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The CoNiCr x (x = 1, 1.4, 1.8, and 2) medium-entropy alloys were synthesized by vacuum arc-melting method to systematically investigate the roles of Cr on microstructures and mechanical properties. The results of microstructural characteristics indicated that both CoNiCr and CoNiCr 1.4 alloys have a single face-centered cubic (FCC) structure, while CoNiCr 1.8 and CoNiCr 2 alloys consist of FCC plus body-centered cubic (BCC) hypoeutectic and eutectic. The formation of FCC and BCC phases could be probably evaluated using the VEC criteria. The tested mechanical properties demonstrated that CoNiCr 1.8 alloy exhibited the best comprehensive properties at room and liquid nitrogen temperatures (RT and LNT), whereas the yield strength and hardness of CoNiCr 2 alloy were higher than those of other alloys, which may be mainly attributed to the second-phase strengthening and the fine and lamellar microstructure. The yield strength of CoNiCr 2 alloy at RT and LNT was 3.8 and 1.9 times (514 MPa and 825 MPa) that of CoNiCr alloy. In addition, the precipitation of dense and ordered FCC (L1 2) phase could primarily contribute to the enhance of yield strength of CoNiCr 1.4 alloy. • Cr addition facilitates the formation of L1 2 nano-particles and BCC/B2 phase. • The CoNiCr 1.8 hypoeutectic medium-entropy demonstrates an excellent strength and ductility synergy at liquid nitrogen temperature. • A CoNiCr 2 lamellar eutectic medium-entropy with FCC/L1 2 and BCC/B2 is designed. • L-C lockers dominate the deformation of lamellar eutectic MEAs under cryogenic condition. [ABSTRACT FROM AUTHOR]

A novel κʹ-strengthened FCC-based Al 0.5 CoFeNiC 0.1 high entropy superalloy (HESA) with minor B2 phase was prepared by arc melting. The FCC/κʹ dual-phase microstructure with thermal stability is similar to γ/γʹ microstructure of Ni-based superalloys. Surprisedly, the large volume fraction (nearly 50%), high density and nano-scale coherent κʹ particles with uniform distribution are easily accessible only by direct solidification, which gives rise to superior compressive yield strengths at both room and elevated temperatures, reaching 1130 MPa at 25 °C and 830 MPa at 700 °C, respectively. The sustained high-temperature yield strength is contributed to both the antiphase boundaries (APBs) and stacking faults (SFs) shearing κʹ particles. Further, optics and electron microscopy analyses revealed the changes (transition point about at 0.6 T m , where T m is melting temperature) of the dominant deformation and failure mechanisms, dependent on the temperature. The dominant deformation mode is planar slip and SFs at relatively low temperatures, while the cross slip and dislocation dissociation above 800 °C (0.6 T m), coupled with the diffusion-controlled recovery mechanisms. The failure mechanism shows failure mode from shearing fracture to thermal softening. Our κʹ-strengthened FCC-based superalloy provides a route for elevated-temperature applications. [ABSTRACT FROM AUTHOR]