Crystal structure, Curie temperature, and electromagnetic absorption properties in Fe_xCo₃₀Ni_60−xSi₅Al₅ (x = 30, 35, 40, 45) high entropy alloys

Abstract In order to meet the requirements of future national defense for high temperature electromagnetic (EM) absorbing performance, a series of Fe_xCo₃₀Ni_60−xSi₅Al₅ (x = 30, 35, 40, 45) high-entropy alloys (HEAs) powders was prepared and their Curie temperatures (T_C) were measured by a self-made Wheatstone bridge. According to the results, varying the Fe/Ni ratio affected the crystal structure, Curie temperature, oxidation resistance, and electromagnetic absorbing properties of the above compounds. Since Fe has a BCC structure and is thus easier to form the solid solutions with Si and Al, the crystal structure of the alloy has changed from FCC toward BCC with increasing Fe dopant content. In turn, the Curie temperature (T_C) decreased from 473.68 °C to 358.07 °C, being lower than their initial oxidation temperature (>800 °C). The reflection losses (RL) of powders at room temperature and high temperatures (≤500 °C) were calculated as well. It was found that the flake powders after ball milling gained a larger aspect ratio, resulting in the better absorption effect, which was due to high toughness and low strength characteristics of the initial FCC structure. Furthermore, the permittivity and permeability of alloys upon heating reached impedance matching at a certain temperature, thus achieving the greater RL_max value. Finally, the high-temperature EM absorption characteristics of HEAs were shown to merit a thorough study.