Revisiting the pinning sites in 2:17-type Sm-Co-Fe-Cu-Zr permanent magnets

It is still an open debate whether the 1:5 H cell boundaries(CBs)or the intersections of 1:3 R platelets and1:5 H CBs are the strong pining sites for the cellular nanostructured 2:17-type Sm-Co-Fe-Cu-Zr high temperature permanent magnets despite that they have been widely applied in advanced industries since 1970 s.Herein,through tuning the volume fraction of Zr-enriched 1:3 R platelets by varying the second-step aging time,the pinning behavior in a model magnet Sm_(2.5)Co_(44.9)Fe_(21.5)Cu_(5.6)Zr_(3.0)(wt%)was investigated.The results show that the volume fraction of 1:3 R platelets can be effectively enlarged without changing the cell size(i.e.the volume fraction of CBs)by extending the aging time at 400℃.Micro scopic TEM characterizations co mbined with macro scopic magnetic measurements reveals that the locally thickened 1:3 R platelets after long-term second-step aging reduce the effective pinning area by interrupting the magnetic domain walls at CBs,weakening the average pinning strength and the coercivity of the magnet.Consequently,our work supports that the 1:5 H CBs act as the dominating pinning sites instead of the intersections of 1:3 R platelets and 1:5 H CBs,which may provide an important insight towards understanding the hard magnetism of pinning-controlled permanent magnets.

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 Sm25Co47.9Fe18.5Cu5.6Zr3.0(wt%)was selected to study.Through comparison with the as-solution-treated state,it is found that pre-aging for 2 h at 550℃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 2:17 R ordering degree).Further studies reveal that after aging for the same time(10 h)at the same temperature(830℃),the reduced density of defects by preaging also leads to slower precipitation/phase transformation kinetics when co mpared with the non-preaged one,which was characterized by the lower 2:17 R 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.