Enhanced magnetic properties of Fe-rich Sm-Co-Fe-Cu-Zr magnets by compressive stress-aging

Precipitation of Sm/Cu-enriched cell boundary phase during isothermal aging plays a crucial role on the hard magnetic properties of the cellular nanostructured 2:17-type Sm-Co-Fe-Cu-Zr permanent magnets. However, this method offers only limited magnetic hardening effect in Fe-rich magnets as conventional thermal aging usually leads to decreased density of cell boundary precipitates. Here we report a stress-aging approach to rectify this limitation. As exhibited by a model magnet Sm₂₅Co_44.9Fe_21.5Cu_5.6Zr_3.0 (wt.%), aging under 50 MPa uniaxial compressive stress for the same time at the same temperature can simultaneously enhance the intrinsic coercivity from 17.51 to 25.68 kOe and maximum energy product from 28.39 to 31.39 MGOe when compared with conventional isothermal aging. Detailed microstructural and microchemical investigations revealed that the magnetic performance enhancement stems from the retained [001] texture, the increased number density of Cu-enriched cell boundary precipitates, and the reduced stacking faults density at cell edges, which can significantly strengthen the magnetic domain wall pinning. Such microstructural improvements are contributed from the increased density of defects that promote the precipitate nucleation at early stage and the stress-induced dislocation re-arrangements that accelerate defects dissociation and atomic diffusion at the following stage, i.e. optimizing the thermodynamic-kinetic compromise of precipitation. Further study revealed that aging under higher stress leads to abnormal cell growth and weakened [001] texture due to the dynamic recrystallization effect, deteriorating the magnetic properties. Consequently, stress-aging provides a fresh freedom to manipulate the microstructure of Sm-Co-Fe-Cu-Zr magnets, but the stress magnitude should be carefully controlled to improve the magnetic performance.