RETM5 and RE2TM17 (RE=rare earth, TM=transition metal) permanent magnets are discussed in this review. The scientific and technological considerations which have guided their development are described. Along with a discussion of the various research efforts reported in the literature, they help establish the framework within which most of the work can be integrated and understood. In regard to the RETM5 alloys, the review limits itself to the technologically significant SmCo5 materials; others are discussed minimally, and only to the extent that they contributed to overall magnet development efforts. Likewise, from among the fabrication technologies, the review is limited to those processes which have been demonstrated as capable of producing magnets in bulk useful form. Areas covered in this review include phase diagrams, crystal structures, magnetocrystalline anisotropy, coercivity mechanisms, fabrication technologies, processing-microstructure-property interrelationships, and thermal effects related to stability of magnet flux. Directions for future, additional work are discussed briefly as well.

A value of 67.5 kOe has been measured for the room temperature coercivity of an isotropic, heat treated, 1-5 samarium-cobalt plasma sprayed deposit. This value is believed to be the largest published, so far, for a permanent magnet. Sprayed magnets are generally found to be better than 93 percent dense with coercivities of 35 to 60 kOe. Deposits comprised of fine microstructures yield broad, unresolved x-ray diffraction peaks. Low temperature crystallization of these deposits yields diffraction patterns representative of the high temperature equilibrium structure. The emergence of peaks, ascribed to SmCo5, from a low temperature 700 °C heat treatment implies both formation and stability of the SmCo5 phase at this temperature. These results dispute the existence of the generally accepted eutectoid decomposition reaction for SmCo5 at 750–800 °C. If such a reaction were present the products of crystallization should have included Sm2Co7 and not SmCo5. High temperature investigations on sprayed magnets indicate that temperatures of 1100 °C result in low values of coercivity, possibly due to grain growth.

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