion of Ti, which minimizes void formation and facilitates the development of a dense and protective Cr₂O₃ oxide layer. These findings offer a promising strategy for designing HEAs with superior comprehensive performances for demanding high-temperature environments.