Transformation induced plasticity and hetero-deformation induced strengthening enable superior strength-ductility combination of a novel Ni-Mn-Ti-Fe-Co eutectic high entropy alloy

Achieving an optimal balance between ductility and strength in high-entropy alloys (HEAs) remains a significant challenge. In this study, a series of eutectic high-entropy alloys (EHEAs) with the nominal compositions of Ni35Mn25Ti18-xFe12+xCo10 (x = 0, 2, 3, 4, 6; at%) were prepared by arc melting under an argon atmosphere. All alloys exhibit a heterogeneous microstructure composed of FCC and B2 dual-phase structures, which undergo sequential phase evolution: from dual-phase to hypoeutectic, followed by near-eutectic, and ultimately hypereutectic with increasing Fe content. Notably, the Ni35Mn25Ti15Fe15Co10 alloy exhibits a nearly complete lamellar eutectic structure, achieving an exceptional combination of strength and ductility. All alloys exhibit a yield strength exceeding 950 MPa. In particular, the hypereutectic alloy maintains a yield strength of 990 MPa while exhibiting an excellent fracture strength of 2417 MPa and a fracture strain of 30.2%. The superior mechanical properties of these alloys can be attributed to transformation-induced plasticity (TRIP) via B2 → L1₀ martensitic transformation during deformation, as well as to hetero-deformation induced (HDI) strengthening arising from interactions between the soft and hard phases. These findings provide critical insights for designing HEAs with optimized strength-ductility synergy.