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Thermomagnetic treatment effects on microstructure in Nd-Fe-B-type sintered magnets
Smith, Catherine Paige
Smith, Catherine Paige
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2016
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Optimizing the microstructure of sintered Nd-Fe-B type magnets has become increasingly important. Sintered magnets are key components to the growing industry of alternative energy, particularly wind turbines and electric car generators. With rising costs and limited supply of rare earth elements, special attention has been dedicated to improving magnetic properties of these magnets through processing rather than compositional modifications. The magnetic property needing the most improvement in Nd-Fe-B type sintered magnets is coercivity. Coercivity dictates the performance of magnets at temperatures still below the demagnetizing threshold temperature. It has been shown that annealing sintered magnets in a magnetic field can enhance coercivity when compared to conventional post-sinter annealing in the absence of a magnetic field. However, little is known about the microstructural changes that occur in sintered magnets as a result of these thermo-magnetic treatments. This work presents the microstructural characterization of Dy-free and Dy-containing sintered magnets that have undergone annealing in a magnetic field of 9T. Microstructural characterization techniques were used to identify phases, analyze the texture of phases identified, and to study intergranular phases. Microstructural findings were then used to propose a solidification scheme during annealing and connect such findings to magnetic property results, as well as to make suggestions for optimizing the manufacturing process. In addition to the magnetic phase, phases identified in the Dy-free and Dy-containing Nd-Fe-B sintered magnets consisted of Nb-rich precipitates, Nd-rich phases, and phases from the Nd-Fe-Cu ternary system. Nd-Fe-Cu phases included α-Nd, NdCu, and the τ (Nd6Fe13Cu). The Nd-rich phases were specifically identified to be NdOx precipitates in an α-Nd matrix, and contained an orientation relationship described by (0001)α-Nd
(111)NdOx and [112 ̅0]α-Nd
[1 Ì…10]NdOx. This orientation relationship, as well as the other phases found in the microstructure, were unaffected by the processing treatments. While no effects of the magnetic field were observed in either magnet; the coercivity improved with thermal annealing in the Dy-free sintered magnet. The coercivity in the Dy-containing sample remained unchanged. It was concluded that the coercivity enhancements in the Dy-free sintered magnet were likely due to improved grain boundary phase characteristics. It was proposed that cooling rates of the Nd-Fe-Cu ternary system phases largely affect the grain boundary phases and the wetting by the liquid during thermal treatments. It is suggested that solidification formation of the Ï„ phase prior to eutectic final solidification enriches the final liquid, found in grain boundary regions, in Cu. Cu-enrichment is known to assist in grain boundary wetting which may smooth reduce the surface roughness of Nd2Fe14B grains, and thus improve coercivity by reducing nucleation sites for reverse magnetic domains.
(111)NdOx and [112 ̅0]α-Nd
[1 Ì…10]NdOx. This orientation relationship, as well as the other phases found in the microstructure, were unaffected by the processing treatments. While no effects of the magnetic field were observed in either magnet; the coercivity improved with thermal annealing in the Dy-free sintered magnet. The coercivity in the Dy-containing sample remained unchanged. It was concluded that the coercivity enhancements in the Dy-free sintered magnet were likely due to improved grain boundary phase characteristics. It was proposed that cooling rates of the Nd-Fe-Cu ternary system phases largely affect the grain boundary phases and the wetting by the liquid during thermal treatments. It is suggested that solidification formation of the Ï„ phase prior to eutectic final solidification enriches the final liquid, found in grain boundary regions, in Cu. Cu-enrichment is known to assist in grain boundary wetting which may smooth reduce the surface roughness of Nd2Fe14B grains, and thus improve coercivity by reducing nucleation sites for reverse magnetic domains.
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