The lattice friction stress driven temperature-dependent tensile deformation behaviors of CoNiCr2 eutectic medium-entropy alloy

A heterogeneous CoNiCr₂ eutectic medium-entropy alloy (EMEA), comprising soft face-centered cubic (FCC) and hard body-centered cubic (BCC) lamellae, associated with minor acicular hexagonal close-packed (HCP) phase precipitated in BCC phase, was synthesized towards excellent tensile strength and ductility synergy. The tensile mechanical properties demonstrated that this alloy was temperature-dependent, i.e., when the testing temperature reduced from room temperature (RT) to liquid nitrogen temperature (LNT), the yield strength, ultimate strength, and uniform elongation were enhanced from 449 MPa, 821 MPa, and 5.0% to 702 MPa, 1174 MPa, and 8.4%, respectively. The prominent elevation of yield strength at LNT mainly resulted from the dramatically enhanced lattice friction stress (σ₀) and the FCC-BCC interfacial strengthening, while the improved ductility was attributed to the superior crack-arrest capability of FCC matrix stemmed from the accumulation of stacking faults (SFs) and enhanced σ₀ at LNT. Additionally, although the deformation mechanisms were dominated by planar dislocation glides and SFs at both temperatures, the initiation of premature cracks in the BCC phase due to the inferior deformation capability at LNT constrained the better strength-ductility trade-off. The cracks in the BCC phase tended to propagate along the BCC-HCP interfaces because of the strain incompatibility. Furthermore, the sub-nanoscale L1₂ particles in the FCC matrix could not only strengthen this alloy but also improve the stacking fault energy leading to no deformation twinning even at LNT. This work may provide a guide for the design of remarkable strength and ductility synergy EMEAs combined with outstanding castability for applications at cryogenic temperatures.