High-Strain-Rate Strength of AlCoCrFeNi_2.1 High-Entropy Alloy Under Supersonic Impacts Depending on Non-Equilibrium Eutectic Nano-Structures

The emergence of compositionally complex materials—including eutectic high-entropy alloys (EHEAs)—has revolutionized design for extreme environments, offering unprecedented property combinations. However, their mechanical response to far-from-equilibrium remains unexplored. This study investigates dynamic deformation mechanisms of AlCoCrFeNi_2.1 EHEA with distinct manufacturing routes and varying nano-lamellar spacing under extreme strain rates (10⁴-10⁸ s^-1). Using Laser-Induced Projectile Impact Test (LIPIT), we achieve controlled collisions of alumina microprojectiles to EHEA targets with impact velocities of 200-800 m s^-1. This micro-ballistic approach with femtosecond stroboscopic imaging enables precise collision kinetic measurement as impact and rebound velocities. Spectra of the coefficient of restitution reveal the EHEA's elastoplastic responses under extreme strain rate regime. Post-impact characterization exhibits multiple coupled deformation mechanisms: phase fragmentations under severe deformation, adiabatic heating effects, and strain-induced phase transformations in the impact zone. Micro-ballistic hardness measurements show strain rate sensitivity differences between processing routes and nano-lamellar spacing. This research provides critical insights for strengthening behavior and design principles of next-generation materials under extreme.