Remarkable strength of a non-equiatomic Co29Cr29Fe29Ni12.5W0.5 high-entropy alloy at cryogenic temperatures

A non-equiatomic Co29Cr29Fe29Ni12.5W0.5 high-entropy alloy (HEA) strengthened by two-step rolling was fabricated, and its microstructure evolution and tensile behavior at cryogenic temperatures were investigated. When the temperature decreases from 293 K to 173 K, the yield strength increases from 640 MPa to 1017 MPa. At 77 K, an outstanding strength-ductility synergy can be observed, with a yield strength of 1.33 GPa and an excellent ductility of 46%. Prior to tensile testing, the annealed alloy largely has a single face-centered cubic (FCC) structure, while a hexagonal-close packed (HCP) phase is formed in the cryogenically tensile-fractured alloy along the {111} planes. Such high yield strength and tensile plasticity values at cryogenic temperatures are extremely rare in HEAs and even in metal alloys. The deformation micro-mechanism was carefully investigated by a transmission electron microscope, and the results indicated that the cryogenic-temperature properties could be attributed to stacking faults (SFs) and the phase having a hexagonal-close-packed (HCP) structure. The densities of the SFs and the HCP laths have a considerable influence on the work-hardening behavior.