Materials Science for High-Performance Permanent Magnets: Nd-Fe-B: Microstructure and Properties
Sponsored by: TMS Functional Materials Division, TMS: Magnetic Materials Committee
Program Organizers: Satoshi Hirosawa, National Institute for Material Science; Matthew Kramer, Iowa State University; Oliver Gutfleisch, Technische Universität Darmstadt; Hae-Woong Kwon, Pukyong National University

Monday 2:00 PM
February 27, 2017
Room: 24C
Location: San Diego Convention Ctr

Funding support provided by: Elements Strategy Initiative Center for Magnetic Materials

Session Chair: Satoshi Hirosawa, National Institute for Materials Science; Josef Fidler, Vienna University of Technology

2:00 PM  Invited
The Current State and Future of the Rare Earth Magnets: Hajime Nakamura1; 1Shin-Etsu Chemical Co., Ltd.
     Due to the high magnetic performance, Nd-Fe-B sintered magnets have a large share even after encountering the rare earths resource issue in 2010. In these last few years, rare earth magnet manufacturers have focused on saving heavy rare earths (HREs) apart from improvement of magnetic properties. There are three ways to enhance coercivity with less HREs or without HREs: grain size refinement, the Grain Boundary Diffusion process, and modification of the grain boundary phase. The demand for high performance magnets is expected to increase and we welcome alternative magnets containing less or no rare earths. Requirements for such new magnets would be a heat resistance up to around 150 degree C, and higher ratio of remanence to the cost in comparison with the Nd-Fe-B magnets.

2:30 PM  
Quantifying the True Enhancement in Coercivity by Dy Diffusion into Sintered Nd-Fe-B Alloys: Cajetan Nlebedim1; Matthew Kramer1; 1Ames Laboratory, US Department of Energy
    Coercivity and its temperature dependence can be improved by diffusing Dy into sintered Nd-Fe-B. It involves coating sintered Nd-Fe-B with DyF3 and facilitating diffusion by heat treatment. Magnetic properties of Dy-diffused Nd-Fe-B are reported to improve compared to the starting materials. However, this approach assumes that magnetic properties improvements are only due to the diffused Dy. This presentation will show that such assumption over-quantifies the effect of Dy. It will be shown that, with appropriate heat treatment, comparable improvement in magnetic properties can be obtained without Dy diffusion. This study provides insight into quantifying the true gain in magnetic properties via Dy diffusion into sintered Nd-Fe-B magnets. This is important because Dy is a critical magnet manufacturing raw material. Acknowledgement: This work was supported by the Critical Materials Institute, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office.

2:50 PM  
Microstructure and Coercivity in Ultra-fine Grained Nd-Fe-B Sintered Magnets: Taisuke Sasaki1; Tadakatsu Ohkubo1; Yasuhiro Une2; Hirokazu Kubo2; Masato Sagawa2; 1National Institute for Materials Science; 2Intermetallics Co. Ltd.
    Grain size refinement of Nd-Fe-B magnets is one effective way to increase the coercivity, and high coercivity (μoHc) of 2.0 T was reported by refining the grain size to about 1μm using He jet milling and pressless sintering. Further grain size refinement was recently achieved by using the powder prepared by hydrogen- disproportionation-desorption-recombination (HDDR) process; however, the coercivity of the ultrafine grained magnets processed by sintering the HDDR powder remained only 1.3 T. This work presents the microstructure of fine-grained sintered magnets fabricated from HDDR powders and He-jet milled powders to discuss the possible origin of the low coercivity of the sintered magnets fabricated from the HDDR process. The Nd2Fe14B grains in the sintered magnets fabricated from HDDR powders have sharp angular corner, and the Nd-rich grain boundary phase was thinner compared to that fabricated from jet-milled powders. This explains the low coercivity in the sample fabricated from HDDR powders.

3:10 PM  Invited
Grain Size Refinement of Ga-doped Nd-Fe-B Magnet: Yasuhiro Une1; Kazuhiro Kubo1; Tetsuhiko Mizoguchi1; Takahiko Iriyama1; Masato Sagawa1; Masashi Matsuura2; Satoshi Sugimoto2; 1Intermetallics Co., Ltd; 2Tohoku University
    We have investigated the coercivity of sub-micron grained Nd-Fe-B sintered magnets using both hydrogenation-disproportionation-desorption-recombination (HDDR) process and helium (He) jet-milling. The average grain size of the magnet was around 0.8 µm, and the magnet showed the excellent magnetic orientation ratio (Br/Js) of 95% and a good temperature coefficient of coercivity. However, the coercivities of the sintered magnets were around 13 kOe, which were rather lower than expected from the grain sizes. It is thought that these lower coercivities are resulted from the thin intergranular Nd-rich phases. Recently, Ga-doped Nd-Fe-B sintered magnet was reported to create the thick grain boundary and to get high coercivity. We developed fine grained Ga-doped magnets with the coercivity of 23 kOe using helium jet-milling

