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Study On Annealing Process Of Ferro-Based Nanocrystalline Soft Magnetic Materials

Ferro-based nanocrystalline soft magnetic materials have high saturation magnetic induction strength, high permeability, low coercivity and low high frequency loss, good toughness, wear resistance and corrosion resistance, good temperature and environmental stability, its comprehensive magnetic properties are far better than silicon steel, ferrite, Permo alloy and amorphous alloy, etc., is currently recognized in the world as soft magnetic materials with excellent comprehensive performance. It has been widely used in the manufacture of common mode choke, high frequency switching power supply, high frequency inverter, high sensitivity and fidelity magnetic head, high performance magnetic amplifier and other components.

Ferro-based nanocrystalline soft magnetic materials are mainly prepared by amorphous crystallization, while the earlier amorphous materials are generally prepared by rapid cooling and rapid quenching. Through amorphous crystallization annealing, ferro-based nanocrystalline soft magnetic materials become metastable structures with amorphous and nanocrystalline complex phases. Therefore, more and more attention has been paid to the research of improving the comprehensive soft magnetic properties of materials by annealing. In this paper, DSC, XRD and VSM are used to study FeCuNbSiB soft magnetic alloy. Through the comprehensive comparison of magnetic properties after isothermal annealing under different process conditions, the best heat treatment process is determined.

1. Experimental materials and methods

Fe73.5Cu1Nb3Si13.5B9 was selected as the experimental alloy, with a band width of 10mm and a thickness of 25μm. NetzschDSC404 differential scanning calorimeter was used to determine the annealing temperature range. In this range, three different temperature points were selected for vacuum isothermal annealing of the samples. The annealing time was 1h and the samples were cooled in the air. The magnetic properties before and after annealing were measured by JDWM-2000B vibrating sample magnetometer, and the optimum annealing temperature was found by comparison. D/Max2500Pc X-ray diffractometer was used to determine the phase and structure of the samples before and after annealing.

2. Experimental results and discussion
2.1 XRD analysis of samples before annealing
The materials before annealing were tested by XRD. The results show that the experimental material has both nanocrystalline and amorphous diffraction characteristics, which is a complex phase structure of nanocrystalline and amorphous. Among them, Fe3Si phase and a-Fe phase are crystalline phases, but their quantities are small. The lattice constant of a-Fe phase is still 0.2866 nm, indicating that Si is not solidly dissolved in it. In the XRD pattern, the diffraction peak of Fe3Si phase is only implied in a-Fe peak, but does not appear alone, indicating that the phase orientation of Fe3Si is basically the same as that of a-Fe phase.

2.2 Determination of annealing temperature range
DSC curves of the experimental materials were measured by differential scanning calorimeter. It can be seen that the sample is always in an endothermic state at the beginning of heating, and the straight line is in a gentle downward trend. At about 530 ° C, exothermic heat begins, and at about 550 ° C, endothermic heat begins again, at which time there is an exothermic peak. Therefore, it can be estimated that FeCuNbSiB alloy begins to crystallize at about 530℃, and the crystallization rate reaches the maximum at about 550℃. After that, the grains mainly grow up. In order to ensure that the material has a certain degree of crystallization, the appropriate annealing temperature is 550~590℃.

2.3 Magnetic properties detection at different annealing temperatures

Three temperature points (550, 570 and 590℃) were selected in the determined annealing temperature range (550~590℃) for vacuum isothermal annealing, and the isothermal time was 1h. Air cooling. The magnetic properties of the materials annealed at different temperatures were measured and their hysteresis loops were determined. The magnetic properties of the materials annealed at different temperatures were compared with those before annealing.

According to the hysteresis loop obtained, magnetic parameters such as Bs, Br and Hc were determined. If Bs, the larger, and Br and Hc. The smaller it is, the better its comprehensive magnetic properties. Although the Bs of the alloy is the largest after annealing at 590℃, its Br and Hc are the highest among the three annealing temperatures. The data at 550℃ and 570℃ show that the alloy has the best comprehensive magnetic properties after annealing at 550℃. So 550℃ is the best annealing temperature for Fe73.5Cu1Nb3Si13.5B9 nanocrystalline soft magnetic materials.
uring the annealing process, most of the amorphous phase inside the alloy is crystallized to form A-Fe (Si) phase. However, Nb and B are crowded into the amorphous region around the crystallized region due to their limited solubility in α-Fe(Si) phase, which plays a role in stabilizing the amorphous region and inhibiting grain growth. After annealing, the alloy has excellent soft magnetic properties, which is composed of nanocrystalline A-Fe (Si) phase and a few amorphous phases. However, the mechanism is not fully understood, and many scholars believe that the following two factors are the key factors, namely, small grain size. The local anisotropy becomes smaller and the magnetostrictive coefficient is lower than that of iron – based amorphous alloy 18.

The grain size of soft magnetic phase of a-Fe(Si) increases with the increase of annealing temperature, and the magnetic anisotropy increases with the continuous precipitation of hard magnetic phase of Fe2B. The large Angle grain boundaries hinder the displacement of domain walls and the rotation of magnetic moment. Therefore, the soft magnetic properties of the alloy decrease gradually with the increase of annealing temperature.

3. Conclusion

(1) The optimum annealing process of Fe73.5Cu1Nb3Si13.5B9, nanocrystalline soft magnetic material is 550℃ x 1h.

(2) After annealing at 550℃ x 1h for Fe73.5Cu1Nb3Si13.5B9 nanocrystalline soft magnetic material, the composite structure of nanocrystalline A-Fe (Si) phase + a few amorphous phases can be obtained, which has excellent comprehensive magnetic properties.

(3) With the increase of annealing temperature, the grain size of A-Fe (Si) soft magnetic phase increases gradually, in addition to Fe,B hard magnetic phase precipitation, resulting in a decline in the soft magnetic properties of the alloy.

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