Regensburg 2010 – scientific programme

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MA: Fachverband Magnetismus

MA 21: Magnetic Materials

MA 21.9: Talk

Wednesday, March 24, 2010, 17:30–17:45, H23

Grain refinement in HDDR Nd₂Fe₁₄B powders by high pressure reactive milling — •Konrad Güth, Julia Lyubina, Ludwig Schultz, and Oliver Gutfleisch — IFW Dresden, Institut für Metallische Werkstoffe, Postfach 270016, D-01171 Dresden, Germany

The HDDR process (Hydrogenation Disproportionation, Desorption, Recombination) is a unique method to produce highly coercive powders for resin bonded permanent magnets. The process is carried out under carefully controlled hydrogen atmosphere resulting in Nd₂Fe₁₄B crystallites with a size of about 300 nm well-oriented within micrometer-sized particles. A further increase of the energy product in these HDDR powders may be achieved via inter-grain exchange coupling that requires the decrease of the grain size by one order of magnitude. A novel approach is the high pressure reactive milling (HPRM) technique prior to the hydrogen desorption and recombination process. The NdFeBGaNb starting alloy is milled in dedicated ball vial under 50 bar hydrogen pressure for 5 hours. After recombination at elevated temperatures under reduced hydrogen atmosphere the final powder shows a much smaller grain size than that in the HDDR processed powder. Phase analysis and grain size determination were performed using Rietveld refinement of x-ray data. High resolution scanning electron microscopy (HR SEM LEO 1530 GEMINI) was used to study the microstructure. In order to investigate the texture of the final magnet, the powder is aligned applying a transverse magnetic field of 2 T during pressing. The influence of the HPRM on the structure and magnetic properties of the HDDR Nd-Fe-B alloys will be discussed.