Phase Selection in Solidification of Undercooled Co-B Alloys

A series Co-(18.5-20.7) at.% B melts encompassing the eutectic composition (Co81.5B18.5) were solidified at different degrees of undercooling. It is found that the metastable Co23B6 phase solidifies as a substitute for the stable Co3B phase in the alloy melts undercooled above a critical undercooling value of ~60 K. The Co23B6 and α-Co phases make up a metastable eutectic. The corresponding eutectic composition and temperature are Co80.4B19.6 and 1343 K, respectively. On exposure of the metastable Co23B6 phase at a given temperature above 1208 K, it does not decompose even after several hours. But it transforms by a eutectoid reaction to α-Co + Co3B at lower temperature.