Generation and characterization of anisotropic microstructures in rare earth-iron-boron alloys.

摘要

The automotive industry is currently being directed toward electrification of their fleets. In order to provide these hybrid or electric vehicles, lightweight high torque electric motors are needed. Permanent magnet (PM) brushless motors have been identified as the preferred motors for these applications. In order to effectively provide these motors, cost-effective high strength PMs are needed. The use of polymer bonded PMs is one method to reduce processing costs, but performance is decreased unless anisotropic PMs can be formed. New processing methods to form anisotropic mixed rare earth (MRE)-iron-boron PM particulate were studied in this work.;Two primary processing routes were identified and investigated: controlled devitrification through application of uniaxial pressure and rapid directional solidification utilizing a segregating additive. In addition, further control of the melt-spinning process was achieved through control of wheel surface temperature and finish.;Controlled devitrification was found to produce an anisotropic, nanocrystalline microstructure, as observed through TEM and XRD studies. A high defect density within the structure, unprecedented in RE2Fe14B microstructures, was observed. It is surmised that the defects cause soft magnetic behavior.;Stabilization of a columnar, textured microstructure was achieved through the utilization of moderate wheel speeds during melt-spinning, in combination with minor additions of Ag to the alloy. The texture was seen to be altered from that typically seen in RE2Fe14B alloys melt-spun at low-to-moderate wheel speeds. It was observed that this occurs through a modification in the solidification pathway, catalyzed by the addition of Ag. In addition to the altered texture, the presence of fine precipitates within the matrix and varying interdendritic phases was observed.;Alteration of wheel surface temperature and surface finish was seen to have significant effects on the ability to form amorphous material in Nd 2Fe14B + TiC alloys. Counter to the predictions of several theoretical models, increased wheel surface temperatures were seen to increase the glassy fraction. Additionally, utilizing coarser abrasives to finish the wheel surface resulted in greater amorphous fractions. It is proposed that the changes are correlated with alteration of wetting behavior between the wheel and the melt.;The work presented here provided promising directions for the formation of anisotropic particulate suitable for use in polymer-bonded permanent magnets.