热变形磁体力学性能的各向异性和晶界强韧化研究

Hot-deformed (HD) Nd-Fe-B magnet has several advantages, such as high magnetic flux, facile preparation, near-net-shape production, etc.. So far it has been used in EPS motors for cars and IPM motors for compresser in air-conditioner. HD Nd-Fe-B magnet has the characteristics of both laminated structure consisted of Nd2Fe14B platelets and anisotropic grain boundary (GB). They collaborate to determine the strong anisotropy of mechanical performance and thus the poor mechanical performance along the direction parallel to Nd2Fe14B platelets in HD Nd-Fe-B magnet. As a result, HD Nd-Fe-B magnets are easily cracked or broken along the GB parallel to Nd2Fe14B platelets (GB-P). To improve the strength and toughness of GB-P, two methods will be carried out in this project: the aggregation of nano-sized ductile precipitates in GB-P and the dispersion of metal particles at GB-P. Both the two methods will inhibit the crack propagation or change the path of crack propagation in GB-P, which could improve the mechanical performance along the direction parallel to Nd2Fe14B platelets for HD Nd-Fe-B magnet. Meanwhile, the dependence of the anisotropy of mechanical performance and fracture mechanisms on the phases and microstructures will be revealed. Further investigation will be carried out on the lattice structure and the bonding strength of the interfaces between ductile precipitates (or metal particles) and Nd2Fe14B phase (or Nd-rich phase). Such investigation on the interface will accelerate the search for suitable ductile phases as well as the methods for introducing them into HD Nd-Fe-B magnets.

钕铁硼热变形磁体具有磁能密度高、制备流程短、易于实现近终成型等优点，目前已被应用于汽车EPS电机、空调压缩机的IPM电机等方面。但热变形磁体的片状晶堆垛结构和晶界各向异性造成其力学性能呈现明显的各向异性，从而导致沿平行于片状晶方向的力学性能很差，并使其在后续机加工和使役过程中极易沿平行于片状晶方向的晶界(简称平行晶界)开裂和破碎。为此，本项目围绕改善热变形磁体内平行晶界的强韧性，采用在平行晶界中富集韧性纳米沉淀相和弥散分布金属塑性颗粒两种方法来改善平行晶界的力学性能，达到抑制裂纹沿平行晶界扩展或改变裂纹扩展方向的目的，进而提高磁体沿平行片状晶方向的力学性能。与此同时，阐述力学性能的各向异性和断裂机制对微观组织结构的依赖关系，深入研究平行晶界中纳米沉淀相和金属颗粒与基体相(Nd2Fe14B相和晶界富钕相)之间的晶格匹配关系和界面结合强度，为寻找合适的增韧相和增韧方法提供理论指导。

热变形磁体的力学性能具有各向异性，沿垂直于c轴方向的力学性能明显低于平行于c轴方向，这导致磁体的后续加工比较困难。为此，本项目开展了热变形磁体力学性能各向异性及其断裂机理的研究，揭示了磁体沿平行c轴和垂直c轴方向力学性能的差异，阐述了热变形磁体沿条带边界和沿晶粒边界两种断裂机制。针对沿垂直c轴方向力学性能较差的问题，采取了多种方法来解决这一问题。通过减小快淬粉颗粒的尺寸和长厚比，可以有效提高热变形磁体沿垂直c轴方向的抗压强度和抗弯强度，二者的各向异性分别减小了50%和27.6%。通过翻转变形工艺，能够引发粉末条带沿垂直c轴方向产生扭曲和弯折，从而提高沿垂直c轴方向的抗弯强度和断裂韧性。通过改变原料快淬粉中的Nd含量，能够调控热变形磁体中晶界相的组分和含量，进而有效提升了磁体沿垂直c轴方向的抗弯强度和抗压强度。采用条带边界添加金属颗粒(Fe、Co、Cu等)的方法，可以有效改善沿垂直c轴方向的界面结合强度，改变断裂路径，从而提高磁体沿沿垂直c轴方向的力学性能。添加高熔点WC合金纳米颗粒，可以有效抑制条带边界晶粒的异常长大，进而改善磁体力学性能的各向异性。在本项目的资助下，迄今共发表SCI论文8篇，申请发明专利5项，授权发明专利4项；培养硕士生3人，培养博士生1人。其中，毕业硕士生2人。