Magnetic hardening of Pr-Co-B sintered magnets (abstract)

Abstract

It is well known that the Pr2Co14B intermetallic compound has a high magnetocrystalline anisotropy (100 kOe at 294 K). This value is higher than that of the Pr2Fe14B compound (70 kOe at 294 K). Hard magnets, using the rapid-quenching technique, have been prepared for both the Co- and Fe-based materials. In the case of the conventional powder-metallurgy technique, only the Fe-based magnets have been successfully prepared. So far, there is no report of achieving any significant value of coercivity in the Pr-Co-B system. In this paper we report preliminary results on Pr-Co-B sintered magnets with room-temperature intrinsic coersivity, Hci, equal to 5.2 kOe and an energy product, (BH)max equal to 10.3 MGOe. At liquid-nitrogen temperature (77 K) Hci is greater than 18 kOe (which was the maximum available magnetic field) and (BH)max is equal to 12.5 MGOe. These values of (BH)max are twice those reported for the rapidly quenched magnets.1 The Curie temperature of the magnet is 988 K (corresponding to the presence of the Pr2Co14B phase). The microstructure of the magnets with high Hci is markedly different from that of the magnets with low Hci. The major difference is the lower concentration of voids and the well-defined shape of the grains. The grain size is between 5 and 30 μm. The major phases present in the magnet are Pr2Co14B, PrCo4B, and a minor amount of PrCo5. The metallurgical characteristics and phase-diagram features of the Pr-Co-B system seem to be substantially different from those of the Pr-Fe-B system, where the major phases present are the Pr2Fe14B, PrFe4B4, and the Pr-rich phase.