Tuning dielectric response and relaxation processes in phosphate-coated AlCoCrFeNi high-entropy alloy/MgO composites for efficient microwave absorption

High-entropy alloys (HEAs) are promising microwave absorbers; however, their high electrical conductivity often causes severe impedance mismatch, limiting broadband absorption. Here, we propose a dual-modification strategy combining phosphoric-acid passivation and MgO doping (1–5 wt%), to regulate the dielectric response of AlCoCrFeNi HEAs. The phosphate layer suppresses conduction-dominated loss, while MgO introduces heterogeneous interfaces to enhance polarization relaxation. The optimized composite with 3 wt% MgO delivers a minimum reflection loss of −37.64 dB and a maximum effective absorption bandwidth of 6.08 GHz in 2–18 GHz, corresponding to a ∼3.5 × bandwidth improvement over pristine HEA. This work offers a facile and scalable dielectric-regulation route to overcome impedance mismatch in metallic HEA-based broadband absorbers.