添加Nb对烧结Nd-Fe-B永磁体显微结构与磁性能的影响
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摘要
Nd-Fe-B系永磁材料是迄今为止发现的磁性能最高的实用型永磁材料，享有“永磁王”的美誉。因此，多年来一直倍受磁材工作者的关注，大量的研究工作围绕着其成分设计、微量元素添加、显微结构改善以及制备工艺等展开。本实验采用传统的粉末冶金方法制备了烧结Nd_(15)Fe_(78-x)Nb_xB_7(其中x为Nb的原子分数)系列磁体，对其制备工艺及添加元素Nb在磁体中的作用进行了系统的研究。
通过研究具体工艺参数对磁体磁性能及显微结构的影响，总结出适合于该系列磁体的制备工艺如下：按设计成分将原料放入真空感应熔炼炉内进行合金铸锭的熔炼；合金铸锭在高纯氩气保护下粗破碎过40目筛；以15∶1的球料比在120#航空汽油的保护下滚动球磨80min；球磨好的磁体粉末在1.5T取向磁场下以80MPa的压力模压成型，接着在280MPa压力下进行冷等静压；所得的压坯在1110℃烧结1h，900℃回火处理1h，600℃回火处理2h。
三元Nd-Fe-B磁体中添加微量Nb元素，在不明显降低磁体剩磁和磁能积的前提下，不仅可以提高磁体的矫顽力，而且还可以显著改善磁体的温度特性。实验中研究了Nb的添加对Nd_(15)Fe_(78-x)Nb_xB_7系列磁体磁性能的影响，并探讨了这些影响的内在本质，即对合金铸锭及磁体微观结构的影响。研究结果表明：当x=0.75左右时磁体的磁性能较好；磁性能的改善得益于磁体微观结构的改善，添加Nb使烧结磁体的晶粒形状更加规则、尺寸趋于一致，晶间富Nd相分布更加均匀；而磁体微观结构的改善又得益于合金铸锭微观结构的改善，添加Nb使合金铸锭中Nd_2Fe_(14)B相的形状发生变化，由粗大的片状晶逐渐变为细小的颗粒状晶粒，同时还可适当抑制合金铸锭中a-Fe的析出；通过对比烧结磁体常温与高温磁性能，发现添加Nb在一定程度上改善了磁体的温度特性。
烧结Nd-Fe-B系永磁体的宏观磁性能与其微观结构密切相关，对两者之间的关系在定性了解的基础上，如果能进行定量的描述，则对理解烧结Nd-Fe-B系永磁体的矫顽力机理及各种磁学理论都有一定的指导意义。实验中对成问好等人推导出的一个半定量公式进行了讨论，并以实际磁体为例验证了该公式的合理性。
NdFeB-type permanent materials is a kind of practical permanent materials with the best magnetic properties up to now. It is named "the king of permanent magnet" and paid wildly attention for many years. Many of research works focusing on composition designing, elements addition, microstructure improvement and manufacture crafts are carrying out. In this experiment, sintered Nd15Fe78-xNbxB7 (x is the atom contents of Nb) permanent magnets are prepared by conventional powder metallurgical process. Nb addition and manufacture crafts have been carefully researched.
Systematic research about technology parameter's influences on the magnetic properties and microstructure of theses magnets have been done. The better parameters of production have been achieved. They are showed as follows: as-cast alloys with the nominal composition Nd15Fe78-xNbxB7 are prepared by induction-melting under argon atmosphere. As-cast alloys are broken under argon atmosphere and filtered through 40# screen. The coarse grain is ball-milled under 120# avgas for 80 min. The ratio of milling ball and grain is 15:1. Then fine powder is aligned and pressed in a magnet field of 1.5T. The pressure is 80MPa. The green compacts then are shaped by cold isostatic pressing under 280MPa pressure. The last compacts are sintered in vacuum at 1110℃ for 1h and annealed at 900℃ for 1h, then at 600℃ for 2h.
