J. Fidler

Coercivity of precipitation hardened cobalt rare earth 17:2 permanent magnets

Abstract The influence of the microstructure on the coercivity has been investigated by means of transmission electron microscopy. It is shown that a thin coherent (CoCu) 5 Sm-cell boumdary phase, separating cells of 17:2-crystal structure, acts as a pinning centre for magnetic domain walls. The attractive interaction force is interpreted in terms of a micromagnetic theory for domain wall pinning. The coercive force is determined by the domain wall energy gradient and by the magnetoelastic coupling energy between domain wall stresses and lattice deformation strains. The calculated coercive force due to the lattice mismatch, originated by the cellular coherent precipitation structure, is comparable to the experimentally obtained values.

Electron microscopy of Nd-Fe-B based magnets

Abstract Transmission electron microscopy has been used widely to characterize the complex multiphase microstructure of Nd 2 Fe 14 B based permanent magnets. The grain size of the magnets strongly depends on the processing technique. Our study of sintered magnets revealed two types of grain boundaries: one containing no intergranular phase between hard magnetic grains and one composed of nonmagnetic Nd-rich phases. In doped sintered magnets the dopant is dissolved in the hard magnetic φ-phase. In the case where the solubility of the dopant is low at the sintering temperature (Nb, Mo, Zr), precipitates are formed within the φ-phase. Dopants also form new intergranular phases and influence the wetting of the liquid phase and the smoothness of the surface of the φ-grains during sintering and therefore affect the coercivity.

Analytical microscope studies of sintered Nd-Fe-B magnets

Various sintered Nd 15 Fe 77 B 8 permanent magnets supplied by different producers have been examined by analytical electron microscopy. An identical microstructure was found in all of the different magnets. Three phases were detected by means of high resolution electron microscopy, electron diffraction and x-ray microanalysis. Besides the hard magnetic boride phase Nd 2 Fe 14 B, a soft magnetic boride phase close to Nd 2 Fe 7 B 6 , and a Nd-rich sintering aid phase (d.h.c.p. structure) were identified. The Nd-rich phase preferentially occurs along grain boundaries as a thin layer with about 10 nm tickness. The soft magnetic phases determine the magnetization reversal and limit the coercivity of sintered Nd-Fe-B magnets.

Microstructure of precipitation hardened cobalt rare earth permanent magnets

Abstract A transmission electron microscope study of several precipitation hardened cobalt rare earth magnets has been undertaken. The magnets were in peak aged condition and varied in the chemical composition. The studies reveal a fine cellular microstructure. The shape and size of cells depend on the heat treatment and alloying elements. The cell interiors consist of at least two plate-shaped 17:2 phases and are surrounded by a 5:1 boundary phase. The replacement of zirconium with hafnium does not alter the cellular morphology and also leads to a high coercivity. The coercive force is strongly influenced by the shape and size of the cellular structure as well as by the lattice misfit between the 5:1 and 17:2 phases. The influence of the microstructure on the coercivity is discussed.

Electron microscope study of precipitation in a niobium-containing (Nd, Dy)-Fe-B sintered magnet

Abstract The microstructure of a niobium-containing (Nd, Dy)-Fe-B alloy has been investigated using transmission electron microscopy, STEM and SEM X-ray microanalysis and optical microscopy, Magnetic measurements showed an increase in coercivity when niobium was added to a (Nd, Dy)-Fe-B magnet. The microstructure was found to be similar to that of a ternary Nd-Fe-B magnet but with two additional phases, a Laves Fe2Nb compound and a finely dispersed niobium containing coherent precipitate found in the magnetically hard phase. Lorentz microscopy indicated domain wall interactions with the fine precipitates which may be responsible for the enhanced coercivity of the niobium doped magnet.

Analytical TEM study of Al-doped, “two-phase” Nd–Fe–B sintered magnets

The microstructure and the coercivity of sintered Nd 20 Fe (73.5- x ) B 6.5 Al x ( x = 0, 2.5) permanent magnets are influenced by the Al concentration. In Al-containing magnets we found a homogeneous distribution of Al in the hard magnetic phase and the occurrence of an intergranular Nd(Fe, Al) 2 , phase between the hard magnetic grains. Our analytical TEM study revealed that in “Al 2 O 3 -doped” magnets the crystal structure of the Nd-rich intergranular phase partly changes from fcc ( a = 0.52 nm) to hep ( a = 0.39 nm, c = 0.61 nm), whereas the content of iron simultaneously decreases from 5–10 at.% to 1.5–4 at.%. A platelet-shaped phase, embedded in the Nd-rich intergranular phase, was determined as a Nd 5 Fe 2 (B, O) x , phase. The influence of the microstructure on the coercivity in Al-doped magnets is discussed.

Al-doped Nd-Fe-B permanent magnets: wetting and microstructural investigations

The coercivity of Nd-Fe-B sintered magnets has been improved by Al additions. To study the microstructural effect of Al addition, two types of investigation were formed: (1) the change in the wetting behavior of the liquid phase, at the sintering temperature, was studied by measurements of the wetting angle; and (2) the microstructural changes resulting from Al additions were studied by transverse electron microscopy assisted by energy dispersive X-ray analysis. A systematic change was detected that depended on the kind of Al addition, i.e., metallic or as Al/sub 2/O/sub 3/. >

On the role of the ND-rich phases in sintered ND-FE-B magnets

Sintered Nd15-Fe77-B8 magnets contain a multiphase microstructure. Our analytical transmission electron microscope study reveals that the Nd-rich phase, which is an essential part of the liquid phase sintering process can be divided into at least 4 subtypes with different Nd:Fe-ratios. Impurities of the raw material such as Nd-oxides and Nd-chlorides are randomly found in magnets, partly in the form of small inclusions up to 500 nm diameter within 2:14:1-hard-magnetic grains and partly in the form of large grains up to several microns diameter. As consequence of this analytical study a high concentration of boron vacancies as well as iron vacancies, especially in grain boundary regions is proposed. We have found that the degree of continuity of the intergranular Nd-rich phase is proportional to the intrinsic coercivity of the magnet. The intrinsic coercivity of sintered Nd-Fe-B magnets is determined by the nucleation of reversed domains and by the expansion of the reversed muclei, which is hindered by the non-magnetic Nd-rich oxide grain boundary layer phase.

TEM-study of the precipitation of iron in Nd-Fe-B sintered magnets

The microstructure of sintered Nd-Fe-B magnets is studied by analytical electron microscopy. The presence of Fe-rich precipitates was first found in magnets showing a small kink in the demagnetization curve, which is an evidence for soft magnetic nucleation centers for reversed magnetic domains within the magnet. alpha -iron precipitates occurred outside as well as within the hard magnetic 2:14:1 grains and were also found in other commercial-grade magnets with a nominal composition close to Nd15-Fe77-B8. >

Computer-Controlled Domain Detector

This paper is considered a first approach to detecting the domain configuration of coated Si–Fe sheets by computer-controlled field scanning. A topogram of the domains is plotted, and the physical origin of the detected field components is discussed. Also, stray field topograms of magnetized sheets are presented. These indicate that the field intensity cannot be associated with the angle of misorientation of an individual grain.