Study On The Microstructure And Magnetic Properties Of (Nd, Dy, Pr)₂(Fe, Nb)₁₄B/α-Fe Nanocomposite Permanent Magnet Prepared By Melt-Spinning

The (Nd,Dy,Pr)2(Fe,Nb)14B/α-Fe nanocomposite ribbons were prepared by direct melt-spinning and melt-spinning + heat treatment in this work. The influence of the component and preparation parameters on microstructure and magnetic properties of the nanocomposite were investigated. For the element substitution, we studied the effect of Nd substituted by Dy and Pr on the magnetic properties of the nanocomposites. For the preparation technique, we investigated the influence of preparation parameters on magnetic properties of the nanocomposite prepared by direct melt-spinning and melt-spinning + heat treatment. The phase composition, microstructure, crystallization behavior and magnetic properties were studied by X-ray diffraction (XRD), vibrating sample magnetometer (VSM) and scanning probe microscope (SPM). The main work and experimental results are as follows:1. The quenched ribbons of Nd3.8Pr3.5Dy0.7Fe86Nb1B5 nanocomposite were prepared by direct melt-spinning, and the influence of quenching speed on microstructure and magnetic properties were investigated. The experimental results show that for the samples quenched at lower speed, the hard magnetic phases grow along the perpendicular to the ribbon plane and the easy magnetization direction of the sample is perpendicular to the ribbon plane. With increasing the wheel speed, theα-Fe phase induces the growth of c-axis of hard magnetic phase parallel to the ribbon plane direction, the easy magnetization direction to parallel to the ribbon plane. High performance of Br=1.29T, Hcj=503.8kA/m, (BH)max=158.4kJ/m3 , Mr/Ms=0.76 was achieved at wheel speed of 30m/s. SPM observation shows that when the wheel speed is less than 30m/s, the grain size becomes finer and more uniform with quenching speed increasing. A very fine and uniform microstructure with the average grain size less than 20nm was obtained when the quenching speed is 30m/s. The analysis of Henkel plots shows that the exchange interaction in this sample is stronger than in the other samples. This leads to the enhancement of the magnetic properties of the sample.2. The ribbons of Nd3.8Pr3.5Dy0.7Fe86Nb1B5 nanocomposite were prepared by melt-spinning + heat treatment, and the influence of preparation parameters on microstructure and magnetic properties of the samples were investigated. The high heating rate of 460℃/min was adopted in this work. The experimental results show that proper heat treatment plays an important role in determining the high performance of the nanocomposite magnetic material. When the heating temperature is lower, the magnetic phases precipitate incompletely, and amorphous phases exist in the alloys. This leads to the weak exchange coupling interaction between soft and hard grains and results in the poor magnetic properties. When the heating temperature is higher, the exchange coupling interaction is also weak due to the coarse grain ofα-Fe, and the magnetic properties such as Br and (BH)max degraded. The experimental results show that the optimum magnetic properties of Hcj=509kA/m , Br=1.49T ,(BH)max=195kJ/m3 can be obtained in Nd3.8Dy0.7Pr3.5Fe86Nb1B5 sample under the optimum annealing condition 800℃×1min. Those magnetic properties of the ribbon are prior to the similar materials reported. The suitably composition, high heating rate and proper annealing condition are the key factors for preparing high performance nanocomposite magnetic material.3. The experimental results show that the optimum annealing temperature and annealing time increase with increasing Dy content. When Dy content is 0.7at% and Pr content is 3.5at%, the optimum magnetic properties prior to the other component were obtained in Nd3.8Dy0.7Pr3.5Fe86Nb1B5 nanocomposite.