Adding Nb to ternary Nd-Fe-B magnet can enhance the coercivity of magnet without decreasing remanence and maximum energy product obviously. At the same time, Nb addition can improve the stability of temperature. In this experiment, the roles of Nb element in sintered Nd-Fe-B magnets have been studied. The results show that: the magnetic properties at x=0.75 are better. The improvement of the magnetic properties of sintered Nd-Fe-B magnets with Nb addition is due to the improvement of the microstructure of magnets and as-cast alloys. In the magnets with Nb addition the size of Nd2Fe14B phase is homologous and the figure of Nd2Fe14B phase is regularer. The Nd-rich phase is distributed dispersively. Nb addition make the microstructure of as-cast alloys to
change. The figure of Nd2Fe14B phase change from coarse patch to fine particle. Nb addition also can partly prevent the appearance of a -Fe in as-cast alloys. By comparing the magnetic properties in room and high temperature, we find Nb addition can improve the stability of temperature of sintered Nd-Fe-B magnets.
The magnetic properties of sintered Nd-Fe-B magnet strongly depend on its microstructure. Understanding the quantitative and qualitative relationships between the magnetic properties and the microstructure of magnets is useful to grasping the coercivity mechanism and all kinds of magnetic theories. In the paper, we discuss a formula derived by Chen Wenhao and test the reasonability of this formula using a practical magnet.
通过研究具体工艺参数对磁体磁性能及显微结构的影响，总结出适合于该系列磁体的制备工艺如下：按设计成分将原料放入真空感应熔炼炉内进行合金铸锭的熔炼；合金铸锭在高纯氩气保护下粗破碎过40目筛；以15∶1的球料比在120#航空汽油的保护下滚动球磨80min；球磨好的磁体粉末在1.5T取向磁场下以80MPa的压力模压成型，接着在280MPa压力下进行冷等静压；所得的压坯在1110℃烧结1h，900℃回火处理1h，600℃回火处理2h。
三元Nd-Fe-B磁体中添加微量Nb元素，在不明显降低磁体剩磁和磁能积的前提下，不仅可以提高磁体的矫顽力，而且还可以显著改善磁体的温度特性。实验中研究了Nb的添加对Nd_(15)Fe_(78-x)Nb_xB_7系列磁体磁性能的影响，并探讨了这些影响的内在本质，即对合金铸锭及磁体微观结构的影响。研究结果表明：当x=0.75左右时磁体的磁性能较好；磁性能的改善得益于磁体微观结构的改善，添加Nb使烧结磁体的晶粒形状更加规则、尺寸趋于一致，晶间富Nd相分布更加均匀；而磁体微观结构的改善又得益于合金铸锭微观结构的改善，添加Nb使合金铸锭中Nd_2Fe_(14)B相的形状发生变化，由粗大的片状晶逐渐变为细小的颗粒状晶粒，同时还可适当抑制合金铸锭中a-Fe的析出；通过对比烧结磁体常温与高温磁性能，发现添加Nb在一定程度上改善了磁体的温度特性。
烧结Nd-Fe-B系永磁体的宏观磁性能与其微观结构密切相关，对两者之间的关系在定性了解的基础上，如果能进行定量的描述，则对理解烧结Nd-Fe-B系永磁体的矫顽力机理及各种磁学理论都有一定的指导意义。实验中对成问好等人推导出的一个半定量公式进行了讨论，并以实际磁体为例验证了该公式的合理性。
NdFeB-type permanent materials is a kind of practical permanent materials with the best magnetic properties up to now. It is named "the king of permanent magnet" and paid wildly attention for many years. Many of research works focusing on composition designing, elements addition, microstructure improvement and manufacture crafts are carrying out. In this experiment, sintered Nd15Fe78-xNbxB7 (x is the atom contents of Nb) permanent magnets are prepared by conventional powder metallurgical process. Nb addition and manufacture crafts have been carefully researched.
