Effects of pre-aging on defects evolution and magnetic properties of Sm-Co-Fe-Cu-Zr magnets

The heterogeneous precipitation in the 2:17-type Sm-Co-Fe-Cu-Zr permanent magnets has been found to contain complex formation and dissociation of defects. Though low-temperature pre-aging has been utilized to promote the precipitate nucleation by the enlarged chemical driving force, how the defects evolve after pre-aging and how the possibly changed defects state affects the subsequent precipitation behavior remain unclear. In this work, a model magnet Sm₂₅Co_47.9Fe_18.5Cu_5.6Zr_3.0 (wt%) was selected to study. Through comparison with the as-solution-treated state, it is found that pre-aging for 2 h at 550 °C reduces the defects density, which was characterized by less cell boundaries (i.e., larger cell size) and less basal stacking faults inside the cells (i.e., higher 2:17R ordering degree). Further studies reveal that after aging for the same time (10 h) at the same temperature (830 °C), the reduced density of defects by pre-aging also leads to slower precipitation/phase transformation kinetics when compared with the non-pre-aged one, which was characterized by the lower 2:17R ordering degree and smaller coercivity for the former. These findings suggest that pre-aging has a strong influence on the density of defects and their evolution during subsequent isothermal aging process, which should be carefully considered to tailor the microstructure and magnetic properties of Sm-Co-Fe-Cu-Zr magnets.