Ultrahigh Strength with Suppressed Flow Instability at Liquid Helium Temperature via Coherent Nanoprecipitation in a Medium‐Entropy Alloy

Abstract Metallic materials for aerospace and liquid hydrogen technologies need to maintain high strength and ductility under cryogenic conditions. However, conventional strengthening strategies typically increase defect density and promote strain localization, resulting in a strength–ductility trade‐off. This limitation becomes more critical at ultralow temperatures, where it facilitates discontinuous plastic flow and abrupt stress drops, substantially increasing the risk of premature failure. Here, a Co 36 Ni 46 Mo 11 Al 7 medium‐entropy is developed, exhibiting an exceptional combination of tensile strength (2.1 GPa), high ductility (48%), and remarkably low stress drops of ≈99 MPa at 4.2 K. This balance is enabled by two key mechanisms: enhanced lattice friction through compositional tuning and the introduction of coherent L1 2 nanoprecipitates. These features effectively impede dislocation motion while promoting Hirth lock formation, thereby suppressing strain localization. Crucially, cryogenic loading–unloading–reloading tests, rarely performed at 4.2 K, reveal low back stress, directly indicating minimal dislocation accumulation despite the high strength. The findings highlight how dislocation–precipitate interactions can decouple strength from back stress accumulation, enabling a rare combination of ultrahigh strength and suppressed discontinuous plastic flow. This approach establishes a robust alloy design strategy for overcoming the long‐standing conflict between strength, ductility, and mechanical stability in cryogenic environments.