Microstructure and coercivity in alnico 9

Journal Article · 23 September 2018 · Journal of Magnetism and Magnetic Materials

DOI: https://doi.org/10.1016/j.jmmm.2018.09.085 · OSTI ID:1477200

Zhou, Lin ^[1]; White, Emma  ^[1]; Ke, Liqin ^[1]; Cullen, David A.  ^[2]; Lu, Ping ^[3]; Constantinides, S. ^[4]; McCallum, R. W. ^[1]; Anderson, I. E. ^[1]; Kramer, Matthew J. ^[1]

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1. Ames Lab., Ames, IA (United States)
2. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
3. Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)
4. Arnold Magnetic Technologies Corp., Rochester, NY (United States)

Magnetic property enhancement of alnico, a rare-earth free permanent magnet, is highly dependent on both the initial microstructure and the evolution of the spinodal decomposition (SD) inside each grain during the heat treatment process. The size, shape and distribution of the magnetic FeCo-rich (α₁) phase embedded in continuous non-magnetic AlNi-rich (α₂) phase as well as a minor Cu-enriched phase residing in between are shown to be crucial in controlling coercivity. Phase and magnetic domain morphology in a commercial alnico 9 alloy was studied using a combination of structural characterization techniques, including scanning electron microscopy, electron backscatter diffraction, aberration-corrected scanning transmission electron microscopy and Lorentz microscopy. Our results showed that casting created structural nonuniformity and defects, such as porosity, TiS₂ precipitates and grain misorientation, are heterogeneously distributed, with the center section having the best crystallographic orientation and minimal defects. The optimal spinodal is a “mosaic structure”, composed of rod-shape α1 phase with {110} or {100} planar faceting and diameter of ~30–45nm. There is also a Cu-enriched phase residing at the corners of two < 110 > facets of the α1 phase. Furthermore, it was observed that grain boundary phase reverse magnetization direction at lower external magnetic field compared to the SD region inside the grain. Improving grain orientation uniformity, reducing detrimental grain boundary phase volume fraction, and the branching of the α₁ rods, as well as their diameter, are promising routes to improve energy product of alnico.

• Research Organization: Ames Laboratory (AMES), Ames, IA (United States)
• Sponsoring Organization: USDOE Office of Energy Efficiency and Renewable Energy (EERE)
• Grant/Contract Number: AC02-07CH11358
• OSTI ID: 1477200
• Alternate ID(s): OSTI ID: 1542141; OSTI ID: 1636403; OSTI ID: 1490587
• Report Number(s): IS-J--9763; PII: S0304885318303779
• Journal Information: Journal of Magnetism and Magnetic Materials, Journal Name: Journal of Magnetism and Magnetic Materials Journal Issue: C Vol. 471; ISSN 0304-8853
• Publisher: ElsevierCopyright Statement
• Country of Publication: United States
• Language: English