Effect Of Rare Earth Elements On Microstructure And Ac Magnetic Property Of FeCoNi(CuAl)_0.8 High-entropy Alloys

Unlike conventional alloys,high entropy alloys(HEAs)are composed of more than five principal elements in equal or near-equal atomic proportions,which tends to form solid solution and shows excellent mechanical and physical properties.In recent years,ferromagnetic HEAs have been attracted considerable attention as a novel soft magnetic material.Besides direct current(DC)mode,soft magnetic materials are also used in alternating current(AC)conditions.However,the studies on AC magnetic properties of HEAs are relatively limited.Moreover,the influential factors of AC magnetic properties and magnetic mechanism of HEAs are not clear.Alloying is one of the main ways to enhance the properties of materials.In particular,rare earth(RE)elements play an important role in many materials,such as magnesium alloys,steels and permanent magnetic materials.Nevertheless,investigations on the effect of RE elements on the magnetic properties of HEAs are very limited.Therefore,it is necessary to do relative studies.In this work,the sheet samples of FeCoNi(Cu Al)_0.8RE_0.05(RE=Ho,Pr,Nd,Y)and FeCoNi(Cu Al)_0.8Ho_x(x=0,0.03,0.05,0.07)HEAs were prepared by vacuum arc melting and copper mold casting.Firstly,the effects of different RE elements on AC magnetic property and microstructure of FeCoNi(Cu Al)_0.8 HEA are investigated.Secondly,the relationship between AC magnetic property and microstructure of FeCoNi(Cu Al)_0.8Ho_x(x=0,0.03,0.05,0.07)HEAs by changing Ho content is investigated.The influential factors of AC magnetic property are also explored.The main conclusions of this paper are as follows:(1)FeCoNi(Cu Al)_0.8(RE free)and FeCoNi(Cu Al)_0.8RE_0.05(RE=Ho,Pr,Nd,Y)HEAs consist of FCC phase and BCC phase.RE addition can increase the relative content of BCC phase.In comparison to the RE free alloy,RE addition can change the grain size and morphology.The results of the energy dispersive X-ray spectroscopy(EDX)of SEM show that Fe andCoelements are mainly distributed in FCC phase,while Al and Ni elements are distributed in BCC phase.Cu element is mainly segregated at phase boundaries of FCC phase and BCC phase in RE free alloy.The addition of Ho element can reduce the Cu content at phase boundaries,while Nd and Y elements increase it.Moreover,Ho,Pr and Nd elements are segregated at phase boundaries,while Y element is mainly segregated at the region between BCC phases.(2)The results of AC magnetic properties of FeCoNi(Cu Al)_0.8RE_0.05(RE=Ho,Pr,Nd,Y)HEAs show that the total loss(P_s)measured at a fix magnetic flux density(B_m)of 200 m T increases with the frequency in range of 50 Hz to 950 Hz.Ho addition can effectively reduce the value of P_s.At the frequency of 50 Hz,the addition of Ho,Pr,Nd and Y elements can increase the relative content of BCC phase,which is beneficial to reduce the values of P_s and the coercivity(H_c).It results in the improvement of AC magnetic property of HEAs.At 950 Hz,the role of Ho and Pr elements play on the improvement of AC magnetic property is bigger than that of Nd and Y elements.(3)In FeCoNi(Cu Al)_0.8Ho_x(x=0,0.03,0.05,0.07)HEAs,the hysteresis loss(P_h)amounts to approximately 90%suggesting that the hysteresis loss is predominant in the total loss at 50 Hz.The eddy current loss(P_e)amounts to more than 60%indicating that the eddy current loss instead of the hysteresis loss is the main contribution of the total loss at 950 Hz.The x=0.05 HEA shows the minimum P_s in range of 50 Hz to 950 Hz.Furthermore,it exhibits the optimum AC magnetic property with the minimum P_s,P_e,P_h,and H_c at both 50 Hz and 950 Hz.(4)FeCoNi(Cu Al)_0.8Ho_x(x=0,0.03,0.05,0.07)HEAs are composed of FCC phase and BCC phase.Ho addition does not change the phase composition and increases the relative content of BCC phase.The results of SEM-EDX show that the total content of ferromagnetic elements(Fe+Co+Ni)in FCC phase and BCC phase has marginal change with Ho addition.In x=0.05 alloy,the content of Cu element at phase boundaries decreases,which is beneficial to reduce the value of P_s.With further increasing Ho content to x=0.07,the segregated positions of Cu and Ho elements change.The content of Cu and Ho elements in the region between FCC phases is higher than that at phase boundaries,which obviously increases the value of P_sand decreases the AC magnetic properties of HEAs.