SAMARIUM-COBALT PHASE-EQUILIBRIA REVISITE D - RELEVANCE TO PERMANENT-MAGNETS

In the binary system Sm-Co the region between Sm5Co19 (79.2 at.% Co) a nd Co is discussed. SmCo5 is not stable at low temperatures; it is tra nsformed in the anomalous eutectoid reaction SmCo5 --> Sm5Co19 + Sm2Co 17, and the experimental evidence does not support other assumptions a bout this process. In this reaction, the crystallization of Sm5Co19 is suppressed or greatly retarded, leaving the appearance of simple prec ipitation of Sm2Co17. This precipitation, in any form, destroys the hi gh coercivity that is otherwise obtained in sintered magnets. The cond itions for possible spinodal decomposition are described, and it is sh own why it cannot be responsible for the coercivity mechanism in binar y SmCo5. The partial replacement of Co by Cu in SmCo5 also leads to th e precipitation of Sm2Co17 but now, contrary to the behavior in binary Sm-Co, it actually enhances coercivity.