Abstract: Despite the high theoretical energy product and low material cost of Fe-rich 2:17-type Sm-Co-Fe-Cu-Zr magnets, it is still a big challenge to simultaneously achieve high energy product and high coercivity due to damaged cellular nanostructure, i.e. insufficient cell boundary precipitates. In this work, both high energy product (∼30.29 MGOe) and high coercivity (∼26.24 kOe) have been achieved in Fe-rich Sm_24.8Co_balFe_20.5Cu_5.2Zr_x (x wt%) magnets through optimizing Zr content. It reveals that raising Zr content from 1.5 wt% to 2.5 wt% can effectively refine the cellular nanostructure, which corresponds to an increased volume fraction of cell boundary precipitates. However, excess Zr content (e.g. above 2.5 wt%) leads to the formation of micron-sized Zr-rich Zr₆Co₂₃ soft magnetic particles, weakening the hard magnetic performance. In particular, the high Zr-content (3.5 wt%) magnet exhibits strongly inhomogeneous chemistry as well as cellular nanostructure in the vicinity of micron-sized Zr₆Co₂₃ particles (i.e. heterogeneous distribution of cell boundary precipitates), deteriorating both squareness factor and coercivity. As a result, the optimum magnetic property combination is achieved at an intermediate Zr concentration by balancing the contradictive effects between cell refinement and soft magnetic impurities.

Key words: Permanent magnets, Magnetic properties, Microstructure