The role of Cu addition in the coercivity enhancement of sintered Nd-Fe-B permanent magnets
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T. Akiya New Industry Creation Hatchery Center, Tohoku University, Sendai, Miyagi 980-8579, Japan
H. Kato New Industry Creation Hatchery Center, Tohoku University, Sendai, Miyagi 980-8579, Japan; and Department of Applied Mathematics and Physics, Yamagata University, Yamagata 990-8560, Japan
K. Hono National Institute for Materials Science, Tsukuba 305-0047, Japan (Received 30 June 2008; accepted 8 October 2008)
To understand the mechanism of the coercivity enhancement by a trace addition of Cu in Nd-Fe-B sintered magnets, we investigated the microstructure difference between Cudoped and Cu-free alloys using high resolution scanning electron microscopy (HRSEM), transmission electron microscopy (TEM), and laser assisted three dimensional atom probe (LA-3DAP). From a serial sectioning back scattered electron (BSE) images of the Nd-rich phase obtained by an integration of the focused ion beam (FIB) and HRSEM technique, it was found that Cu addition leads to a continuous formation of Nd-rich thin layers along the grain boundaries. 3DAP analysis has shown that a thin Cu-rich layer with a thickness of approximately 2 nm is present at the interface between the Nd2Fe14B and Nd-rich phase grains.
I. INTRODUCTION
The coercivity of Nd-Fe-B based commercial sintered magnets, which typically range from 10 to 15 kOe, is far less than the anisotropy field of the Nd2Fe14B phase (75 kOe). Although the coercivity can be increased by substituting Dy for Nd due to the increase of the anisotropy field, the coercivity increase can only be achieved at the expense of the remanence and energy product. Hence, the improvement of the coercivity without using Dy is strongly desired. The coercivity is known to be strongly related to the microstructure of the sintered magnets, which are typically composed of grains of the Nd2Fe14B hard magnetic phase, Nd-rich phase grains, and the Nd-rich grain boundary phase.1 There are at least two phases in the Nd-rich grains, one is cubic NdOx and the other is dhcpNd.2 The coercivity is believed to be largely influenced by the presence and the morphology of the Nd-rich grain boundary phase, which is typically a thin amorphous layer, formed along the grain boundaries of the Nd2Fe14B grains.3 A lot of effort has been made to increase the coercivity of sintered Nd-Fe-B magnets. Some additives such as Cu, Al, and Ga have long been known to increase the coercivity of sintered Nd-Fe-B magnets after an
L $ NdCu þ Nd at 520 C ;
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0041
L þ Nd2 Fe17 $ Nd6 Fe13 Cu þ NdCu at 512 C ;
J. Mater. Res., Vol. 24, No. 2, Feb 2009
http://journals.cambridge.org
optimal post-sinter annealing.4–6 Among these, Cu addition has been known to show the most pronounced effect on the coercivity increase after optimal post-sinter annealing.4 Even trace Cu addition shows a great beneficial effect on the coercivity.7 By increasing the amount of Cu addition up to 1.5 at.%, the coercivity of the magnet increases gradually,8–10 ther
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