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Medium- and high-entropy alloys based on the CrCoNi-system have been shown todisplay outstanding strength, tensile ductility and fracture toughness(damage-tolerance properties), especially at cryogenic temperatures. Here weexamine the JIc and (back-calculated) KJIc fracture toughness values of theface-centered cubic, equiatomic CrCoNi and CrMnFeCoNi alloys at 20 K. At flowstress values of ~1.5 GPa, crack-initiation KJIc toughnesses were found to beexceptionally high, respectively 235 and 415 MPa(square-root)m for CrMnFeCoNiand CrCoNi, with the latter displaying a crack-growth toughness Kss exceeding540 MPa(square-root)m after 2.25 mm of stable cracking, which to our knowledgeis the highest such value ever reported. Characterization of the crack-tipregions in CrCoNi by scanning electron and transmission electron microscopyreveal deformation structures at 20 K that are quite distinct from those athigher temperatures and involve heterogeneous nucleation, but restrictedgrowth, of stacking faults and fine nano-twins, together with transformation tothe hexagonal closed-packed phase. The coherent interfaces of these featurescan promote both the arrest and transmission of dislocations to generaterespectively strength and ductility which strongly contributes to sustainedstrain hardening. Indeed, we believe that these nominally single-phase,concentrated solid-solution alloys develop their fracture resistance through aprogressive synergy of deformation mechanisms, including dislocation glide,stacking-fault formation, nano-twinning and eventually in situ phasetransformation, all of which serve to extend continuous strain hardening whichsimultaneously elevates strength and ductility (by delaying plasticinstability), leading to truly exceptional resistance to fracture.

Exceptional fracture toughness of CrCoNi-based medium- and high-entropy alloys close to liquid helium temperatures