A perspective on investigating transition metal high-entropy alloys for high-temperature applications

Transition metal high-entropy alloys (TM HEAs) are currently considered as promising structural materials for high-temperature (HT) applications. However, their HT oxidation is a critical issue and must be addressed for hot-end components. This work presents a feasible strategy to identify the HT oxidation behaviors of TM HEAs through experimental investigation and density-functional theory (DFT) calculation. Fe ₂₀Co ₂₀Ni ₂₀Cr ₂₀Cu ₂₀ (H4Cu ₂₀ ) is chosen as the starting model alloy to investigate the HT oxidation behavior of TM HEAs in the ambient atmosphere at the temperature range of 700–900 °C, followed by gradually replacing 5 and 10 at.% Cu with Al, i.e., Fe ₂₀Co ₂₀Ni ₂₀Cr ₂₀Cu ₁₅Al ₅ (H4Cu ₁₅Al ₅) and Fe ₂₀Co ₂₀Ni ₂₀Cr ₂₀Cu ₁₀Al ₁₀ (H4Cu ₁₀Al ₁₀ ). Oxide scales grown on the TM HEAs at HT show a strong temperature and composition dependence and follow a three-stage oxidation growth law. Results show that H4Cu ₂₀ in general suffers from severe localized and internal oxidation, particularly in a secondary phase (FCC2) with low mixing entropy ( Δ S m i x ) and highly positive enthalpy ( Δ H m i x ). The gradual replacement of Cu with Al facilitates the formation of a protective pure or Al ₂O ₃ -rich scale, and significantly reduces the oxidation rate of the TM HEAs. A theoretical investigation is performed from a comprehensive thermodynamics and kinetics perspectives. DFT calculation on the individual constituent phase is conducted to examine the diffusion kinetics of three TM HEAs, and to clarify the postulate of “sluggish diffusion” in high entropy (HE) systems. The diffusivity of constituent elements in each phase follows the empirical “compensation rule”, and diffusion in HE systems shows a strong elemental dependency. Finally, recommendations for selecting alloying elements are given in designing and tailoring TM HEAs for HT applications.