Electromagnetic wave absorption performance of FeCoNiMn0.5Al0.2 high entropy alloys governed by nanocrystal evolution

High entropy alloys (HEA) are promising microwave absorbers because of their multiple attenuation mechanisms on both electrical and magnetic field. Noting that microstructure is the determining factor on microwave absorption for HEA with specific composition, annealing process is performed on the as-milled FeCoNiMn0.5Al0.2 HEA. The obtained HEA powder shows flaky shape with micron-scale length and nanoscale thickness and consists of nanocrystal-amorphous structure with the grain size of 6–30 nm. Results show that altering crystallinity, grain size, lattice constant, phase composition, particle morphology, defects, and internal stress allows to control the magnetic properties, electrical properties, high-frequency electromagnetic parameters, and microwave absorption. To further clarify the absorption mechanism, relaxation polarization, conductive loss, natural resonance, eddy-current loss in 2–18 GHz are systematically discussed associated with the nanocrystal evolution. The increased crystallinity and decreased lattice defects help to increase the real permeability, and shift the natural resonance to GHz, which are great favorable to improve the impedance matching and high-frequency resonance loss. Those changes also lead to great improvement on conductivity, which lead to higher real permittivity, conductive loss, and eddy-current loss. While the drop of internal stress and amorphous phase reduce the dipole polarization and relaxation polarization. The HEA annealed at 500 °C for 0.5 h realize a desirable absorption with a maximumreflection loss of −44.425 dB over a broad effective bandwidth of 3.825 GHz.