Effects of Nb additions on the microstructure and mechanical property of CoCrFeNi high-entropy alloys

• The microstructure evolution of the CoCrFeNi HEA with additions of Nb is characterized.
• All the current criteria for predicting the phase formation in HEA failed to depict the Nb-containing alloy system.
• The strengthening mechanism of the Nb-containing HEAs is clarified.

A series of five-component CoCrFeNiNbx high entropy alloys (HEAs) were synthesized to investigate alloying effects of the large atom Nb on the structure and tensile properties. Microstructures of these alloys were examined using scanning electron microscopy and the phase evolution was characterized and compared using the ΔHmix–δ and ΔX criteria. It was found that the microstructure changes from the initial single face-centered cubic (FCC) to duplex FCC plus hexagonal close-packed (HCP) structure with additions of Nb. The current alloy system exhibits a hypoeutectic structure and the volume fraction of the Nb-enriched Laves phase with the HCP structure increases with increasing the Nb content, which is mainly responsible for the increment in the yield and fracture strength. Particularly, the Nb0.155 alloy containing a 9.3% Nb-enriched Laves phase exhibits the most promising mechanical properties with the yield strength and plastic strain as high as 321 MPa and 21.3%, respectively. The ΔHmix–δ criteria well describe the phase selection for the thermally treated alloys, while the physical parameter ΔX fails to predict the appearance of the Nb-enriched Laves phase in this alloy system.