  Damage-tolerance can be an elusive characteristic of structural materials
requiring both high strength and ductility, properties that are often mutually
exclusive. High-entropy alloys are of interest in this regard. Specifically,
the single-phase CrMnFeCoNi alloy displays tensile strength levels of ~1 GPa,
excellent ductility (~60-70%) and exceptional fracture toughness (KJIc > 200
MPa/m). Here, through the use of in-situ straining in an aberration-corrected
transmission electron microscope, we report on the salient atomistic to
micro-scale mechanisms underlying the origin of these properties. We identify a
synergy of multiple deformation mechanisms, rarely achieved in metallic alloys,
which generates high strength, work hardening and ductility, including the easy
motion of Shockley partials, their interactions to form stacking-fault
parallelepipeds, and arrest at planar-slip bands of undissociated dislocations.
We further show that crack propagation is impeded by twinned, nano-scale
bridges that form between the near-tip crack faces and delay fracture by
shielding the crack tip.
