This article discusses a high-entropy alloy, CrMnFeCoNi, which exhibits exceptional fracture toughness and mechanical properties. The alloy is an equiatomic, multi-element system that crystallizes as a single phase, despite containing elements with different crystal structures. It demonstrates remarkable damage tolerance, with tensile strengths above 1 GPa and fracture toughness values exceeding 200 MPa·m¹/². Its mechanical properties improve at cryogenic temperatures due to a transition from planar-slip dislocation activity to deformation by mechanical nanotwinning, resulting in continuous steady strain hardening. The alloy is composed of five elements (Cr, Mn, Fe, Co, Ni) in equiatomic proportions and forms a single-phase face-centered cubic solid solution. It is mechanically characterized through uniaxial tensile tests and compact-tension fracture toughness tests, revealing increased yield strength, ultimate tensile strength, and ductility with decreasing temperature. The fracture toughness of the alloy remains relatively constant across a wide temperature range, with values of 217–221 MPa·m¹/² at 293 K, 200 K, and 77 K, respectively. The alloy's high fracture toughness is attributed to a 100% ductile fracture by microvoid coalescence, with microvoids and particles acting as initiation sites for void formation. The fracture surfaces show ductile dimpled fracture with Mn-rich or Cr-rich particles, indicating the role of these particles in influencing material ductility. The alloy's fracture toughness is further supported by crack initiation and growth toughness measurements, with values of 250 kJ/m² at 293 K and 221–219 kJ/m² at 200 K and 77 K, respectively. The alloy's mechanical behavior is also characterized by deformation mechanisms such as microvoid coalescence, annealing twins, and nanotwinning, which contribute to its high fracture toughness. At cryogenic temperatures, the alloy exhibits increased ductility and strain hardening, with deformation-induced nanotwinning playing a significant role. The alloy's properties are compared to other materials, including austenitic stainless steels and high-Ni steels, showing that it has a high strength and toughness, comparable to structural ceramics and close to some bulk-metallic glasses. The study concludes that the CrMnFeCoNi high-entropy alloy exhibits remarkable fracture toughness properties at tensile strengths of 730 to 1280 MPa, with fracture toughness values exceeding 200 MPa·m¹/² at crack initiation and rising to over 300 MPa·m¹/² for stable crack growth at cryogenic temperatures down to 77 K. The alloy's toughness levels are comparable to the best cryogenic steels, with outstanding combinations of strength and ductilityThis article discusses a high-entropy alloy, CrMnFeCoNi, which exhibits exceptional fracture toughness and mechanical properties. The alloy is an equiatomic, multi-element system that crystallizes as a single phase, despite containing elements with different crystal structures. It demonstrates remarkable damage tolerance, with tensile strengths above 1 GPa and fracture toughness values exceeding 200 MPa·m¹/². Its mechanical properties improve at cryogenic temperatures due to a transition from planar-slip dislocation activity to deformation by mechanical nanotwinning, resulting in continuous steady strain hardening. The alloy is composed of five elements (Cr, Mn, Fe, Co, Ni) in equiatomic proportions and forms a single-phase face-centered cubic solid solution. It is mechanically characterized through uniaxial tensile tests and compact-tension fracture toughness tests, revealing increased yield strength, ultimate tensile strength, and ductility with decreasing temperature. The fracture toughness of the alloy remains relatively constant across a wide temperature range, with values of 217–221 MPa·m¹/² at 293 K, 200 K, and 77 K, respectively. The alloy's high fracture toughness is attributed to a 100% ductile fracture by microvoid coalescence, with microvoids and particles acting as initiation sites for void formation. The fracture surfaces show ductile dimpled fracture with Mn-rich or Cr-rich particles, indicating the role of these particles in influencing material ductility. The alloy's fracture toughness is further supported by crack initiation and growth toughness measurements, with values of 250 kJ/m² at 293 K and 221–219 kJ/m² at 200 K and 77 K, respectively. The alloy's mechanical behavior is also characterized by deformation mechanisms such as microvoid coalescence, annealing twins, and nanotwinning, which contribute to its high fracture toughness. At cryogenic temperatures, the alloy exhibits increased ductility and strain hardening, with deformation-induced nanotwinning playing a significant role. The alloy's properties are compared to other materials, including austenitic stainless steels and high-Ni steels, showing that it has a high strength and toughness, comparable to structural ceramics and close to some bulk-metallic glasses. The study concludes that the CrMnFeCoNi high-entropy alloy exhibits remarkable fracture toughness properties at tensile strengths of 730 to 1280 MPa, with fracture toughness values exceeding 200 MPa·m¹/² at crack initiation and rising to over 300 MPa·m¹/² for stable crack growth at cryogenic temperatures down to 77 K. The alloy's toughness levels are comparable to the best cryogenic steels, with outstanding combinations of strength and ductility