On the microstructure of SmCo5 magnets

SmCo5-based magnet microstructure is more complex than might be expected from the binary Sm-Co phase diagram, as other elements like oxygen and carbon intervene and as local heterogeneities occur due to the powder metallurgy method of production. This paper investigates the effect of sintering temperature, samarium and oxygen content on its microstructure and properties. Six constituents were found: SmCo5, Sm2Co7, oxides, pores and the carbide SmCoC2 are found in high coercivity magnets, whereas Sm2Co17 is found in low coercivity magnets. A samarium-depleted surface region is always present and eventually large Go-rich particles occur. A microstructural modeling based on the magnet density, samarium, oxygen and carbon content was able to predict the Sm2Co7 volume fraction of magnets. The model permits the calculation of the necessary amount of Sm-rich additive to obtain high coercivity. The presence of Sm2Co7 in the microstructure is not detrimental to high coercivity and high H-K, as H-K=12 kOe was obtained in samples with 7% volume fraction of Sm2Co7. The effect of heat-treatment on coercivity is reversible: resintering with no heat-treatment lowers coercivity. As an oxygen solubility change was not found between 1150 and 850 degrees C, it cannot be the cause of coercivity improvement due to heat-treatment.