Effects of Annealing on the Atomic-Scale Structures and Mechanical Properties
in Single Crystals of the Equiatomic Cr-Co-Ni Medium-Entropy Alloy

The equiatomic Cr-Co-Ni medium-entropy alloy exhibits many extraordinary mechanical properties, such as excellent combination of strength and ductility that increase at cryogenic temperatures and exceptional fracture toughness. The local chemical inhomogeneity derived from preferred bonding between specific atoms, usually termed short-range ordering (SRO), is widely believed to be one of the reasons for its peculiar properties. The effect of SRO on the atomic-scale structures have been investigated extensively in theoretical calculations. In experimental work, however, 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. In the present study, we investigate the existence of SRO and its possible effects in the Cr-Co-Ni single crystals from various aspects, including mechanical properties, dislocation dissociation width variation, electron and X-ray diffraction patterns. 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. Compression tests were conducted at room temperature. 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.