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Sepehri-Amin H.,Japan National Institute of Materials Science | Ohkubo T.,Japan National Institute of Materials Science | Hono K.,Japan National Institute of Materials Science | Guth K.,Fraunhofer Institute for Materials Recycling and Resource Strategies | And 2 more authors.
Acta Materialia | Year: 2015

Microstructure evolution in Nd12.8Fe80.1B6.6Ga0.3Nb0.2 alloy powders at different hydrogen pressures during the hydrogen-disproportionation process PH2HD has been studied in order to understand the underlying mechanism of the texture development during the dynamic hydrogen-disproportionation-desorption-recombination (d-HDDR) process. Transmission electron microscopy showed that Fe2B grains "memorize" the crystallographic orientation of the initial Nd2Fe14B phase and transfer it to the recombined Nd2Fe14B grains in the highly textured sample. 3-D tomography of backscattered electron SEM images showed that recombined Nd2Fe14B grains nucleate at the interfaces of Fe2B/NdH2 phases, which grow through the interfaces of NdH2/α-Fe phases during the DR process. Boron segregation was found at the NdH2/α-Fe interfaces, which serve as a boron source for the growth of the recombined Nd2Fe14B grains. The mechanism of the texture development is discussed based on the microstructure observations and the corresponding evolution of magnetic properties. © 2014 Acta Materialia Inc. Source


Pal S.K.,Leibniz Institute for Solid State and Materials Research | Guth K.,Leibniz Institute for Solid State and Materials Research | Guth K.,Fraunhofer Institute for Materials Recycling and Resource Strategies | Woodcock T.G.,Leibniz Institute for Solid State and Materials Research | And 2 more authors.
Journal of Physics D: Applied Physics | Year: 2013

Textured, polycrystalline Nd2Fe14B powders, produced by dynamic hydrogenation disproportionation desorption and recombination (d-HDDR) were further processed by wet and surfactant-assisted ball milling. After 4 h of milling at 400 rpm in absolute ethanol and heptane + oleic acid, the polycrystalline d-HDDR particles had disintegrated, via intergranular fracture, into the individual grains i.e. isolated single crystalline particles of size 200 to 500 nm. An excellent degree of alignment was produced in the single crystalline particles using an applied magnetic field. This was reflected in the remanence of the field-aligned single crystalline powder (148.1 emu g-1) which was far higher than that of field-aligned un-milled d-HDDR powder (119.5 emu g-1). Milling the single crystalline powder further at 800 rpm in the same media produced polycrystalline flakes of size 0.2 to 1.0 μm. The polycrystalline flakes showed (0 0 l) in-plane texture and thus oriented edge to edge in an applied field. © 2013 IOP Publishing Ltd. Source


Loewe K.,TU Darmstadt | Brombacher C.,Vacuumschmelze GmbH and Co. KG | Katter M.,Vacuumschmelze GmbH and Co. KG | Gutfleisch O.,TU Darmstadt | Gutfleisch O.,Fraunhofer Institute for Materials Recycling and Resource Strategies
Acta Materialia | Year: 2014

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. © 2014 Acta Materialia Inc. Source


Sawatzki S.,TU Darmstadt | Dirba I.,TU Darmstadt | Schultz L.,Leibniz Institute for Solid State and Materials Research | Gutfleisch O.,TU Darmstadt | Gutfleisch O.,Fraunhofer Institute for Materials Recycling and Resource Strategies
Journal of Applied Physics | Year: 2013

The effect of deformation and DyF3 additions on the electrical resistivity and the magnetic performance has been studied in hot-deformed Nd-Fe-B melt-spun ribbons and correlated with respective microstructures. Despite the nanocrystallinity of hot-compacted magnets, the specific electrical resistivity measured by four-point-method was shown to be comparable with that of sintered magnets. Die-upsetting reduces electrical resistivity within the magnetically hard plane because of an enhanced shape anisotropy of the grains. The addition of DyF3 overcompensates this reduction due to the presence of electrically insulating Dy-F rich inclusions and thus reduces eddy current losses within the magnet. Magnetic measurements reveal an increase in coercivity without a change in remanence for die-upset magnets with a total height reduction of 63% and 1.2 wt. % Dy (1.6 wt. %DyF3). Both properties, remanence and coercivity, demonstrate an effective reduction in heavy rare earth Dy for Nd-Fe-B magnets. © 2013 AIP Publishing LLC. Source


Krautz M.,Leibniz Institute for Solid State and Materials Research | Krautz M.,TU Dresden | Krautz M.,TU Darmstadt | Skokov K.,TU Darmstadt | And 8 more authors.
Journal of Alloys and Compounds | Year: 2014

Fully hydrogenated Mn containing LaFeSi alloys provide long time stability of attractive magnetocaloric properties; issues of previously reported induced phase co-existence can be avoided. In this paper, two series LaFe 11.8-xSi1.2Mnx and LaFe11.6-xSi1.4Mnx (x = 0, 0.1, 0.2, 0.3, 0.4), before and after hydrogenation are presented. In the non-hydrogenated parent samples, microstructural evolution and phase formation with increasing Mn content are studied. It is found that materials combination within the sample series can provide constant MCE over a large working temperature range. As a peculiarity, the hydrogenated compounds show a different dependence of the transition temperature as a function of Si content than the parent compounds. This behaviour is explained by comparing the amount of hydrogen incorporated in samples with different Si content. Moreover, an estimation for the maximum adiabatic temperature change and optimal magnetic field is given exemplarily for a non-hydrogenated and a hydrogenated compound. © 2014 Elsevier B.V. All rights reserved. Source

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