| Nd-Fe-B permanent magnets are widely used in automotive, wind power and other fields for its high coercivity, high remanence and excellent magnetic energy product. Sm-Co permanent magnets are widely used in fields of aerospace and other special environments for its high curie temperature and good thermal stability. Among various Sm-Co based permanent magnetic materials, Tb Cu7-type Sm-Co alloys have atttracted much attention in recent years. As is well known, compared to the isotropic permanent magnets, the anisotropic magnets have larger values of the remanence Mr and maximum energy product(BH)max. The investigation on the minimotor s market show s that the motor volume and weight can be reduced by 50% or more by using anisotropic permanent magnets instead of isotropic magnets. These can save the resources and reduce the costs, which improve the competitiveness of the motor products in the market.In this work, a nisotropic Nd-Fe-B and Sm(Co,Zr)7 nano-flakes are prepared by oleic acid assisted ball milling. The influences of milling parameters on the structure and magnetic properties of as-milled Nd-Fe-B powders by milling Nd-Fe-B alloy ingots or ribbons have been investigated. Anisotropic and isotropic melt-spun Nd-Fe-B ribbons were prepared to study the influence of the ribbon s’ anisotropy on the phase, morphology and magnetic properties of the ball milled powders. In addition, the influences of wheel speed on the microstructure and magnetic properties of melt-spun Sm(Co,Zr)7 ribbons have been studied. The phase structure, morphology and magnetic properties of as-milled powders are studied. Finally, the structure and magnetic properties of the Sm(Co,Zr)7 powders obtained by milling the ingots for various time have been studied.Firstly, the anisotropic Nd-Fe-B nano-flakes with length of 0.2-1 μm are prepared by oleic acid assisted ball milling Nd13.5Fe80Nb0.3B6Al0.2 alloy ingots. The influences of milling time and oleic acid concentration on the microstructure and magnetic properties of as-milled powders have been studied. The as-milled powders prepared without oleic acid are isotropic particles, but those prepared with oleic acid are anisotropic. The remanence ratio Mr/Ms of as-milled powders is gradually increasing with the increasing concentration of oleic acid. The powders have the maximum Mr/Ms of 0.75 when the concentration of oleic acid is 40%. With the increase of the milling time, the coercivity of as-milled powders first increases and then decreases. The as-milled powders have the maximum coercivity of 221 k A/m when they are milled for 5 h with 40 % concentration of oleic acid. When vacuum dried in a magnetic field, the as-milled powders can keep the anisotropy with the c-axis alignment, but the powders are easy to be oxidized without the coating of oleic acid, which may lead to the decrease of coercivity.Secondly, based on the oleic acid assisted ball milling melt spun Nd-Fe-B ribbons, the effects of the ribbon anisotropy on the ball milling powders have been studied. The Nd13Fe80B7 ribbons were prepared by melt-spinning at various wheel speeds. The anisotropic structure and relatively low hard magnetic performance have been obtained in the ribbon spun at 8 m/s, but the ribbon s prepared at 20 m/s are almost isotropic with high magnetic properties. It has been found that the anisotrop y of the melt-spun ribbons has almost no obvious effects on the phase structure and morphology of the powders prepared by oleic acid assisted ball milling. Generally, the higher coercivity of the melt-spun ribbons leads to the higher coercivity of as-milled powders.Thirdly, the anisotropic Sm(Co,Zr)7 nano-flakes were prepared by oleic acid assisted ball milling Sm Co6.8Zr0.2 melt-spun ribbons. The influences of wheel speed on the microstructure and magnetic properties have been studied. For the melt-spun ribbons, the grains show a long axis along one direction for the ribbons prepared with the wheel speeds of 5 m/s and 15 m/s, but an isometric structure is found in the ribbons prepared with a 30 m/s wheel speed. The coercivity increases with the increasing wheel speed. The coercivity of melt-spun ribbons with 30 m/s wheel speed is 162 k A/m in the ribbon plane and 381 k A/m out of the ribbon plane. As-milled Sm(Co,Zr)7 flakes with the length of 2-5 μm and the width of 300-450 nm show some anisotropy. Its coercivity values are between the in-the-plane and out-of-the-plane values of the melt spun ribbons. The grains size of as-milled flakes is smaller than that of the melt-spun ribbons. In addition, compared to the as-milled powders prepared by oleic acid assisted ball milling, the powders prepared by normal ball milling have more content of the amorphous phase, which results in the decrease of coercivity and Mr/Ms.Finally, the anisotropic Sm(Co,Zr)7 nano-flakes are also prepared by oleic acid assisted ball milling Sm(Co,Zr)7 alloy ingots. The influences of milling time on the microstructure and magnetic properties of as-milled powders have studied in detail. With the increasing milling time, the size of powders and size of grains get smaller. The coercivity of as-milled powders first increases and then decreases. The as-milled powders get the maximum coercivity at the milling time of 9h. The Mr/Ms of as-milled powders is in the range of 0.72-0.85, which means a good magnetic anisotropy.Overall, this thesis describes the systematic investigations on the preprations of two types of rare earth-based hard magnetic materials, Nd-Fe-B and Sm-Co alloys, by a top-down approach, which can contribute to the development of anisotropic rare earth permanent magnetic powders with high properties. |
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