Design and development of CoCrFeNiVx EHEAs for structural applications

Abstract

Single solid solution alloys with face centered cubic (FCC) crystal structure have good ductility but lower strength, while body centered cubic (BCC) and intermetallic alloys have higher strength but lower ductility. High entropy alloys (HEAs) can achieve a better balance of strength and ductility by combining multiple principal elements. The CoCrFeNiVx eutectic HEA system was selected to further improve this balance. The addition of V and its effects were studied using calculated phase diagrams. Seven compositions (x=10, 20, 23.2, 25, 30, 35, 40) were produced by vacuum arc melting. Phases were identified by XRD, microstructures by optical and SEM. Mechanical properties were evaluated by compression and hardness testing. Heat treatment was performed to further improve properties. With increasing V, the microstructure transitioned from single FCC phase to eutectic mixture of FCC + brittle σ phases (x=10, 20, 23.2, 25) and finally to solely σ phase (x=30, 35, 40). CoCrFeNiV10 had maximum ductility while CoCrFeNiV20 showed good strength-ductility balance. CoCrFeNiV23.2 exhibited highest yield strength and ductility due to its ultrafine eutectic structure. Heat treated CoCrFeNiV23.2 maintained maximum yield strength with increased ductility. CoCrFeNiV25 and higher V compositions showed brittle behavior due to the σ phase. In summary, careful microstructural design and V additions in the CoCrFeNiVx eutectic HEA system allowed tuning of strength-ductility balance. Ultrafine eutectic structure and optimal V concentration achieved a synergistic combination of high strength and good ductility.