热变形(Nd,Pr)-Fe-Nb-B磁体取向度及矫顽力机制研究

Orientation Degree and Coercive Force Mechanism of Die-Upset (Nd,Pr)-Fe-Nb-B Magnets

采用X射线衍射(XRD)、扫描电镜(SEM)分析了热变形Nd_(10.5)Pr_(2.5)Fe_(80)Nb_1B_6磁体断口等显微组织,并用磁滞回线仪测量了磁体的磁性能,系统研究了磁体显微组织、磁性能与取向度的变化规律。估算了250~380μm和<250μm Nd_(10.5)Pr_(2.5)Fe_(80)Nb_1B_6粉、变形量60%的热变形磁体等效易磁化轴与热变形压力方向夹角α,随α从26.0°降至9.1°,磁体密度从7.04 g·cm^(-3)增加至7.49 g·cm-3,M_r/M_s(M_r为剩余磁化强度,Ms为饱和磁化强度)从0.78增大到0.91,磁能积持续增加,矫顽力先从1346 k A·m^(-1)增加至1499 k A·m^(-1),然后矫顽力下降到1216 k A·m^(-1),矫顽力增加的原因是磁体单位体积内磁性主相体积分数增加导致对矫顽力的贡献增大,更多的等轴晶参与热变形、晶粒发生碎化、晶粒尺寸细化;随着α的继续降低,热变形磁体的剩磁、矫顽力将分别进一步增加、减小,矫顽力减小的原因是片状晶的比例增加,片状晶的晶粒尺寸增大,单畴晶粒占总磁畴晶粒比例减小,片状晶出现退磁场。实验统计热变形Nd_(10.5)Pr_(2.5)Fe_(80)Nb_1B_6矫顽力与等效平均晶粒尺寸的关系拟合曲线为Hci=1523.3786+0.3280D-0.0031D2,矫顽力随等效平均晶粒尺寸的增大而降低。

Ndl0.5Pr2.5Fe80Nb1B6 magnets were die-upset at height reduction of 60%, with compacts of 250 - 380 μm or 〈 250 μm particles. The magnets were detected by X ray diffraction （XRD） , scanning electron microscopy （SEM） , and measured by a hystere- sis loop tester at room temperature. The microstructures, magnetic properties, orientation degree and coercive force mechanism were studied and discussed, α, the angle between the equivalent easy magnetization axis of the magnets and the die-upset pressure direc- tion, was calculated. As α decreased from 26.0° to 9.1°, the density of the magnets increased from 7.04 to 7.49 g.cm-3 , Mr/Ms （Mr, MS was the remnant magnetization, and the saturation one, respectively） was from 0.78 to 0.91, the magnetic energy increased, but the coercive force climbed first from 1346 to 1499 kA m-1, and then reduced to 1216 kA m-1. The climbing reasons were that more grains of the magnetic matrix phase in unit volume contributed to the coercive force, and more equiaxed grains joined the die-up- set process and were broken. And the reducing reasons were that more and larger lamellae grains were obtained while the bigger demag- netizing fields of the grains appeared, and the proportion of single-domain grains in magnets decreased. The curve fitting between coer- cive force of Nd10.5Pr2.5Fe80Nb1B6 die-upset magnets and their equivalent average grain size D, was Hci = 1523. 3786 ＋ 0. 3280D - 0. 0031D2. And the coercive force reduced as the grain size increased.