3:40 PM Break

4:00 PM  Invited
High-coercivity Dy-free Nd-Fe-B Permanent Magnets: Kazuhiro Hono1; 1National Institute for Materials Science
    In this talk, we will overview our recent progresses at NIMS toward the development of high coercivity Dy-free Nd-Fe-B permanent magnets. To obtain better understandings of the microstructure-coercivity relationships, we revisited the microstructures of Nd-Fe-B sintered and hot-deformed magnets using aberration-corrected STEM complemented by atom probe tomography (APT), magneto-optical Kerr microscopy and finite element micromagnetic simulations. We found that the intergranular phase parallel to the c-planes are mostly crystalline with a higher Nd concentration in contrast to that lying parallel to the c-axis that contains higher Fe content with an amorphous structure. Micromagnetic simulations suggest the reduction of the magnetization in the latter is critical to enhance the coercivity. Based on these new experimental findings, we developed a method to increase the coercivity of Nd-Fe-B hot-deformed magnets while keeping relatively high remanence.

4:30 PM  
Microstructural Engineering of Nd-Fe-B Permanent Magnets with Significantly Reduced Dy: Matt Tianen1; Catherine Galligan1; Jie Li1; Peter Moran1; Yongmei Jin1; 1Michigan Tech
    Nd-Fe-B magnets with good high temperature coercivity are desirable for applications like traction motors in electric vehicles. Reducing Dy content in these magnets is critical to lowering their cost. Results from our recent proof-of-concept study indicate that Nd-Fe-B with 2.5%Dy exhibits comparable coercivity to commercial magnets containing 7%Dy. This is due to the engineered microstructure where well-aligned submicron grains are magnetically isolated by nonmagnetic intergranular Nd-Cu phase. The magnet was produced by uniaxial hot pressing of Nd-Fe-B and Nd-Cu melt spun ribbons. The Dy content can be further reduced by microstructure optimization. In particular, the grain shape and the thickness of the intergranular phase both impact the coercivity. To quantitatively evaluate the effects of these factors and optimize them, micromagnetic modeling is employed to simulate magnetic domain evolutions in the two-phase Nd-Fe-B/Nd-Cu composite systems under various microstructure conditions. The findings from computer simulations and implications for future experimentation are discussed.

4:50 PM  
Electrical Resistivity Enhancement by Doping with Eutectic DyF3–LiF Salt Mixture in Nd-Fe-B Die-upset Magnet: Hae-Woong Kwon1; Kyung Min Kim1; Dong Hwan Kim2; Jung Gu Lee3; Ji Hoon Yu3; 1Pukyong National University; 2Star-group Ind. Co.; 3Korea Institute of Materials Science
    As an excessively high operating temperature of Nd-Fe-B magnet for traction motor in HEV, EV is due largely to eddy current, enhancing electrical resistivity of the magnet would be useful for decreasing the operating temperature. Enhancement of electrical resistivity in the Nd13.6Fe73.6Co6.6Ga0.6B5.6 die-upset magnet was attempted by doping with low-melting point eutectic DyF3–LiF salt mixture (eutectic temperature ≈ 700 °C). The salt mixture doping (> 4 wt%) led to a remarkable enhancement of electrical resistivity from ca. 190 μΩ.cm to over 400 μΩ.cm. During die-upset (735 °C) the added eutectic salt mixture became liquid. Remarkable enhancement of the electrical resistivity was attributed to homogeneous and continuous coverage of the inter-flake boundaries by the easily-melting dielectric eutectic salt mixture. Coercivity was also slightly enhanced from 13.5 kOe to 14.6 kOe. The enhancement of electrical resistivity and coercivity were achieved at the expense of remanence (from 13.0 kG to 10.9 kG).

5:10 PM  
Coercivity Enhancement of Hot-deformed NdFeB Magnets by GBDP with NdHx and Metallic Nanoparticles: Junggoo Lee1; Heeryoung Cha1; Younkyoung Baek1; Jihun Yu1; Haewoong Kwon1; 1Korea Institute of Materials Science
    Recently, much attention has been focused on HRE-free high coercive NdFeB magnets. Hot deformation has been proven quite useful to achieve this goal. However, hot-deformed magnets exhibit a very large specific interfacial area per unit volume due to ultra-fine grain size. So it is significantly important to control the phase along the interfacial area after hot-deformation to achieve high coercivity. In this study, NdHx and metallic nanopowder were employed for grain boundary diffusion process (GBDP). For GBDP, NdHx and metallic nanopowder were mixed in ethanol as diffusion source. The hot-deformed samples were dipped into diffusion source. After dried, the dip-coated samples were heat treated at 550 - 700℃ for GBDP. GBDP at lower temperature resulted in higher enhancement of coercivity with the same amount of Nd-Cu source. In addition, the GBDPed sample showed weak dependence of the coercivity on the grain alignment.