2009 Volume 50 Issue 4 Pages 723-726
Nd-Fe-B sintered magnets show high energy products and have started to be used for motors of hybrid electric vehicles (HEVs). For the use of the magnets, the understanding of the coercivity mechanism is required for obtaining high coercivity. The Nd-rich phase in Nd-Fe-B sintered magnets plays an important role in cleaning the surface of Nd2Fe14B grains for decreasing the number of nucleation sites of reverse domains, which leads to high coercivity. In this study, the phase equilibria including oxygen in Nd-Fe-B sintered magnets are discussed in view of the wettability between Nd-rich phase and Nd2Fe14B phase. It is considered that Cu addition decreases the free energy of Nd-rich liquid phase, which leads to the shift of liquidus line of Nd-O system to lower temperature and the increase in solubility limit of oxygen. Due to improvement of wettability and increase in solubility limit, Cu-added Nd-rich liquid spread easily and form homogeneous liquid phase during the sintering process. These phenomena enhance cleaning effect of Nd-rich phase and contribute to the increase in coercivity of Nd-Fe-B sintered magnets.