Development And Performance Research Of High B_s FeSiBNbCu Nanocrystalline Soft Magnetic Alloys

Fe-based amorphous nanocrystalline soft magnetic alloys stand out from many soft magnetic materials due to their low coercivity（H_c）,high permeability（μ）and low loss（P_c）,and are considered to be energy-saving and environmentally friendly materials in the 21st century.They are widely used in technical fields such as transformers,servo motors and sensors.In response to the market requirements of high efficiency and miniaturization of devices,it is one of the urgent problems to improve the saturation magnetic induction（B_s）of amorphous/nanocrystalline soft magnetic alloys.In order to solve this problem,a series of high-B_s Fe Si BCu M amorphous/nanocrystalline soft magnetic alloys have been studied systematically in this thesis.The optimization of soft magnetic properties can be achieved through the composition design and heat treatment process control,and as a result,the nanocrystalline soft magnetic alloys,which meet the requirements of industrial production,have been developed successfully.In addition,through the in-depth observation of the nanocrystalline microstructure and the analysis of non-isothermal crystallization kinetics process,the crystallization process and nucleation&growth mechanism of the alloy were deeply investigated,thereby establishing the related mechanism among soft magnetic properties,microstructure and crystallization,which reveals the crystallization mechanism of FINEMET-like nanocrystalline alloys with high Fe content.The main research work of this thesis includes:1.The soft magnetic properties of Fe_78+2xSi_7.2-xB_13-xCu_0.8Nb₁（x=0,1,2）nanocrystalline alloy are regulated through conventional heat treatment and rapid heat treatment.The research results show that the Fe₇₈Si_7.2B₁₃Cu_0.8Nb₁ and Fe₈₀Si_6.2B₁₂Cu_0.8Nb₁ nanocrystalline alloys obtained by the two heat treatments have general magnetic properties,while the Fe₈₂Si_5.2B₁₁Cu_0.8Nb₁ nanocrystalline alloys have excellent magnetic properties.Fe₈₂Si_5.2B₁₁Cu_0.8Nb₁ alloy obtains the best magnetic properties by rapid heat treatment:H_c of 5.3 A/m,μof 20000,and B_s of 1.80 T.The present studies have shown that the excellent magnetic properties of Fe₈₂Si_5.2B₁₁Cu_0.8Nb₁nanocrystalline alloy should be attributed to a large number of pre-existing nuclei in the as-quenched alloy.These pre-existing nuclei and Cu clusters will serve as heterogeneous nucleation points to induce the precipitation of a large amount ofα-Fe.Under the triple effects of the inhibition of Nb with a large atomic size,competition mechanism with a large number of nucleation points and a large concentration gradient,the coarsening of grains can be suppressed,the grains will be refined,and thus the fine and dense nanocrystals are obtained,which results in excellent magnetic properties.2.Industrial production of alloy ribbons requires high amorphous forming ability（AFA）.We attempt to optimize the magnetic properties by adjusting the Ge content,and as a result,Fe_80.2Si₃B_12-xP₂Nb₂Cu_0.8Ge_x（x=0,1,2）nanocrystalline soft magnetic alloy are successfully developed in this work.The studies have shown that with the increase of Ge element,the crystallization activation energy of the alloys gradually decreases,and so the precipitation ofα-Fe is easier and the heat treatment window is greatly expanded.Non isothermal crystallization kinetics analysis indicates that the crystal growth of the alloy belongs to the type of pre-existing nucleus growth.The growth process corresponds to the direct growth of a large number of pre-existing nuclei without the precipitation of new crystal nuclei as well as the restriction of vertical growth of nanocrystals due to the thin thickness of the ribbons.The magnetic properties of the prepared nanocrystalline soft magnetic alloys become better with the increase of Ge content.Among the present nanocrystalline alloys,the Fe_80.2Si₃B₁₀P₂Nb₂Cu_0.8Ge₂ nanocrystalline alloy exhibits the best magnetic properties with H_c of 3 A/m,μof 22000,and B_s is 1.65 T,as well as the AFA to meet the requirement of industrial production,and thus has a good commercial perspective.