Effect of Fe and/or Cu contents on the intrinsic coercivity of Sm2Co17-type coercive powders for the bonded magnet application

Abstract

The microstructure of conventionally cast Sm(CobalFe0.28CuvZr0.022)8.0 alloys, where v=0.043–0.092, were examined by optical microscopy and scanning electron microscope with energy dispersed analytical x ray in the as-cast state. In addition to the regular 2:17 matrix phase, the Sm-rich grain boundary phase and the needle-like Zr-rich phases are present in most commercial Sm(Co,Fe,Cu,Zr)z alloys. A grayish Sm- and Cu-rich grain boundary phase was found in alloys with v≥0.07. Attention was focused on the impact of increasing Cu content to the microstructure and process parameters required to produce coercive powder for bonded magnets. An increase of the Cu content from the v value from 0.043 to 0.092 shifts the liquid+2:17→TbCu7-type transformation temperature from slightly above 1180 to approximately 1140 °C. The Hci of optimally prepared magnet was found to increase significantly while the Br remained relative constant when the Cu content is increased. When the Cu content was increased to beyond v=0.07, a slight decrease in the BHmax was noticed. A magnetizing field of 15 kOe and more than 50 kOe were determined to be necessary to charge magnets with a Hci of 12 and 25 kOe, respectively, to about 90% of their full potential. For a composition of Sm(CobalFeuCu0.07Zr0.022)8.0, a slight increase in Fe concentration from u=0.22 to 0.28 was found to increase the Br of an optimally prepared magnet from 8.07 to 8.28 kG and decrease the Hci from 11.8 to 9.7 kOe. A BHmax of 15 MGOe was obtained on a bonded magnet with a composition of Sm(CobalFe0.28Cu0.07Zr0.022)8.0 when the data are normalized to a specific density of 7.0 g/cm3.