Effect of C content on microstructure evolution and mechanical properties of CoCrFeNiTa0.1 high entropy alloy

A series of CoCrFeNiTa0.1Cx (x = 0.1, 0.2, 0.3, 0.4, 0.5) high entropy alloys (HEAs) with different C content were prepared by vacuum melting technology. The influence of different C element contents on the alloy microstructure and mechanical properties have been systematically investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and a universal testing machine system. It is found that the introduction of trace amounts of C element content facilitates the precipitation of Laves phase in C0.1 alloy. As the C element content further increases, the volume fractions of both FCC matrix and Laves phases gradually decrease, while the volume fraction of M7C3 carbide increases. Notably, the CoCrFeNiTa0.1C0.3 HEA shows a ternary eutectic structure. Compression tests reveal an increase in yield strength from 342MPa to 1475MPa and hardness from 182 HV to 458 HV as the C content increases. The precipitation of Laves phase and M7C3 carbide is principally involved in the enhancement of strength and hardness. Nanoindentation tests indicate that the M7C3 carbide has a higher nano-hardness compared to FCC matrix and Laves phase. The fracture microstructure exhibits a mode transition from ductile fracture to cleavage fracture.