Magnetic field control of microstructural development in melt-spun Pr2Co14B

Abstract

In the processing of commercial rare earth permanent magnets, use of external magnetic fields is limited mainly to the alignment of anisotropic particles and the polarization of the finished magnets. Here we explore the effects of high magnetic fields on earlier stages of magnet synthesis, including the crystallization and chemical phase transformations that produce the 2:14:1 phase in the Pr-CoB system. Pr2Co14B alloys produced by melt-spinning were annealed in the presence of strong applied magnetic fields (H = 90 kOe). The resulting materials were characterized by x-ray diffraction, electron microscopy, and magnetization measurements. We find that magnetic fields suppress the nucleation and growth of crystalline phases, resulting in significantly smaller particle sizes. In addition, magnetic fields applied during processing strongly affects chemical phase selection, suppressing the formation of Pr2Co14B and α-Co in favor of Pr2Co17. The results demonstrate that increased control over key microstructural properties is achievable by including a strong magnetic field as a processing parameter for rare-earth magnet materials.

Effects of High Magnetic Fields on Phase Transformations in Amorphous Nd2Fe14B

“…Although a more detailed study would be required to fully understand this interesting observation, we can speculate that this may arise due to simultaneous thermodynamic and kinetic effects associated with the field and temperature. The thermodynamic stabilization of high moment phases by large magnetic fields has been previously reported in the literature [16] and theoretically predicted based on the free energy of formation calculations [7]. The annealing temperature of 600 °C is well above the Curie temperature of Nd2Fe14B (~320 °C) and below that of Fe (770 °C), which would be consistent with the promotion of Fe formation.…”

“…The ferromagnetic Nd 2 Co 17 phase and α-Co were also promoted and dominated above the Curie temperature of Nd 2 Co 14 B resulting in lower coercivity compared to the zero-field annealing. In the Pr-Co-B system [16] the magnetic field exhibited a deleterious effect on coercivity due to the stabilization of the Pr 2 Co 17 phase to higher annealing temperatures, though significant microstructural refinement was observed. Both of these Co-containing systems have a high moment, high Curie temperature phase (2:17) that appears to affect the phase evolution at high fields.…”

“…Some less intuitive effects are microstructural refinement, stabilization of phase fields to higher and/or lower temperatures, and hastening or slowing of kinetics. Several thermomagnetic processing studies on systems targeting the 2:14:1 hard magnetic phase are found in literature [11][12][13][14][15][16].…”

Manufacturing Processes for Permanent Magnets: Part II—Bonding and Emerging Methods

“…101,102 Perhaps the most consistently observed phenomenon observed when applying strong magnetic fields during annealing treatments is a refinement of the microstructure when compared with no-field annealing. Exploration into thermomagnetic processing of ferrous alloys have the greatest body of literature demonstrating this effect, 95,103,104 though several magnet material systems have demonstrated it as well, including Nd-Fe-B 105,106 and Pr-Co-B 107 systems. The mechanisms underlying this effect are not well understood due to the interplay of thermodynamic and kinetic processes.…”

A rapid heating and high magnetic field thermal analysis technique

“…Further motivating experiments in high static field thermal analysis are the observations of significant rate and magnetic field-dependent changes in microstructural morphology and phase equilibria that have been observed from microstructural analysis and dilatometry. These observations have been described theoretically through evaluation of the magnetic component in Gibbs free energy [16][17][18][19][20][21][22][23][24][25][26].…”