Hierarchical Ordering Induced Ultrahigh Cryogenic Strength and Strain Hardening in a Ni₂CoFeV Medium-Entropy Alloy

Conventional metallic materials for cryogenic engineering are typically single-phase face-centered cubic (fcc) alloys with limited yield strength, whereas ordered phases enhance strength at the expense of ductility, in some cases even causing a ductile-to-brittle transition. Here, we overcome this long-standing limitation by designing a non-equiatomic fcc Ni₂CoFeV medium-entropy alloy featuring hierarchically ordered, intragranular κ and L1₂ intermetallic phases. The resulting tri-phase alloy achieves a high yield strength of 1.4 GPa at 77 K, enabled by severe lattice distortion from V enrichment together with synergistic strengthening from coherent phase interfaces. Notably, exceptional strain hardening, characterized by an ultrahigh strain hardening rate of ∼7 GPa and a large exponent of 0.85, drives a tensile strength exceeding 2 GPa while retaining a high ductility of 28% at 77 K, surpassing that of most reported fcc-based high-/medium-entropy alloys. The superior cryogenic performance arises from the sequential activation of multiple deformation mechanisms, involving high-density superlattice dislocations and stacking faults in the coupled fcc/L1₂ phases and dislocation multiple slip in the κ phase. This work establishes hierarchical intermetallic ordering as an effective paradigm for cryogenic materials design.