Temperature-dependent microstructural evolutions and deformation mechanisms of (Ni₂Co₂FeCr)₉₂Al₄Nb₄ high-entropy alloys

Precipitation-strengthened high-entropy alloys (HEAs) with tailored phase structures are expected to possess excellent strength-ductility combinations for advanced structural applications. In this work, we systematically studied the microstructural evolutions and mechanical behaviors of (Ni₂Co₂FeCr)₉₂Al₄Nb₄ HEAs at different aging temperatures (700–900 ℃). It was revealed that only spherical L1₂ phase existed in the specimen peak-aged at 700 °C without other precipitates, while in the specimen peak-aged at 800 °C, irregular precipitation of ε phase appeared on the grain boundaries in addition to the intragranular L1₂ phase. In strong contrast, upon peak aging at 900 ℃. the fine interleaving Widmanstätten-type ε phase with an ordered hexagonal structure (D0₁₉) became the dominated precipitate. We further revealed that the coherent L1₂ phase provided a more effective strengthening effect than that of the ε phase, leading to the highest yield strength of ~950 MPa in the alloy peak-aged at 700 °C. The stacking faults shearing of L1₂ phase and the planar dislocations shearing of ε phase were identified as the main deformation mechanisms of the specimens peak-aged at 700 and 900 °C, respectively. Interestingly, the ε phase was determined to have certain plastic deformability, enabling the 900 °C peak-aged alloy to exhibit excellent tensile ductility. These findings would provide valuable guidelines for the design of precipitation-strengthened HEAs with optimized microstructures and superior mechanical properties.