On the Origin of the Superior Mechanical Properties of the Equiatomic Cr-Co-Ni Medium-Entropy Alloy

High- and medium-entropy alloys (HEAs/MEAs) have attracted extensive interest for their extraordinary and promising mechanical performance, such as excellent combination of strength and ductility that increase at cryogenic temperatures and exceptional fracture toughness. Systematic work has revealed that the equiatomic Cr-Co-Ni MEA exhibits remarkably higher strength and ductility than the Cr-Mn-Fe-Co-Ni and other subsets. The improved strengths have motivated intensive detailed research to explore their micro-scale origins via experimental approaches and theoretical calculations. These results have implied that the type and bonding of atoms, instead of number of elements, are more important for solid solution hardening in HEAs. The high strength is believed to be related to the severe and ubiquitous crystal lattice distortion, such as the mean-square atomic distortion (MSAD) model. The excellent tensile ductility of the Cr-Co-Ni MEA is generally attributed to the high propensity of deformation twinning resulting from its low stacking fault energy. Thin layers of the hexagonal close packed (HCP) structure are formed in association with deformation twins in the Cr-Co-Ni MEA. This indicates that not only TWIP (twinning induced plasticity) but also TRIP (transformation induced plasticity) are responsible for the excellent tensile ductility of the Cr-Co-Ni MEA as in the case of high-Mn steels. On the other hand, the local chemical inhomogeneity derived from preferred bonding between specific atoms, usually termed short-range ordering (SRO), is widely believed to be another reason for its peculiar properties. The effect of SRO on the atomic-scale structures have been investigated extensively in theoretical calculations. In experimental work, a few studies have indicated that the formation of SRO can act as additional obstacle to dislocation motion, while no measurable effect of SRO on strength are claimed in other studies. However, it is well known that in polycrystals grain size and boundaries induce scattered results of mechanical response. Therefore, single crystals are of prime important to investigate the critical resolved shear stress (CRSS), stacking fault energy (SFE) and twinning shear stress as well as the possible effects of SRO on these parameters. In the present study, we investigate the plastic deformation behavior of bulk single crystals of the equiatomic Cr-Co-Ni after various heat treatment, in order to experimentally deduce materials parameters (such as CRSS for slip, twinning shear stress as well as SFE) and the contribution of SRO in Cr-Co-Ni MEA. Single crystals of Cr-Co-Ni were grown from a polycrystalline rod with an optical floating-zone furnace. The single crystals were annealed at 1473 K for 168 hours as the beginning materials. Then, the specimens were further annealed at 573-1373 K for various duration to introduce SRO. Tensile and Compressive tests were conducted at room and liquid nitrogen temperatures. A careful tilting observation along various zones were conducted to get the diffraction information by transmission electron microscopy, and the results were compared with those obtained by theoretical simulation for atomic structures with various degrees of SRO. Synchrotron X-ray diffraction was also applied to characterize the atomic-scale structures.