Critical undercoolings for the formation of metastable phase and its morphologies solidified from undercooled Fe–Co melts

The maximum undercoolings of 304, 318, 308, and 296 K were achieved, respectively, in Fe-22, 26, 30, and 34 at.% Co alloys. The metastable bcc phase nucleated from melts when undercoolings exceeded critical ones. The critical undercoolings for the formation of metastable bcc phase from Fe-22, 26, 30, and 34 at.% Co melts were 104, 156, 204, and 248 K, respectively. The morphologies of as-obtained metastable bcc phase exhibited five typical patterns: dendrite cores with primary and second arms, well-developed second arms, and radiated, lath, and platelike structures. Based on the classical nucleation theory, the solidification behavior of the melts was analyzed with regard to the metastable phase formation when the melts were undercooled greater than critical undercoolings. The formation of various morphologies was also evaluated to consider the solidification behavior of the undercooled melts.