Effect of Ti on Microstructure and Mechanical Properties of CoFeNiVTi x High-Entropy Alloys

A series of CoFeNiVTi x ( x = 0, 0.25, 0.5, 0.75) high-entropy alloys (HEAs) were prepared by vacuum arc melting, based on which the influence of Ti dopant on the microstructure and mechanical properties of the CoFeNiV-based alloy was intensively analyzed. The results show that the phase of the CoFeNiVTi x HEAs evolved from single-FCC to FCC + BCC, BCC + (Ni, Co) 3 Ti, and finally to single-BCC phase with increasing Ti content. The high mixing entropy facilitates the formation of solid solutions in the HEAs with x = 0, 0.25, and 0.75, while the specific atomic ratio of Ti and Ni + Co in the x = 0.5 HEA seemingly breaks the high mixing entropy effect and promotes the formation of (Ni, Co) 3 Ti intermetallic phase. The microstructures of the CoFeNiVTi x HEAs experienced a columnar-to-equiaxed transition (CET), during which the formation of vast needle-shaped precipitates in the CoFeNiVTi 0.5 alloy as heterogeneous nucleation sites further promoted the grain refinement and CET. Additionally, the (Ni, Co) 3 Ti phase in the BCC matrix significantly improved the strength at the expense of the plasticity of the CoFeNiVTi 0.5 alloy due to its brittle essence. Comparatively, the symbiotic soft FCC dendrites and hard BCC interdendrites in the CoFeNiVTi 0.25 alloy achieved an excellent strength and plasticity balance: a high yield strength of 738 MPa and a great plastic strain more than 40%.