The effects of niobium (Nb) addition on the Suzuki segregation and high-temperature creep properties in Co-Ni-Cr-Mo-Nb medium-high-entropy alloys were systematically investigated. Tensile and creep tests were conducted at 700 °C under applied stresses of 200 and 400 MPa to evaluate the mechanical properties and deformation behavior at elevated temperatures. The results demonstrated that increasing the Nb content from 1 to 2 wt% significantly enhanced the creep resistance and improved the yield and ultimate tensile strengths, while maintaining a stable elongation of approximately 40 %. Transmission electron microscopy (TEM) observations confirmed that higher Nb content promoted pronounced Suzuki segregation and the formation of d-phase precipitates along stacking faults, effectively reducing the stacking fault energy (SFE) and stabilizing extended stacking faults. This phenomenon led to a substantial suppression of dislocation motion, resulting in an enhanced high-temperature creep strength. Although the calculated configurational entropy (∼11.49 J/(mol·K)) categorizes the alloy technically as a medium-entropy alloy (MEA), the observed microstructural stability, deformation mechanisms, and mechanical properties closely resembled those of high-entropy alloys (HEAs). These findings indicate that the optimal Nb addition provides a significant strengthening effect through Suzuki segregation and precipitation mechanisms, making Co-Ni-Cr-Mo-Nb alloys promising as candidates for advanced structural materials suitable for high-temperature applications in aerospace, energy production, and other demanding environments. • Nb addition enhances creep resistance via Suzuki segregation and δ-phase precipitation. • 2 wt% Nb significantly reduces stacking fault energy and stabilizes fault structures. • Extended stacking faults and δ-phase particles suppress dislocation motion. • The alloy exhibits high strength and ductility at 700 °C across all strain rates. • The microstructure shows characteristics similar to high-entropy alloys (HEAs).
Wang et al. (Wed,) studied this question.