The Formation Mechanism Of Primary Crystal Phase In High B_s Nanocrystalline Alloys

Fe-based nanocrystalline/amorphous soft magnetic alloys exhibite excellent magnetic properties such as a relatively high saturation magnetization（B_s）,a near zero magnetostriction coefficient（λ_s）,a low coercive force（H_c）and high permeability（μ_e）,and thus have been widely used in energy-saving and power electronics and other fields.The work of this thesis includes the development of new FeSiBPCCu nanocrystalline alloy prepared using industrial grade raw materials and the application of an external magnetic field during the stripping process to improve amorphous forming ability（AFA）of FeSiBCu alloy.The specific research contents and results are summarized as follows:（1）High B_s Fe_82.5Si₃B₁₃P_0.5C_0.2Cu_0.8 nanocrystalline alloy ribbons with high impurity tolerance are successfully developed.By comparing the samples prepared using industrial grade and pure raw materials,it is found that the impurities in the commonly used industrial grade raw materials have a significant effect on the AFA,but little effect on crystallization behaviors and magnetic properties,thus indicating a high impurity tolerance.The Fe_82.5Si₃B₁₃P_0.5C_0.2Cu_0.8 nanocrystalline alloy ribbons prepared with industrial grade raw materials exhibit excellent magnetic properties,including high B_s over 1.79 T,low H_c of 9.5 A/m and maximum permeability（μ_m）of4×104,which is similar to the sample with the same composition but prepared using pure raw materials.（2）By applying an external magnetic field to the two sides of the copper disk of the single-roller ribbon stripper machine,Fe_82.65Si₄B₁₂Cu_1.35 melt-spun ribbon can be prepared by the method of stripping with a magnetic field.The effect of the strength（H_A）of the applied magnetic field at different rotation speeds（v）on the amorphous forming ability（AFA）of alloy is studied.The results show that,as the applied magnetic field strength increases,the AFA of alloy first increases and then decreases,and for the rotation speed v=25 m/s,30 m/s,35 m/s,and 40 m/s,the H_A corresponding to the maximum AFA of alloy achieved through stripping with a magnetic field is 240 mT,180 mT,120 mT,and 120 mT,respectively.In addition,the external magnetic field can stabilize the amorphous phase and inhibit the phase separation of the alloy.A theoretical model has been established to explain the experimental results.Our theory shows that the existence of the external magnetic field produces two effects,namely increasing the viscosity of the alloy melt and the Joule heat generated by cutting the magnetic lines of the copper disk.The former effect is favorable to the alloy AFA,and the latter is harmful.Due to the two opposite effects,the maximum AFA of alloy in the method of stripping with a magnetic field can only be reached under a suitable strength of external magnetic field.Based on the present theoretical model,we quantitatively calculate the strength of external magnetic field corresponding to the maximum AFA of alloy achieved in the method of stripping with a magnetic field at different rotational speeds,and the calculated results are in good agreement with the experimental results.