Microsc. Microanal. 23, 425–430, 2017 doi:10.1017/S1431927616012770
© MICROSCOPY SOCIETY OF AMERICA 2017
The Effects of the Addition of Dy, Nb, and Ga on Microstructure and Magnetic Properties of Nd2Fe14B/ α-Fe Nanocomposite Permanent Magnetic Alloys
Kezhi Ren,1 Xiaohua Tan,1,* Heyun Li,1 Hui Xu,1 and Ke Han2
1School of Materials Science and Engineering, Institute of Materials, Shanghai University, Shanghai 200072, P. R. China 2National High Magnetic Field Laboratory, Florida State University, 1800 E. Paul Dirac Drive, Tallahassee, FL 32310, USA
Abstract: We study the effects of Dy, Nb, and Ga additions on the microstructure and magnetic properties of Nd2Fe14B/α-Fe nanocomposites. Dy, Nb, and Ga additions inhibit the growth of the soft magnetic α-Fe phase. Dy and Nb additions are able to refine the microstructure, whereas Ga addition plays only a minor role in prohibiting crystal growth. The magnetic properties are sensitive to Dy, Nb, and Ga additions. The Dy-containing alloy enhances the intrinsic coercivity of 872kA/m because Dy partially replaces Nd, forming (Nd, Dy)2Fe14B. Nb addition refines the microstructure, and consequently increases the exchange coupling between magnetic grains. The Nd9.5Fe75.4Co5Zr3B6.5Ga0.6 alloy exhibits the highest remanence (0.92 T) due to Ga addition.
Key words: atom probe, nanocomposite permanent alloy, melt spinning, microstructure, magnetic property
INTRODUCTION Nanocomposite permanent magnetic materials, first repor- ted by Coehoorn et al. (1988), attracted much attention because of their high remanence, high theoretical maximum magnetic energy product (Schrefl et al., 1994b) and low rare earth content. Nd2Fe14B-based nanocomposites are composed of a fine mixture of hard magnetic Nd2Fe14B phase and soft magnetic α-Fe phase. The exchange coupling between soft and hard magnetic grains with nanometer size enhances the remanence, however, it decreases the coercivity (Schrefl et al., 1994a). The substitution of Dy for Nd is an effective method to improve the coercivity in nanocomposite alloys by increasing the anisotropy field of the hard magnetic Nd2Fe14B phase (Chen et al., 2004; The et al., 2007). Unfortunately, Dy has been considered as an unfavorable element because of its increased price and uncertain avail- ability. Researchers have, therefore, been making efforts to reduce Dy content or search for Dy-free nanocomposite alloys. Nb and Ga were reported to optimize the microstructure and improve the magnetic properties in the nanocomposite alloys (Ping et al., 1999, 2002). In melt-spun nanocomposite ribbons, however, the role of Nb and Ga additions on the magnetic properties is unclear. Therefore, the effects of Nb and Ga on the magnetic properties of nanocomposite alloys need to be studied further. In this work, Nd2Fe14B/α-Fe nanocomposite alloys with
minor Dy addition (Nd8.5Fe76Co5Zr3B6.5Dy1) and Dy free (Nd9.5Fe76Co5Zr3B6.5,Nd9.5Fe75Co5Zr3B6.5Nb1,andNd9.5Fe75.4 Co5Zr3B6.5Ga0.6) were prepared by melt spinning. The effects of Dy, Nb, and Ga on both the microstructure and the mag- netic properties were investigated. The partitioning of Dy, Nb,
*Corresponding author.
tanxiaohua123@shu.edu.cn Received June 22, 2016; accepted December 21, 2016
and Ga in Nd2Fe14B/α-Fe nanocomposites was examined by atom probe tomography (APT) and related to magnetic prop- erty improvement. This work provides a route for designing nanocomposite permanent magnets with high performance and low cost.
EXPERIMENTAL
Ingots with nominal composition Nd9.5Fe76Co5Zr3B6.5 (A-0), Nd8.5Fe76Co5Zr3B6.5Dy1 (A-Dy), Nd9.5Fe75Co5Zr3B6.5Nb1 (A- Nb), and Nd9.5Fe75.4Co5Zr3B6.5Ga0.6 (A-Ga) (at%) were pre- pared by arc-melting pure metals and Fe–Balloy
in
an argon atmosphere. Ribbon samples were obtained by melt spinning in an argon atmosphere at a wheel speed of 10–30 m/s. The melt-spun ribbons were annealed between 963 and 983K for 4–5min at 3×10−3 Pa in order to develop a nanocrystalline microstructure. The samples with optimum heat treatment are shown in Table 1. The magnetic properties of the ribbon samples were measured using a Lake Shore 7407 (Lake Shore Cryotronics, Westerville, OH, USA) vibrating sample magnetometer with a maximum applied field of 1.8T. X-ray powder diffraction (XRD) patterns were recorded in a D/Max-2550 diffractometer (Rigaku Corporation, Tokyo, Japan) with Cu-Kα radiation. Transmission electron micro- scopy (TEM) was performed using a JEM 2010F (JEOL Ltd, Tokyo, Japan),witha field emission gun operating at 200 kV. Planar TEM samples were prepared by grinding ribbons to a thickness of 30 µm and subsequent electropolishing in a solu- tion of 5% HClO4+95% ethanol at 20V at 243±5K. APT samples were prepared by a two-stage electropolishing proce- dure. The first stage was performed in an electrolyte of 25 vol% perchloric acid in 75 vol% acetic acid at 20V, and the second stage was performed in an electrolyte of 2% perchloric acid in 2-butoxyethanol at 20V. The APT characterizations were carried out in a CAMECA Instruments LEAP4000×-HR
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