Study On The Magnetocaloric Effect Of TbDyHoEr-based High Entropy Alloy And The Effect Of Entropy On The Stability Of ThMn₁₂ Phase

The content of this paper is mainly divided into two parts around the role of high entropy.The first part mainly studies the effect of high mixing entropy on the crystal structure and magnetocaloric effect of TbDyHoEr-based rare earth high entropy alloys,and studies the magnetocaloric effect of high-entropy alloy TbDyHoEr enhanced by melt-spun treatment.The magnetic transition was studied and the magnetic phase diagram was drawn.In the second part,the stabilization effect of high entropy on the metastable phase ThMn₁₂ phase was studied.The 1:12 compounds include stable element Ti and multi-element rare earth to provide high entropy were prepared.The stabilization effect of the two methods on the ThMn₁₂ phase was compared,and the magnetic properties of these samples were studied.Specifically,the following work has been done:Equimolar ratio high-entropy alloy TbDyHoEr composed of high magnetic moment rare earth elements was designed,and prepared by arc-melting,the crystal structure and phase composition of TbDyHoEr were studied by XRD,SEM and EDS.It is found that TbDyHoEr has a single-phase close-packed hexagonal（HCP）structure.An ideal’table-like’magnetocaloric effect wasf_Sou_mn_axd in the high entropy alloy TbDyHoEr,the peak value of magnetic entropy change（^M）is 6.82 J·kg^-1K^-1 and the RC value is 883.19 J·kg^-1 under the magnetic field change of 5 T.After the melt-spun treatment,the high-entropy alloy TbDyHoEr extends the working temperature range to lower temperature while the magnetic entropy change maintaining,so the RC value is expanded to 1049.22 J·kg^-1,the magnetocaloric effect is significantly improved by melt-spun treatment.In addition to excellent magnetocaloric properties,it is also found that the high-entropy alloy TbDyHoEr has extremely high saturation magnetization and small hysteresis,which can be considered as an alternative for soft magnetic materials.The high-entropy alloys Gd Tb Dy Ho and Gd Tb Dy Er were also prepared by arc melting,and the saturation magnetization,hysteresis of the above three rare earth high-entropy alloys at 10 K were compared.Under comp REnsive comparison,TbDyHoEr has the largest saturation magnetization of 292.19emu/g and smaller hysteresis,which is the best candidate for soft magnetic materials.In order to further enhance the magnetocaloric effect of high entropy alloy TbDyHoEr and improve the economy,transition elements Ti and Co were added to prepare high-entropy alloys TbDyHoEr Ti and TbDyHoEr Co.It was found that TbDyHoEr Ti also has a HCP structure,while TbDyHoEr Co is composed of R₃T phase and HCP phase.The magnetic entropy change peak and RC values of the two samples are 6.66,11.32 J·kg^-1K^-1 and 779.22,482.66 J·kg^-1,respectively.Due to the stability effect of high entropy,the use of high entropy is expected to make the thermodynamically metastable phase stable.Therefore,two 1:12-type compounds,REFe₁₂ compound using high entropy to try to stabilize the phase and(Sm_1-xHR_x)Fe₁₁Ti compound using stable element Ti to stabilize the phase,were prepared.The stabilizing effect of stable elements and high entropy on ThMn₁₂ phase was compared.It was found that(Sm_1-xHR_x)Fe₁₁Ti formed ThMn₁₂ phase,but due to the volatilization and preparation process,α-Fe and the second phase were produced in large quantities,and the coercivity of the magnets was reduced.The coercivity of(Sm_1-xHR_x)Fe₁₁Ti series magnets was between0.97-1.77 KOe.For the REFe₁₂compound,samples with A sites containing 5,6 and 7elements were prepared.It was judged by XRD and EDS that no ThMn₁₂ phase was found in the 5-element sample,and the corresponding composition ratio of ThMn₁₂ appeared in the6-element and 7-element samples.It is concluded that the high entropy value has the possibility of stabilizing the ThMn₁₂ phase,and the stabilization effect increases with the increase of configuration entropy.It is preliminarily speculated that the A-site configuration entropy of 1.79 R is the critical value of stability,and the increased entropy value of Fe site partial doping can further stabilize the ThMn₁₂ phase.