Systematic research about technology parameter's influences on the magnetic properties and microstructure of theses magnets have been done. The better parameters of production have been achieved. They are showed as follows: as-cast alloys with the nominal composition Nd15Fe78-xNbxB7 are prepared by induction-melting under argon atmosphere. As-cast alloys are broken under argon atmosphere and filtered through 40# screen. The coarse grain is ball-milled under 120# avgas for 80 min. The ratio of milling ball and grain is 15:1. Then fine powder is aligned and pressed in a magnet field of 1.5T. The pressure is 80MPa. The green compacts then are shaped by cold isostatic pressing under 280MPa pressure. The last compacts are sintered in vacuum at 1110℃ for 1h and annealed at 900℃ for 1h, then at 600℃ for 2h.
Adding Nb to ternary Nd-Fe-B magnet can enhance the coercivity of magnet without decreasing remanence and maximum energy product obviously. At the same time, Nb addition can improve the stability of temperature. In this experiment, the roles of Nb element in sintered Nd-Fe-B magnets have been studied. The results show that: the magnetic properties at x=0.75 are better. The improvement of the magnetic properties of sintered Nd-Fe-B magnets with Nb addition is due to the improvement of the microstructure of magnets and as-cast alloys. In the magnets with Nb addition the size of Nd2Fe14B phase is homologous and the figure of Nd2Fe14B phase is regularer. The Nd-rich phase is distributed dispersively. Nb addition make the microstructure of as-cast alloys to
change. The figure of Nd2Fe14B phase change from coarse patch to fine particle. Nb addition also can partly prevent the appearance of a -Fe in as-cast alloys. By comparing the magnetic properties in room and high temperature, we find Nb addition can improve the stability of temperature of sintered Nd-Fe-B magnets.
The magnetic properties of sintered Nd-Fe-B magnet strongly depend on its microstructure. Understanding the quantitative and qualitative relationships between the magnetic properties and the microstructure of magnets is useful to grasping the coercivity mechanism and all kinds of magnetic theories. In the paper, we discuss a formula derived by Chen Wenhao and test the reasonability of this formula using a practical magnet.
引文
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[2] Tebble R. S., Craik D. J.. In: Magnetic Materials. London: Wilery-Interscience, 1969.
[3] Strnat K., Hoffer G., Olson J., et al. A Family of New Cobalt-Base Permanent Magnet Materials. J. Appl. Phys., 1967, 38(3): 1001
[4] Velge W. A. J. J., Buschow K. H. J.. Magnetic and Crystallographic Properties of some Rare Earth Cobalt Compounds with CaZn_5 Structure. J. Appl. Phys., 1968, 39(3): 1717-1719
[5] Das D. K.. IEEE Trans. Magn., 1969, MAG-5:214
[6] Bentz M. G, Martin. D. L.. Appl. Phys. Lett., 1970, 17(1): 176
[7] Martin D. L.. Cobalt, 1971, 50:11
[8] Ojima T., Tessahito T., Tetsao Y.. J. Appl. Phys., 1977, 16:671
[9] Mishra R. K., Thomas G., Yoneyama T., et al. Microstructure and properties of step aged rare earth alloy magnets. J. Appl. Phys., 1981, 52(3): 2517-2519
