Electromagnetic Wave Absorbing Performance Of FeCoNiSiAl Series Of High Entropy Alloys

With the rapid development of 5G communication and aviation radar technology,electromagnetic(EM)pollution and national defense security issues have become increasingly prominent.As wave-absorbing materials can fundamentally eliminate the harm of EM waves,they have become the current research hotspot.The traditional alloy-type EM wave absorbents have higher conductivity,the dielectric constant increases obviously after flaking,and the impedance mismatch reduces the absorbing property of the material.Due to the large number of constituent elements of high entropy alloys(HEA),lattice distortion leads to the decrease of conductivity and the increase of skin depth,which makes the HEA absorbent easier to absorb EM waves.In addition,HEAs are prone to generate nanocrystalline structure,which makes the improvement of absorbent performance not limited in changing the aspect ratio of alloy particles,but the regulation of microstructure can make it obtain high permeability and stronger absorption in lower frequency band.At the same time,the anti-corrosion and anti-oxidation properties of HEA make it possible to be used as a new generation of EM wave absorbent under harsh environmental conditions.In this paper,FeCoNiSiAl high entropy alloy powders(HEAP)are prepared by mechanical alloying method and as-cast milling method(dry/wet milling),and the effects of alloy composition and microstructure on EM absorbing properties are investigated.Firstly,the effects of Si element on EM absorption performance are clarified through the change of alloy composition so as to determine the optimal ratio.Secondly,the effects of aspect ratio and element uniformity on the magnetic and EM properties of HEAPs are explored through the exploration of the preparation process.Then,HEAP particles with nano-crystalline and nano-amorphous microstructure and large aspect ratio are prepared by as-cast dry-milling method,realizing the strong absorption in low frequency band.Finally,the mechanism for the magnetic transformation of Mn in HEA system and the effect of Mn addition on EM properties are explored.Main research contents and conclusions are as follows:(1)FeCoNiSixAl0.4 HEAPs are prepared by mechanical alloying method,and the alloy powders have a large amount of FCC phase and a small amount of BCC phase.After annealing at 673 K,more BCC phase and CoFe2O4 phase are precipitated.The Ms of FeCoNiSi0.4Al0.4 increases from 95.7 emu/g to 114.1 emu/g after annealing,while the coercive force(Hc)decreases from 142 Oe to 92.7 Oe.The EM parameters of the alloy powder after milling show a trend of increasing first and then decreasing with the increase of Si content.When Si content is 0.4,the EM wave absorption intensity reaches-36.44 dB(11.84 GHz),and the absorption peak intensity of other samples improved significantly after annealing.The optimal absorption peaks of the two HEAs before and after annealing are located in the X and Ku bands.(2)FeCoNiSi0.4Al0.4 HEAPs are prepared by mechanical alloying method and as-cast wet-milling method.Both techniques have different phase formation rules,and the mechanical alloying powder and the as-cast wet-milling powder have more FCC and BCC phase,respectively.By comparison,the as-cast wet-milling powder have larger aspect ratio,better elemental distribution uniformity and fewer defects,which leads to smaller Ms(85.8-92.1 emu/g),larger Hc(111.1-173.1 Oe),and larger EM parameters of ?' up to 1.90(2 GHz).The strongest absorption peaks are all located in the C-band with low frequency,and the absorption intensity is up to-41.93 dB at 5.30 GHz.(3)FeCoNiSixAl0.4 HEAPs with dual-phase nano-crystalline and nano-amorphous microstructures and large aspect ratio are prepared by as-cast dry-milling method,and the crystallinity could be regulated by controlling the FCC phase ratio of the milling precursor(alloy ribbon).When x<0.3(FCC phase ratio is greater than 40%),the crystallinity of HEAPs are inversely proportional to the FCC phase ratio of the alloy ribbon.When x>0.3,they are proportional.The FCC phase ratio of FeCoNiSi0.4Al0.4 alloy ribbon increase with the extension of annealing time,and HEAPs with adjustable crystallinity of the same composition can be prepared by the same technology.In the dry-milling process,multiple phase transition effects coexist.BCC phase amorphization,FCC phase amorphization,and amorphous crystallization lead to the formation of nano-crystalline and nano-amorphous microstructure.A large aspect ratio(up to 1000),a small Hc(14.05 Oe-23.43 Oe),and a large ?'(2.41-2.80,1 GHz)can be obtained,and the EM wave absorption intensity reach-17.43 dB at the S-band(2.53 GHz)with a lower frequency.(4)FeCoNiSi0.4Al0.4Mnx HEAs are prepared by induction melting.When Si0.4/Al0.4 exists alone,the addition of Mn reduces the Ms of the alloy,while Ms increases when Si0.4Al0.4 co-exists,with a maximum Ms increment of 25.79 emu/g.DFT calculation based on EMTO system show that the synergistic effect between SiAl elements induce the magnetic transformation of Mn from antiferromagnetism to ferromagnetism,and the interaction between Fe-Mn,Co-Mn and Ni-Mn are changed from antiferromagnetic coupling to ferromagnetic coupling.Adding Mn0.4 into FeCoNiSi0.4Al0.4 alloy system can increase the absorption peak intensity from-13.01 dB(3.72 GHz)to-22.83 dB(3.55 GHz),thus achieving strong absorption in S-band.