Grain boundary diffusion in nanocrystalline Nd-Fe-B permanent magnets with low-melting eutectics. (15th August 2016)

• Record Type: Journal Article
• Authors: Sawatzki, Simon; Kübel, Christian; Ener, Semih; Gutfleisch, Oliver
• Is Part Of: Acta materialia. Volume 115(2016)
• Page Start: 354
• Page End: 363
• Publication Date: 2016-08-15
• Subjects: Permanent magnet -- Nd-Fe-B -- Hot-deformation -- Grain boundary diffusion process (GBDP) -- Low melting eutectic; Materials -- Periodicals; Materials science -- Periodicals; Materials -- Mechanical properties -- Periodicals; Metallurgy -- Periodicals; Chemistry, Inorganic -- Periodicals; 620.112
• DOI: 10.1016/j.actamat.2016.05.048
• Languages: English
• ISSNs: 1359-6454

Abstract

Abstract: In order to combine the good thermal stability of nanocrystalline Nd-Fe-B magnets with the efficient grain boundary diffusion process (GBDP), low-melting eutectics have been mixed with Nd-Fe-B melt-spun ribbons, hot-compacted and subsequently die-upset. Transmission electron microscopy (TEM) analysis revealed the formation of 5–10 nm thick Dy-shells and a crystallography dependent diffusion into the individual solid grains on the nanoscale. Subsequent annealing at 600 °C leads to an enhanced diffusion but also some undesired Nd-O phases and induced grain growth depending on the annealing time, degree of deformation, strain rate and position within the sample. An optimized distribution of the low-melting eutectic was realized by milling the precursor powder and by using ternary alloys with reduced melting points. As a consequence a much higher effective increase in coercivity per wt%Dy was obtained compared to a homogeneous Dy-distribution which demonstrates the validity of this new approach. Graphical abstract: Highlights: Formation of Dy-rich nano shells was observed with HR-TEM. Dy-diffusion into the individual solid grain depends on crystallograph-ic orientation. Powder milling and the usage of ternary alloys increase homogeneity and effective increase in coercivity. Faster die-upsetting suppresses grain growth and undesired phase formation and increases absolute coercive values.