Phase stability of Co5Sm

Diffusion couples formed between cobalt and samarium provide useful structures for studying the Co
5
Sm eutectoid decomposition. The diffusion zones consist of Co, Co
17
Sm
2
, Co
5
Sm, Co
7
Sm
2
, Co
3
Sm, and Co
2
Sm when formed in the 820° to 1130°C range. The absence of the Co
5
Sm phase in the 775° couple provides supporting evidence for the eutectoid transformation and locates the eutectoid temperature at about 800°C. Couples formed above 820°C and then heat-treated below 800°C provided information on the eutectoid transformation which does not follow the classic Fe-C pearlite reaction. The possible effects of the decomposition on properties of permanent magnets are discussed.