Initial Magnetization Behavior of Rapidly Quenched Neodymium-Iron-Boron Magnets
- PDF / 981,130 Bytes
- 17 Pages / 420.48 x 639 pts Page_size
- 13 Downloads / 192 Views
INITIAL MAGNETIZATION BEHAVIOR OF RAPIDLY QUENCHED NEODYMIUM-IRON-BORON MAGNETS F. E. Pinkerton Physics Department, General Motors Research Laboratories, Warren, MI 48090-9055
ABSTRACT Initial magnetization and demagnetization data are reported for three forms of rapidly solidified Nd-Fe-B permanent magnet materials: melt-spun ribbons, hot pressed magnets, and die upset magnets. In all three materials the results are consistent with domain wall pinning at grain boundary phases as the coercivity mechanism. Optimally quenched ribbons are comprised of randomly oriented single domain Nd 2Fel 4B grains, and both initial magnetization and demagnetization are controlled by strong domain wall pinning at grain boundaries. Maximum coercivity is accompanied by a low initial permeability. Coercivity is reduced in overquenched ribbons by partial retention of a magnetically soft amorphous or very finely crystalline microstructure. Coercivity decreases in underquenched ribbons because wall pinning weakens as the grain size increases above optimum. Correlation of magnetization and demagnetization behaviors suggests that maximum coercivity in ribbons is largely determined by the resistance to domain wall formation in grains smaller than the single domain particle limit. Grain size is much less important in the aligned die upset magnets. Domain walls are initially free to move until they become strongly pinned at grain edges, and complete magnetization requires an applied field greater than the coercive field. Hot pressed magnets show a mixture of ribbon and die upset behavior.
1. INTRODUCTION The recent development of permanent magnet materials based on the ternary compound Nd 2 Fe1 4B[1] has sparked considerable scientific and technological interest.[2,3] Excellent permanent magnet properties have been achieved by both rapid solidification[4] and powder metallurgy[5] techniques. Rapid solidification by melt-spinning produces magnetically isotropic ribbon with a remanence Br and intrinsic coercivity Hri of 8 kG and 15 kOe, respectively. Ribbons can be consolidated into fully dense bulk magnets by hot pressing[6] to obtain a nearly isotropic magnet with an energy product (BH)maz.15 MGOe. Subsequent hot deformation, or die upsetting, induces alignment into a previously hot pressed magnet, yielding energy products in excess of 40 MGOe. This paper reviews the initial magnetization and demagnetization process in the three stages of rapid solidification-based magnet materials.[7,8] The strong links between magnetization behavior and microstructure are discussed, together with the implications for the coercivity mechanism in each stage. Much of the emphasis in this paper will be on the properties of melt-spun Nd-FeB ribbons, which have a unique finely microcrystalline microstructure with a grain size smaller than the estimated single domain particle limit.[9] While this microstructure is suggestive of a material in which the coercivity mechanism is of the single-domain particle type, it is known that ribbons do not strictly adhere to th
Data Loading...
