Microstructural studies and micromagnetic analysis of nanocrystalline NdFeCoMB (M = Ga, Ge) melt-spun ribbon
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-Fe-B–type magnets are well known because of their interesting hard magnetic properties. In the past two decades the improvement of various magnetic properties of this alloy such as saturation magnetization (Ms), maximum energy product (BH)max, intrinsic coercive field (iHc), and Curie temperature have been achieved by applying various processing routes and the addition of various alloying elements.[1,2] There are two major processing routes for the production of Nd-Fe-B–based magnets; one is the powder metallurgy route for producing anistoropic sintered magnets and the other is the rapid solidification route for producing isotropic bonded magnets. Fidler et al.[2,3] presented a typical category for the additives; type 2 elements such as Ga, Al, Cu, and Ge with low solubility in the Nd2Fe14B phase, which concentrate in Nd-rich phase and improve the wettability of the phase at grain boundaries and the decoupling of Nd2Fe14B grains. Type 2 elements play an important role in controlling the final magnetic properties by modifying the microstructure. Type 1 elements such as Co, Ni, Dy, and Tb modify the intrinsic magnetic properties by substituting in the Nd2Fe14B phase.[2–5] Liquid phase sintering of microcrystalline sintered Nd-Fe-B magnets and hot-compaction and deformation in nanocrystalline Nd-Fe-B magnets take place due to the existence of intergranular Nd-rich phase. This phase is mainly observed as a triple junction phase or a surrounding intergranular ultrathin layer (1 to 2 nm) in sintered Nd-Fe-B magnets. An even distribution of Nd-rich phase around the matrix grains is known to give rise to an improved magnetic decoupling of the grains; that is, smoother boundaries cause higher coercivities.[4–6] Therefore, detailed chemical and structural analysis of the grain boundary phase is essential for the understanding of the microstructure–coercivity relationship in these magnets. R. GHOLAMIPOUR, Researcher, and A. BEITOLLAHI, Associate Professor, are with the Nanomaterial Group, Department of Metallurgy and Materials Engineering, Iran University of Science and Technology, Narmak, Tehran, Iran. Contact e-mail: [email protected] V.K. MARGHUSIAN, Professor, is with the Department of Metallurgy and Materials Engineering, Iran University of Science and Technology, Narmak, Tehran, Iran. T. OHKUBO, Senior Researcher, and K. HONO, NIMS Fellow, are with the National Institute for Materials Science (NIMS), Tsukuba, Japan. Manuscript submitted July 18, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS
Extensive microstructural studies have been carried out on the Nd-rich phase using various techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), and electron energy loss spectroscopy (EELS).[6–12] However, due to the limitations of these methods originating from the convolution effect in the analysis of nanoscale particles embedded in the matrix phase, most of the analyses were carried out focusing on the Nd-rich phase formed at triple junctions rath
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