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On the reversible and fully repeatable increase in coercive field of sintered Nd–Fe–B magnets following post sinter annealing

Woodcock, T. G. and Bittner, F. and Mix, T. and Müller, K.-H. and Sawatzki, S. and Gutfleisch, O. (2014):
On the reversible and fully repeatable increase in coercive field of sintered Nd–Fe–B magnets following post sinter annealing.
In: Journal of Magnetism and Magnetic Materials, Elsevier Science Publishing, pp. 157-164, 360, ISSN 03048853,
[Online-Edition: http://dx.doi.org/10.1016/j.jmmm.2014.02.025],
[Article]

Abstract

The effect of different heat treatments on coercivity (Hc) has been investigated in three different Nd–Fe–B magnets. These were: commercially available Dy-free and Dy-containing Nd–Fe–B sintered magnets, both with minor additions of Al and Cu, and a third Nd–Fe–B sintered magnet which contained no measurable Cu but a similar amount of Al to the other materials. Heat treating the materials in the as-sintered state at 500 °C for 1 h resulted in an increase in Hc of ≈30% in all cases. Taking these annealed materials and heat treating them at 1050 °C for 1 h resulted in a reduction in Hc of a similar magnitude. This process was repeated several times and Hc changed by ≈30% each time. The Hc increase was therefore not only reversible but also fully repeatable. The remanence and oxygen content were stable throughout. Experiments with annealing duration showed that the beneficial effects to Hc were complete after <5 min at 500 °C. A simple mechanism to explain the results was proposed, which implied that differences in the distribution of the Nd-rich phases between the low and high Hc states should be present. Scanning and transmission electron microscopy confirmed the presence of such differences. In the Cu doped materials, the increase in Hc was linked to the melting of a Nd–Cu eutectic. This reaction was not observed in the low-Cu material, indicating that it was not essential for the increase in Hc.

Item Type: Article
Erschienen: 2014
Creators: Woodcock, T. G. and Bittner, F. and Mix, T. and Müller, K.-H. and Sawatzki, S. and Gutfleisch, O.
Title: On the reversible and fully repeatable increase in coercive field of sintered Nd–Fe–B magnets following post sinter annealing
Language: English
Abstract:

The effect of different heat treatments on coercivity (Hc) has been investigated in three different Nd–Fe–B magnets. These were: commercially available Dy-free and Dy-containing Nd–Fe–B sintered magnets, both with minor additions of Al and Cu, and a third Nd–Fe–B sintered magnet which contained no measurable Cu but a similar amount of Al to the other materials. Heat treating the materials in the as-sintered state at 500 °C for 1 h resulted in an increase in Hc of ≈30% in all cases. Taking these annealed materials and heat treating them at 1050 °C for 1 h resulted in a reduction in Hc of a similar magnitude. This process was repeated several times and Hc changed by ≈30% each time. The Hc increase was therefore not only reversible but also fully repeatable. The remanence and oxygen content were stable throughout. Experiments with annealing duration showed that the beneficial effects to Hc were complete after <5 min at 500 °C. A simple mechanism to explain the results was proposed, which implied that differences in the distribution of the Nd-rich phases between the low and high Hc states should be present. Scanning and transmission electron microscopy confirmed the presence of such differences. In the Cu doped materials, the increase in Hc was linked to the melting of a Nd–Cu eutectic. This reaction was not observed in the low-Cu material, indicating that it was not essential for the increase in Hc.

Journal or Publication Title: Journal of Magnetism and Magnetic Materials
Volume: 360
Publisher: Elsevier Science Publishing
Uncontrolled Keywords: Permanent magnets, Nd–Fe–B, Coercivity, Post sinter annealing, Microstructure, Transmission electron microscopy, Scanning electron microscopy, Reversibility, Intergranular phase
Divisions: 11 Department of Materials and Earth Sciences > Material Science > Functional Materials
11 Department of Materials and Earth Sciences > Material Science
11 Department of Materials and Earth Sciences
Date Deposited: 31 Mar 2014 08:20
Official URL: http://dx.doi.org/10.1016/j.jmmm.2014.02.025
Identification Number: doi:10.1016/j.jmmm.2014.02.025
Funders: The authors gratefully acknowledge financial support from the Toyota Motor Corporation.
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