Effects of Hf on the microstructure and mechanical properties of CoCrFeNi high entropy alloy

In this study, the effects of hafnium addition on the microstructure and mechanical properties of CoCrFeNi high entropy alloy were investigated. The CoCrFeNiHf_x (x = 0, 0.1, 0.2, 0.3 and 0.4, x values in molar ratio) high entropy alloys were prepared by vacuum arc melting. The microstructure and phase identities were studied by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. With increasing Hf contents, the structure of CoCrFeNiHf_x HEAs changed from FCC single phase to Ni₇Hf₂ + FCC, then to Ni₇Hf₂ + C15 Laves + FCC, and finally to C15 Laves + FCC phases when x values changed from 0 to 0.4. Hypoeutectic microstructures were observed in CoCrFeNiHf_x (x = 0.1–0.3) HEAs. Fully eutectic structure with lamellaes of FCC phase and C15 Laves phase formed in CoCrFeNiHf_0.4 HEA. The Vickers hardness and compressive yield strength of the alloys increased while the plastic strain reduced with increasing Hf contents, which were resulted from solid solution strengthening and second phase strengthening. The CoCrFeNiHf_0.1 showed a better combination of strength and ductility in tensile tests. Lateral fracture surfaces indicated that the primary FCC phase in CoCrFeNiHf_0.2 could effectively hinder the propagation of micro-cracks to prevent catastrophic failure during compression. The results of this study can provide a better understanding about the phase development in Hf-containing HEAs and its effects on the mechanical properties of such systems.