Study on the enhancement of strength-ductility of CoCrFeNiMn_0.5 high-entropy alloys by cryogenic-temperature induced nano-twinning and 9R phase formation

The present study systematically investigated the microstructure and mechanical properties of cold-rolled and annealed CoCrFeNiMn_0.5 high-entropy alloys (HEAs) during tensile deformation at cryogenic temperatures (∼123 K). The results showed that after annealing at 1273 K for 1 h, the strength and ductility and work-hardening capacity of the cold-rolled alloys (CR98) at 123 K were significantly improved. The yield strength (R_p0.2), tensile strength (R_m) and elongation at break (A) reached 325 ± 21.8 MPa, 1015 ± 23.5 MPa, and 88 ± 8.1 %, respectively. After cold-rolled and annealed treatment, the strengthening mechanisms of the alloys changed from plastic deformation dominated by dislocation slip at room temperature to the synergistic deformation of dislocation slip and stacking faults dominated by nano-twinning and the 9 R phase at cryogenic temperatures, thereby significantly enhancing the alloy's work-hardening capacity and strength-ductility. At 123 K, the stacking fault energy (SFE) of the alloys decreased, leading to the formation of the 9 R phase through the dissociation of incoherent twin boundaries (ITBs) in nano-twinning. At cryogenic temperatures, Lomer-Cottrell (L-C) locks were formed inside the alloys, which pinned and stored dislocations, thereby enhancing the alloy's strength-ductility. During cryogenic-temperature tensile deformation at 123 K, the fracture surface of the annealed alloys showed typical ductile fracture, and the distribution density of dimples was greater than that at room temperature because of the increase in dislocation slip resistance and the reduction of dislocation proliferation and cross-slip at cryogenic temperatures. In brief, the prepared CoCrFeNiMn_0.5 HEAs exhibit excellent cryogenic-temperature mechanical properties and can be applied in aerospace vehicles, deep-sea exploration, and specialized equipment operating under demanding conditions.