[10] Buschow K. H. J.. A Handbook on the properties of Magnetically Ordered Substances. In: Wohlfarth E. P., eds. Ferromagnetic Materials. North-Holland, Amsterdan: 1980. 297
[11] Sagawa M., Fujimura S., Togawa M., et al. New Materials For Permanent Magnets on a based of Nd and Fe. J. Appl. Phys., 1984, 55(6):2083
[12] 周寿增．超强永磁体—稀土铁系永磁材料．北京：冶金工业出版社，1999：15
[13] Kubaschewski O.. Iron-Binary phase diagrams, 1982:96
[14] Buschow K. H. J., Wohlfarth E. P. Permanent magnetic materials based On 3d-Rich Ternary Compounds in Ferromagnetic Materials. Elsevier Science Publishers B. V., 1988, 4:8
[15] Burzo E., Kirchamayer H. R.. Physical properties of R_2Fe_(14)B Based alloys. In: Gschneidner K. A., Eyring J. and L, eds. Handbook on the physics and chemistry of rare earth, Vol. 12. Elesvier Science publisher B. V., 1989.71
[16] Herbst J. E., Croat J. J., Pinkerton F. E., et al. Relationships between crystal structure and magnetic properties in Nd_2Fe_(14)B. Phys Rev, B, 1984, 29:4176
[17] Givord D., Li H. S., Moreau L. M.. Magnetic properties and crystal structure of Nd_2Fe_(14)B. Solid state commun, 1984, 50, 497
[18] Shoemaker C. B., Shoemaker D. P., Fruchart R.. The structure of a New magnetic
phase Related to the Sigma phase: Iron Neodymium Borides Nd_2Fe_(14)B. Acta Crystallogsect, 1984, C40:1665
[19] Bezinge A., Braun H. F., Muller J., et al. Tetragonal Rare Earth Iron Borides R_(1+ε) Fe_4B_4(ε=0.1) with incommensurate Rare Earth and Iron substructure. Solid state commum, 1985, 55:131
[20] 周寿增，唐伟忠，王润．烧结Nd-Fe-B永磁合金的边界显微结构与磁硬化．金属学报，1990，26(4)：290
[21] Kronmuller H., Durst K. D., Sagawa M.. Analysis of the magnetic hardening mechanism in RE-FeB permanent magnets. J. Magn. Magn. Mater., 1988, 74: 291-302
[22] Kronmuller H., Fischer R., Hertel R., et al. Micromagnetism and the microstructure in nanocrystalline materials. J. Magn. Magn. Mater., 1997, 175: 177-192
[23] Givord D., Lu Q., Rossignol M. F. Experiment approach to Coercitity analysis in hard magnetic materials. J. Magn. Magn. Mater., 1990, 83:183-188
[24] Kou X. C., Kronmuller H. K., Givord D.. Coercivity mechanism of sintered Pr_(17)Fe_(75)B_8 and Pr_(17)Fe_(53)B_(30) permanent magnets. Phys. Rev., 1994, 50(6): 3849-3860
[25] Gao Rewei, Zhou Shouzeng, Zhang Deheng. Coercitity and its dependence on the strength of alignment magnet field in Nd-Fe-B sintered magnets. J. Appl. Phys., 1995, 78(2): 1156
[26] Gao Rewei, li Wei, Ji Changguo, et al. Chinese Sci. Bull., 1998, 42:107
[27] 周寿增，郭灿杰，呼琴等．高磁能积低温度系数的铁基永磁合金的磁性能与组织结构．北京钢铁学院学报，1988，10(3)：317
[28] Katter M., Zapf L., Blank R., et al. Corrosion Mechanism of RE-Fe-Co-Cu-Ga-Al-B Magnets. IEEE Trans. Magn., 2001, 37(4): 2474
[29] Ma B. M., Krause R. F.. In: Bad Soden, eds. Proc. 5th Int. Symposium on Anisotropy and Coercivity in Rare Earth Transition Metal Alloys. Deutche Physikalishe: FRG, 1987. 141
[30] Zhou Shouzeng, Guo C, Hu Q. Magnetic Properties and Microstructures of Iron-Based Rare Earth Magnets with low Temperature Coefficients. J. Appl. Phys., 1988, 63(8): 3327
[31] Endoh M., Tokumaga M.. Nd-Fe-B Ba sed sintered Magnets with low temperature Coefficients. Proc. 10th Int. Workshop on Rare Earth Magnets and
Their Application. Tokyo, Japan: 1989. 449
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