High impact toughness of CrCoNi medium-entropy alloy at liquid-helium temperature

We show that face-centered-cubic CrCoNi medium-entropy alloy sustains a Charpy V-notch impact toughness (A_K) as high as 340 J at 4 K. Deformation twinning (DT) is rendered exceptionally profuse by this alloy's low stacking fault energy, combined with liquid-helium temperature and high strain rates that both incur high stresses to favor DT across a large plastic zone. This efficiently dissipates imposed mechanical energy, accumulates defects to sustain high strain hardening, and restrains strain localization events from evolving into major shear bands. Martensitic transformation to hexagonal phase is insignificant. CrCoNi is therefore a strong competitor to the best existing cryogenic alloys.