Effect of post-solutionizing cooling rate on microstructure and magnetic properties of 2:17-type Sm-Co-Fe-Cu-Zr magnets

The fabrication of 2:17-type Sm-Co-Fe-Cu-Zr magnets that have served as the strongest high-temperature permanent magnets for nearly half a century requires a strict processing control to form full cellular nanostructure. Considering that slow cooling after solution-treatment may enable a more homogeneous temperature field inside the chamber and more slight magnetic properties difference among the magnets in mass production than rapid cooling, here we performed a comparative study on a model magnet Sm₂₅Co_46.9Fe_19.5Cu_5.6Zr_3.0 (wt%) to investigate how the post-solutionizing cooling rate affects the microstructure and magnetic properties. In comparison with the rapid cooling condition, slow cooling produces coarser cellular nanostructure and lower defects density at the solution-treated state. Such initial microstructural difference leads to slower 1:5 H growth kinetics and slower defects dissociation kinetics during the subsequent aging process, characterized by the smaller fraction of 1:5 H cell boundary phase and the higher density of remanent defects in the slowly-cooled final magnets. Since the 1:5 H phase plays a dominant role on the coercivity and the remanent defects are harmful to hard magnetic properties, further work reveals that longer aging time can promote the formation of 1:5 H phase and to reduce the harmful defects for achieving better magnetic performance in the slowly-cooled magnets. These findings may be helpful for achieving uniform magnetic performance in mass production of 2:17-type Sm-Co-Fe-Cu-Zr magnets.