| Correlative Characterization from Atoms to Magnetic Fields in Tb-Doped Nd-Fe-B Magnets Kristina Zagar Soderznik1; Saso Sturm2; Andras Kovacs3; Aleksei Savenko4; Marko Soderžnik5; Rafal Dunin Borkowski6; Joachim Mayer7; Spomenka Kobe8; 1JOZEF STEFAN INSTITUTE, Ljubljana, Slovenia; 2HEAD OF DEPARTMENT FOR NANOSTRUCTURED MATERIALS, Ljubljana, Slovenia; 3ERNST RUSKA-CENTRE FOR MICROSCOPY AND SPECTROSCOPY WITH ELECTRONS, Juelich, Germany; 4THERMO FISHER SCIENTIFIC, Erlangen, Germany; 5JOžEF STEFAN INSTITUTE, Ljubljana, Slovenia; 6FORSCHUNGSZENTRUM JüLICH, Jülich, Germany; 7ERNST RUSKA-CENTRE FOR MICROSCOPY AND SPECTROSCOPY WITH ELECTRONS, Jülich, Germany; 8JOSEF STEFAN INSTITUTE, Ljubljana, Slovenia; PAPER: 347/SISAM/Regular (Oral) SCHEDULED: 14:00/Sat. 26 Oct. 2019/Dr. Christian Bernard ABSTRACT: High coercivity Nd-Fe-B permanent magnets play an important role in the rapidly-growing renewable energy sector. To retain the coercivity at high operating temperatures, heavy-rare-earth elements (HRE), such as Dy and Tb, are added using the grain-boundary diffusion (GBD) process. The addition of HRE results in a significant improvement of the coercivity due to the increase of the intrinsic resistance to demagnetization. [1] In the present study, we report on the correlation between magnetic properties and the distribution of Tb<sub>4</sub>O<sub>7</sub> in the Nd<sub>2</sub>Fe<sub>14</sub>B magnet. The Nd<sub>2</sub>Fe<sub>14</sub>B magnet was coated with Tb<sub>4</sub>O<sub>7</sub> powder and annealed. During the annealing process, Tb diffused along grain boundaries (GB) into the outer parts of Nd-Fe-B grains, thus forming core-shell grains with the Tb-rich shell and Nd-Fe-B core. Magnetometry measurements were performed to observe the Tb concentration gradient from the surface of the magnet into its central part. It was found that the coercivity gradually decreases towards the central part where it is still around 30% higher when compared with the untreated magnet. [2,3] Although magnetic measurements clearly indicate the presence of Tb, it is not clear what the actual amount of Tb is in central regions of magnets or how they are distributed in the microstructure and if it is possible to distinguish the magnetisation flux between soft magnetic shells and hard magnetic cores. For that purpose, we applied the Cs-corrected STEM: FEI Titan 80-200 equipped with SuperX electron dispersive X-ray (EDX) spectrometer and electron energy-loss (EEL) spectrometer and FEI Titan 80-300 equipped with electron biprism to perform electron holography. As a complementary method, atom probe tomography (APT) was used using 3D atom probe LEAP 4000x HR. In order to analyse the core-shell region, a lamella was prepared from the representative core-shell grains and the interface between the shell and the core was examined using EELS and APT. Detailed line-scans and spectrum image maps were performed at this interface. The estimated width of the transition area between the shell and the core was 20 nm. Further studies focused on the electron holography of core-shell grains. The magnetic fluxes were within the core and the shell was determined. The thickness and the composition of the shell were determined as a function of the specimen position within the magnet. References: [1] P. J. McGuiness, et al., JOM 67 (2015) 1306-1317. [2] M. Soderžnik, et al., Intermetallics 23 (2012) 158-162. [3] M.Soderznik, et al., Acta Materialia 115 (2016) 278-284 |
