Magnets with a nominal composition of Nd14.5Fe79.5B6 have been prepared by the conventional powder metallurgy technique. Precursor alloy powders with average particle sizes of 2.5, 3.0, 3.5, and 4.2 μm were included in this study. Average particle size and oxygen content were effectively manipulated to control the average grain size in the sintered magnets. Typically, for NdFeB sintered magnets, the corrosion resistance of these magnets was improved with increasing oxygen content. The corrosion resistance of magnets varied significantly with the average particle size of precursor alloy powders. For a fixed oxygen content, magnets made from powders of larger sizes exhibited a higher weight loss (a poor corrosion resistance) when compared to those made from smaller alloy powders. However, the Hci of magnets made from 2.5, 3.0, and 3.5 μm precursor alloy powders was found to decrease drastically with increasing oxygen content while magnets made from the 4.2 μm powder was found to remain relatively constant with increasing oxygen content. To optimize magnetic performance, one needs to compromise the corrosion resistance and the Hci obtained by balancing the average particle size of the precursor alloy powder for magnet fabrication, as well as the oxygen content and the average grain size in the finished magnet.
The combination of chemical composition and microstructure of cast alloy has been found to be critical to the performance of NdFeB sintered magnets. Maximizing the amount of Nd2Fe14B phase (or minimizing the amount of secondary phase) by reducing the Nd content more closely to the stoichiometric composition appears to be essential for obtaining high BHmax magnets. However, α-Fe precipitation has been found to increase with decreasing Nd content and severely hinders the development of high BHmax magnets. A two-step method, incorporating ingot casting and isothermal annealing, has been developed to minimize the amount of precipitated α-Fe in low Nd content alloys. This method provides a drastic improvement in the Br and BHmax of sintered magnets obtained. By decreasing the Nd content to 13 at. % in the cast alloy, incorporating better particle control during fine milling, and controlling grain growth during sintering; magnets with a Br of more than 14.5 kG and a BHmax of 50 MGOe have been consistently obtained. Furthermore, because of the reduction in the amount of Nd-rich grain boundary phase, a significant improvement in the corrosion resistance of magnets was also observed.
Temperature-dependent magnetic properties of nine NdFeB sintered magnets with various Co-Nb, Co-V, or Co-Mo additions have been measured up to 175 °C using both closed-loop and open-circuit methods. The irreversible loss of induction, reversible temperature coefficient of induction (α), and temperature coefficient of intrinsic coercivity β have been related to the Tc, the Br, and the Hci at 25 and 175 °C. The irreversible loss of induction is strongly affected by the Hci at 25 °C and the α has been found to be strongly dependent upon the Tc. Intrinsic coercivity of more than 25 and 7.5 kOe at 25 and 175 °C, respectively, are essential to bring the β to better than −0.5%/ °C. A magnet with a composition of Nd12Dy3Fe70Co5Nb2B8 has been found to exhibit an outstanding thermal stability: a Br of 8.5 kG (comparable to that of SmCo5 sintered magnet) and a Hci of 12.9 kOe when measured at 175 °C. An α of −0.10%/ °C and a β of −0.4%/ °C have been obtained.
In Nd2Fe14B-based permanent materials, the intergranular phase has a strong influence on magnetic properties. Here, we study the effect of partial substitution of Fe by Co on the microstructure to gain insight into the mechanism of enhancing magnetic properties of (Nd0.8Pr0.2)2.2Fe14−xCoxB (x = 0, 1.75, 2, 2.25) alloys. Our results show that the substitution Co for Fe changes the magnetic properties obviously by tuning the chemistry and distribution of the intergranular phase between hard magnetic grains. In particular, for (Nd0.8Pr0.2)2.2Fe12Co2B (x = 2) alloy, no obvious intergranular phase is observed. And the through-thickness homogeneity and ultrafine microstructure with an average size of ~25 nm is obtained, which produces maximum product ((BH)max) of 141 kJ/m3, 29% higher than that of quaternary alloy. Our findings provide a new idea to design prospective permanent alloys with increased magnetic properties by tuning the distribution and chemical composition of the intergranular phase.
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