Dynamically reversible shear transformations in a CrMnFeCoNi high-entropy alloy at cryogenic temperature

Shear transformation, such as twinning and martensitic phase transformation, is generally unidirectional under monotonic thermal or mechanical loading. Here, we report the dynamically reversible shear transformations in a CrMnFeCoNi high-entropy alloy (HEA) under uniaxial tension at the extremely low temperature of 4.2 K. Since stacking fault energy (SFE) of CrMnFeCoNi HEA with a face-centered cubic (fcc) structure is low and decreases with decreasing temperature, plastic deformation is accommodated by dislocation slips and shear transformation bands, such as {111} stacking faults (SFs), {111} nano-twins and fcc → hcp (hexagonal close-packed structure) shear transformation bands. When deformed at 4.2 K, the lower SFE promotes fcc → hcp shear transformation, forming hcp grains. Besides basal and non-basal dislocation slips in hcp grains, high-density {0001} SFs and {1 0 1 ¯ 1} nano-twinning are activated to accommodate plastic deformation. More intriguingly, reverse hcp → fcc shear transformations are stimulated by deformation-induced local dissipative heating. The reversible fcc ↔ hcp shear transformations and both {1 0 1 ¯ 1} and {111} nano-twinning lead to dynamic nano-laminated dual-phase (NL-DP) structures, which advances the monotonic “dynamic Hall-Petch” effect in enhancing strength, strain-hardening ability, and ductility by dynamically tailoring the type and width of shear transformation bands. Our work provides a promising strategy for evading the strength-ductility dilemma via dynamically developing NL-DP structures through activating reversible shear transformations.