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Temperature-dependent Dy diffusion processes in Nd–Fe–B permanent magnets

Loewe, K. and Brombacher, C. and Katter, M. and Gutfleisch, O. (2015):
Temperature-dependent Dy diffusion processes in Nd–Fe–B permanent magnets.
In: Acta Materialia, Elsevier Science Publishing, pp. 248-255, 83, ISSN 13596454,
[Online-Edition: http://dx.doi.org/10.1016/j.actamat.2014.09.039],
[Article]

Abstract

Nd–Fe–B permanent magnets have been coated with 0.6 wt.% dysprosium and annealed at various temperatures to study the impact of the temperature-dependent Dy diffusion processes on both the magnetic properties and the microstructure. When optimum annealing conditions are applied the Dy processed magnets with initial coercivity of ∼1100 kA m−1 yield coercivity increases which can exceed 400 kA m−1 without a significant reduction of the remanent magnetic polarization. The improved stability against opposing magnetic fields can be observed up to a depth of ∼3 mm along the diffusion direction, restricting the application of the Dy diffusion process to either thin magnets or magnets with tailored coercivity gradients. While in the proximity of the Dy-coated surface, each grain has a Dy-enriched shell with a Dy content of ∼6 at.%; the Dy concentration decreases exponentially to ∼1.8 at.% after a diffusion depth of 400 μm and to ∼1 at.% after a diffusion depth of 1500 μm, as was found with wavelength dispersive X-ray spectroscopy and scanning transmission electron microscopy–energy dispersive X-ray spectroscopy, respectively. In the vicinity of the Dy-coated surface, the mechanism of the Dy-shell formation is attributed to the melting/solidification of a heavy-rare-earth-rich intermediate phase during high-temperature annealing. This is based on the observation that a constant Dy concentration over the width of the shells was found. Also an epitaxial relation between the Dy-poor core and the Dy-rich shell was observed by electron backscattered diffraction, which is supported by results obtained with Kerr microscopy.

Item Type: Article
Erschienen: 2015
Creators: Loewe, K. and Brombacher, C. and Katter, M. and Gutfleisch, O.
Title: Temperature-dependent Dy diffusion processes in Nd–Fe–B permanent magnets
Language: English
Abstract:

Nd–Fe–B permanent magnets have been coated with 0.6 wt.% dysprosium and annealed at various temperatures to study the impact of the temperature-dependent Dy diffusion processes on both the magnetic properties and the microstructure. When optimum annealing conditions are applied the Dy processed magnets with initial coercivity of ∼1100 kA m−1 yield coercivity increases which can exceed 400 kA m−1 without a significant reduction of the remanent magnetic polarization. The improved stability against opposing magnetic fields can be observed up to a depth of ∼3 mm along the diffusion direction, restricting the application of the Dy diffusion process to either thin magnets or magnets with tailored coercivity gradients. While in the proximity of the Dy-coated surface, each grain has a Dy-enriched shell with a Dy content of ∼6 at.%; the Dy concentration decreases exponentially to ∼1.8 at.% after a diffusion depth of 400 μm and to ∼1 at.% after a diffusion depth of 1500 μm, as was found with wavelength dispersive X-ray spectroscopy and scanning transmission electron microscopy–energy dispersive X-ray spectroscopy, respectively. In the vicinity of the Dy-coated surface, the mechanism of the Dy-shell formation is attributed to the melting/solidification of a heavy-rare-earth-rich intermediate phase during high-temperature annealing. This is based on the observation that a constant Dy concentration over the width of the shells was found. Also an epitaxial relation between the Dy-poor core and the Dy-rich shell was observed by electron backscattered diffraction, which is supported by results obtained with Kerr microscopy.

Journal or Publication Title: Acta Materialia
Volume: 83
Publisher: Elsevier Science Publishing
Uncontrolled Keywords: Nd–Fe–B, Permanent magnet, Coercivity enhancement, Grain boundary diffusion process, Microstructural characterization
Divisions: 11 Department of Materials and Earth Sciences
11 Department of Materials and Earth Sciences > Material Science
11 Department of Materials and Earth Sciences > Material Science > Functional Materials
Date Deposited: 17 Nov 2014 09:55
Official URL: http://dx.doi.org/10.1016/j.actamat.2014.09.039
Identification Number: doi:10.1016/j.actamat.2014.09.039
Funders: Financial support from the Federal Ministry of Education and Research (BMBF) via the PerEMot project (No. 03X4621A) is gratefully acknowledged.
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