Magnetic Properties and Microstructure of FeCoNiM_x(M=AlCu,AlMn,AlSi and MnSi) High-Entropy Alloys

Author:Li Zhong

Supervisor:xu zuo


Degree Year:2019





As new type of alloys,high-entropy alloys(HEAs)have broken the traditional alloy design concept,which based on one principal elements.In recent years,high entropy alloys have attracted much attention due to their unique phase structure and excellent physical and chemical properties.The studies on magnetic high-entropy alloys are mainly focused on saturation magnetization(Ms)and coercivity(He).However,there have been lack of research on other magnetic parameters,such as initial permeability(μi),maximum permeability(μmax),maximum magnetic flux density(Bm),hysteresis loss(Pu),specific total loss(Ps),specific apparent loss(Ss).The magnetic properties of materials are usually closely related to their phase composition and microstructure,and if the relationship between them can be established,a more ideal magnetic high-entropy alloy material with specific requirement can be designed.Therefore,in this work,FeCoNi(CuAl)0.8Gax(x=0-0.08),FeCoNi(MnSi)x(x=0-0.4)and FeCoNi(AlM)x(M=Cu、Si、Mn)HEAs were prepared via arc-melting high-purity metals,and then they were sucked into a water-cooled Cu mold to ensure chemical homogeneity.The microstructure,phase constitution,magnetic and mechanical properties of them were studied.Besides,the phase transition mechanism from FCC to BCC of the FeCoNi(CuAl)0.8Ga0.06 HEA was also investigated.We can draw the following conclusions:All the FeCoNi(CuAl)0.8Gax(0≤x≤<0.08)HEAs contain a mixture of FCC+BCC phases and the addition of Ga can promote the formation of BCC phases,and this will futher promote the increase of yied strength,maximum compressive strength and hardness.Further analysis shows that the magnetic parameters of the HEAs are closely related to the phase composition of the alloys.Moreover,the formation of BCC phases in the alloys will be conducive to increase the values of Br,Hc,Pu,Ms and Bm,while the formation of FCC phases could help increase the values of μiandμmax.After annealing at the range of 623 K-723 K,the phase transformed from FCC to BCC,which leads to the increase of Ms、Bm、Br、Hc and Pu,but the decrease of μiand μmax.Based on the XRD,SEM,TEM,and STEM results,we can summarize that the phase transition is mainly due to the diffusion and movement of the Cu atoms in the phase boundary region.And the formation and growth of Cu-rich nano-precipitates in the BCC matrix is due to the different solubility of Cu atoms in FCC and BCC phases.The FeCoNi(MnSi)x(0≤x≤0.4)alloys contain single FCC structure.Some Mn6Ni16Si7 precipitates in the FCC matrix when x>0.3,and the volume fraction of it increases with the increasing of x.The addition of MnSi is beneficial to Br、AC Br、AC Hc and Ps while it is detrimental to Ms、Bm、AC Bm、μa、μi and μmax.Moreover,the hysteresis losses play an important role when the frequency is 50 Hz,while the eddy current losses are dominate in the total losses at a frequency of 950 Hz.This provides the direction of effects to reduce the Ps.The phase composition,Vickers hardness and resistivity of FeCoNi(AIM)x(M=Cu、Si、Mn)HEAs were studied,and the results as follow:with the increase of AIM,in terms of promoting BCC phase formation ability,Si>Mn>Cu;in terms of promoting Vikers hardness,Si>Mn>Cu;in terms of promoting the increase of resistivity,Mn>Si>Cu.Based on the study of magnetic properties results,it was found that when x=0.5,the alloy with Si element shows a higher μi、μm、and Ms,as well as a lower Hc and Pu;however,the alloy with Cu element shows a lower AC Br、AC Hc、Ps、Ss and δ,as well as a higher AC Bm.when x=1.0,the alloy with Mn element shows a higher μi、μm、Bm and AC Bm,as well as a lower Hc、Pu、AC Br、AC Hc、Ps、Ss and δ.