Magnetic hardening in Zr-substituted 2 : 17 rare-earth permanent magnets

The effect of Zr on the hard magnetic properties of Sm(Co0.745−xFe0.200Cu0.055Zrx)7.5 permanent magnets has been examined by correlating the coercive field with the microstructure. Optical and transmission electron microscope studies show that Zr suppresses the precipitation of a SmCoCu phase and controls the size of the grains, the size of the cells, and the formation of the lamella phase. In the Zr-free samples large spherical precipitates of a SmCoCu phase have been observed to be uniformly distributed in the matrix. The microstructure of the matrix consists of very fine precipitates with a size below 60 Å. As the Zr concentration increases, the SmCoCu phase disappears, the cell size is increased, and the coercivity increases. At a Zr content around 1.5%, low density lamellae are observed to coexist with the cellular microstructure and coercivity is maximum. At higher Zr concentrations a CoZrFe phase is formed, the density of lamellae is increased, and the coercivity decreases.