Preparation, Structure And Magnetic Properties Of Melt-spun And Copper Mold Suction Cast Boron-Rich Rare-earth Nanocomposite Permanent Magnets

Nanocomposite permanent magnetic material is a kind of attracting new permanentmagnet. It consists of both nano-scale hard magnetic phase with high anisotropic field andsoft magnetic phase with large saturation magnetization, leading to excellent magneticperformance through exchange coupling between their neighboring atomic magnetic moments.In addition to the high maximum energy product that may be achieved, nanocompositemagnets are of commercial interest because they require less of an expensive rare earthelement, low cost and high corrosion resistance.In this dissertation, nanocomposite REFeB/Fe3B type hard magnetic alloys have beenprepared by melt-spinning technique with subsequent crystallization annealing and coppermold water cooling method. The microstructure and magnetic properties of the alloys havebeen investigated. The magnetization and demagnetization behavior of nanocomposite alloyshave also been studied.REFeCoBM ribbons were fabricated by single copper rolling at speed of15m/s. In theNdFeCoBM alloys, for Nd₉Fe_71.5B_15.5Zr₄component, amazing magnetic properties ofjHc=891kA/m, J_r=0.77T, J_s=1.37T, J_r/J_s=0.56, and (BH)_max=82.9kJ/m³were achieved for as-spunalloy ribbons. The doping of Co and refractory elements such as Nb, Zr enhances the GFA(Glass Forming Ability) to get fully amorphous ribbons, and also improves the magneticproperties after heat-treatment. For Pr_9.5(FeCo)_71.5B₁₅M₄alloys, fully amorphous or hardmagnetic properties failed in as--spun ribbons. The binary Fe2B phase and soft magneticphase Fe₃B were exhibited in ribbons.REFeCoBM ribbons were fabricated by single copper rolling at speed of50m/s. InNd(FeCo)BM(Nb, Zr) alloys, the addition of Nb and Zr can enhance the GFA, refine thestructure after annealing and improve magnetic properties. The addition of Co can enhancethe Jrand the temperature stability of Jr, still improve the maximum magnetic energy productat the expense of slightly coercivity decreasing. The contents of9.5at%Nd have highercoercivity than the9at%, due to the increasing of hard magnetic phase Nd2Fe14B. Thecoercivity mechanism was analyzied based on experiments and theory. Furthermore, thehenkel plots were analyzed to testify the exchange coupling in alloys. The structure andmagnetic properties of Pr_9.5(FeCo)_71.5B₁₅M(Nb, Zr)₄were also studied. With the substitutionof Nb by Pr the alloy has a lower crystallization temperature, which means that Pr makes thealloy instabler in crystallization kinetics. So the crystallization transition and the grain growthtake place easier, which results in coarse and inhomogeneous microstructure and a lower remanence. For DyFeBM(Nb, Zr) alloys, the high anisotropic field of Dy₂Fe₁₄B brings in highcoercivity, due to its low saturation magnetization and short exchange length, improves therequirement of the distribution of soft magnetic phase and grain size, which are hard to getfully exchange coupling, then leads to lower remanence and less maximum energy product.(Y, Dy, Nd)₄FeB₂₂(Ta, Nb)₂alloys of2mm diameter rod were prepared by a coppersuction casting method in water cooled crucible. Dy₄Fe₇₂B₂₂Nb₂was fully amorphous by theXRD test in the core of the rod. The reason of the bulk metal glass (BMG) forming wasanalyzed by Senkov principle and Inoue empirical rules. Meanwhile, the50m/s as-spunribbons of same components were fabricated, and the crystallization activation energy ofDy₄Fe₇₂B₂₂Nb₂was calculated by thermal analysis and Kissinger equation. The crystallizationactivation energy of two exothermal peaks were572and484kJ/mol, corresponding to Fe3Band Dy₂Fe₁₄B, separately. The magnetic propertied of as-cast rod and annealed ribbons weremeasured; the coercivity was low, due to low rare earth and Dy2Fe14B phase.(Nd, Pr)(Fe, Co)B(Nb, Zr) alloys of2mm and1mm diameter rod were prepared by acopper suction casting method in water cooled crucible. The Nd(Fe, Co)B(Nb, Zr) alloysmainly consist of hard magnetic phase Nd₂Fe₁₄B and soft magnetic phase (Fe₃B, Fe). Themagnetic properties of ofjHc=568kA/m, J_r=0.56T, and (BH)max=35.9kJ/m³are achieved forNd₉Fe_71.5B(15.5)Nb₄as-cast2mm rod. After annealed at650oC for10minutes, the coercivityreaches up to1151kA/m, and the remanence also has slight increase. It attributes to theexistence of binary phase FeNb. The FeNb particles distributing in the grains hold the reversedomains wall and meanwhile preserve the strong exchange coupling between soft and hardmagnetic phases. The Pr(Fe, Co) B(Nb, Zr) alloys mainly consist of hard magnetic phasePr2Fe14B and soft magnetic phase (Fe₃B, Fe). The value of coercivity is between40-159kA/mfor as-cast rod samples. After annealed at650oC for5minutes, the magnetic properties ofPr_9.5Fe_51.5Co₂₀B₁₅Nb₄isjHcof1210kA/m, Jrof0.61T,(BH)maxof65.1kJ/m³. The Fe₂Nbparticles pin the domain walls and hinder the reverse domains from growing and moving.For (Nd, Pr)(Fe, Co)B(Nb, Zr) as-cast rods of1mm diameter, the component ofNd₉Fe_71.5B_15.5Nb₄shows the best hard magnetic properties, i.e.jHc=307kA/m, Jr=0.57T,(BH)max=34.8kJ/m³. In Pr series, The magnetic properties of Pr_9.5Fe(71.5)B₁₅Nb₄isjHcof1113kA/m, Jrof0.61T and (BH)maxof56.2kJ/m³, and Pr_9.5Fe_71.5B₁₅Zr₄isjHcof1118kA/m, Jrof0.69T and (BH)maxof80.7kJ/m³.