Research On The Preparation Of Fe₈₄(NbV)₇B₉ Nanocrystalline Soft Magnetic Materials And Related Fundamental Theory

Preparation methods for Fe₈₄(NbV)₇B₉ nanocrystalline soft magneticalloys and related fundamental theory were investigated in this paper,supported by the National Key Technologies R & D Program of Chinaduring the 10th Five-year Plan Period (No.2001BA310A03-1).First, the mixing heat of formation for Fe and B that combine withother elements to form binary ordered alloys or solid solution wascalculated on the basis of thermodynamic theory. It's found that thenegative maximum value appears in the Fe-Nb and B-Nb systemsrespectively, next are Fe-V and B-V systems, and the energy requiredreaches its minimum for the formation of ordered alloys. Furthermore,considering the influence of strain energy, elastic energy and structureenergy, and combining the relationship of Slater-Pauling, the possibilityof high magnetic Fe₈₄(NbV)₇B₉ prepared by mechanical alloying or rapidsolidification- crystallizing amorphous solid is predicted.Accordingly, Fe₈₄(NbV)₇B₉ nanocrystalline bulk was obtained bymechanical alloying-high pressure forming. It shows that with theincreasing of milling time, Fe,Nb(V),B mixing powders tend to higheralloying lever and the grain size decreases gradually. In the end,nonequilibrium Fe₈₄(NbV)₇B₉ nanocrystalline solid solution with bccstructure, microthin-layer morphology and grain size of 10～15nm isformed. The addition of V element can accelerate the alloying process.Completely alloyed powders exhibit high M_s of 150～170Am²/kg.Amorphous layer generated on the boundary of powders can benefit theexchange of coupling, which improves the coercive force of alloys. Withthe increment of annealing temperature, grain size of Fe₈₄(NbV)₇B₉nanocrystalline powder increases gradually and its inner stress relaxes.However, when annealed below 750℃, the nanocrystalline size growsslowly without new phases formed. For Fe₈₄Nb₃V₄B₉ alloy, its grain sizekeeps within 10～20nm. When the annealing temperature beyond 750℃, the grain size increases rapidly and new phases, such as NbFeB appear.During mechanical alloying, the dislocation pump mechanism andlayer structure supply fast channel for atom diffusion between elements.Mechanical alloying forces plentiful micro-layer structure inside powders,and atoms can diffuse via interlayer interface to form nonequilibriumsolid solution. During the alloying, lots of dislocations are accumulated,which results in the formation of dislocation cell and later develops intonanocrystalline structure. When the system free energy caused by latticedistortion and dislocation density rises high enough, amorphous structurecan be obtained.Based on the above work, Fe₈₄(NbV)₇B₉ nanocrystalline bulk withrelative density over 97ï¼…was successfully produced under 5.SGpapressure and 1530w heating power. Its grain size is about 10～15nm,saturation magnetization is about 150Am²/kg and coercive force is about0.85KA/m. During annealing, its regulation of the growth ofnanocrystalline grain, stress relaxation and formation of new phase fornanocrystalline bulk is well constant with that of nanocrystalline powders.It's also found that the increase of forming pressure during high pressureprocess can effectively restrain the formation of impurity phases, such asFe-Nb and Fe-B, and prevent the growth of bulk nanocrystalline grains aswell. Such conclusion is meaningful to the technique development ofsuperhigh pressure forming.The Fe₈₄(NbV)₇B₉ bulk with amorphous and nanocrystalline binarystructure was also successfully prepared by rapid solidification method inthe cooling copper mould. Its grain size is about 10～20nm, distributedhomogenously in the amorphous matrix. During annealing, amorphousparts in the bulk is crystallized gradually, and the soft magnetic propertyof the bulk is improved. Optimal soft magnetic data can be obtained whenannealing at 550℃, namely, B_s=1.52～1.54T, H_c＜5.0～8.0 A/m,μ_e (1KHz, 0.4A/m)=18000～20000. The shortened process of rapidsolidification by cooling copper mould plus amorphous crystallization israther promising for the preparation of high performance nanocrystallinesoft magnetic bulks. The soft magnetic properties of Fe₈₄(NbV)₇B₉nanocrystalline bulk prepared by this method are much better than nanocrystalline bulks obtained by other methods. Especially, our bulkseven perform as well as those nanocrystalline soft magnetic ribbons thatprepared by rapid solidification ribbon plus amorphous crystallization.Therefore, it exhibits remarkable application potential.According to the Hill's thermodynamic theory for small systems, aCurie temperature model for magnetic particles was developed to showboth size effect and shape effect. Using this, the prediction of Curie pointfor freestanding and embedding magnetic particles Fe,Co,Ni is in goodagreement with experimental results. Meanwhile, the model for meltingand superheating of nanocrystals was developed on the basis ofnanocrystal cohesive energy. Accordingly, the minimum critical size andmelting temperature of Fe,Co,Ni magnetic nanocrystals were calculated,and their thermal stability during application was discussed. At last,models for higher surface energy and vacancy formation energy ofnanocrystals were developed. The predicted results for Fe,Co,Ni metalsare agreeable with mostly corresponding experimental values and othertheoretical calculations found in the literature. Based on this, Fe-M-Bbinary structure alloy was described by the nanocrystal cohesive energymodel with embedded interface, and its melting temperature, meltingentropy and melting enthalpy can be further predicted by our melting andsuperheating equivalent model.