A novel strategy for architecting low interfacial energy transition phase to enhance thermal stability in a high-entropy alloy

Alloys with high strength and thermal stability are attractive for crucial applications in aerospace and power-generation sectors. In this paper, based on the thermodynamics, the microstructure, hardness, and compressive properties of the (FeCoNiCrCu_1.71)₉₆Nb₄ and (FeCoNiCrCu_1.71)₉₀Nb₁₀ (at%) high-entropy alloys (HEAs) upon high-temperature aging were studied. The results indicate that the addition of Nb increases the amounts of the Nb-rich precipitates and concentration of Nb within the matrix, enhancing the hardness and yield strength. With prolonged aging time, the hardness and yield strength of the (FeCoNiCrCu_1.71)₉₆Nb₄ and (FeCoNiCrCu_1.71)₉₀Nb₁₀ HEAs remain almost unchanged, showing high thermal stability. It is discussed from the synergistic of high-density secondary Cu-rich precipitates and the low-energy Nb-rich precipitate/transition precipitate/matrix and Cu-rich precipitate/matrix interfaces. From a thermodynamical point of view, the free energy of the matrix is decreased to be lower than that of the Nb-rich precipitates, changing the diffusion direction of Nb and forming a gradient distribution of Nb. It provides a convenient strategy to architect an fcc metastable transition precipitate linking Nb-rich precipitates and Cu-depleted matrix, reducing interface energy. Moreover, Cu and the coherent interfaces with low interfacial energy effectively retard the coarsening of the Nb-rich precipitates. It may serve as guidance for the future alloy design strategy of high-entropy superalloys with high thermal stability.