Influence of Zr and Cu content on the microstructure and coercivity in Sm(CobalFe0.1CuyZrx)8.5 magnets

The effect of Cu and Zr on the microstructure and coercivity of Sm(CobalFe0.1CuyZrx)8.5 magnets has been studied using magnetometry and transmission electron microscopy. For Zr-free samples, when y=0.088, a cellular microstructure is not formed. Instead a rod-like 1:5 phase is observed, distributed in the 2:17 matrix phase. For a sample with y=0.168, the cellular microstructure develops after a short aging. However, the cellular microstructure coarsens and becomes highly nonuniform and finally, breaks down with further aging. This leads to a reduction in coercivity from an optimal value of 5.6–2 kOe. After adding Zr(x=0.04), the cellular microstructure along with a lamellar phase can be formed even with a Cu content as low as 0.048 and the coercivity increases dramatically. Coercivities of up to 8.9 and 38.4 kOe are obtained for the y=0.048 and 0.168 samples, respectively. These results show that the formation of cellular structure strongly depends on the Cu content. But the formation of the lamellar phase can stabilize a uniform cellular microstructure over a wide range of cell size and help the redistribution of Cu at the cell boundaries, which is a key to obtain appropriate microstructures with high coercivity.