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Face centered cubic (FCC) high-entropy alloys (HEA) exhibit excellent ductility while body centered cubic (BCC) HEAs are characterized by high strength. Development of fine two-phase eutectic microstructure (consisting of a tough phase such as fcc and a hard phase such as bcc/intermetallic) can help in obtaining an extraordinary combination of strength and ductility in HEAs. Designing eutectic high entropy alloys is an extremely difficult task for which different empirical and non-empirical methods have been previously tried. In the present study, the possibility of developing a eutectic microstructure by the addition of Mo to CoCrFeNi was evaluated by calculation of the pseudo-binary phase diagram. Experimental results validated the presence of eutectic reaction in the calculated phase diagrams; however, small changes in the calculated phase diagrams were proposed. It has been shown that calculated pseudo-binary phase diagrams can provide a very good starting point for the development of eutectic HEAs and help in exponentially reducing the amount of experimental effort that may be required otherwise. Eutectic mixture consisting of FCC (A2) phase and intermetallic phases (σ and μ) was successfully obtained by the addition of Mo to the CoCrFeNi system. The development of the eutectic microstructure showed a profound effect on the mechanical properties. Hardness of the samples increased from 150 HV for CoCrFeNiMo0.1 to 425.5 HV for CoCrFeNiMo1.0, whereas yield strength increased from around 218 MPa for CoCrFeNiMo0.1 to around 1100 MPa for CoCrFeNiMo1.0.

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