Synergistic Dynamic Phase Transformation and Multistage Structure Enabling Ultra‐Broadband Electromagnetic Wave Attenuation in High Entropy Alloy

Abstract Developing high entropy alloy (HEA) based electromagnetic wave absorption (EMWA) materials with ultra‐broadband remains challenging, primarily due to the inherent trade‐off between impedance matching and attenuation efficiency. Herein, Co 4 Fe 2 AlMn (CFAM) HEA flake‐like powders are fabricated through induction melting, melt spinning, and high‐energy ball milling, followed by annealing. The ball milling induces severe plastic deformation, forming micron‐scale flakes with surface‐stacked nanoflakes and lattice distortions, significantly enhances polarization effects and synergistic multiple scattering and reflection for electromagnetic waves. Subsequent annealing triggers dynamic phase evolution, including BCC/B2 to DO 3 transitions and nano octahedral MnO precipitation, creating heterogeneous interfaces for optimized impedance matching. The synergistic effects of multistage structure, defect polarization, and various magnetic loss mechanisms endow the CFAM60‐600 with excellent EMWA performance, a minimum reflection loss ( RL min ) of −71.34 dB at 1.51 mm, and CFAM48‐400 achieves an effective absorption bandwidth (EAB) of 8.07 GHz at 1.67 mm. The stepped structure achieves an EAB of 15.92 GHz, and the radar scattering cross‐section simulation demonstrates a reduction exceeding −10 dB at multiple angles. This work demonstrates the synergy of dynamic phase and multistage structure engineering of CFAM, advancing the development of HEA‐based EMWA materials with practical prospects.