Microstructure evolution and metastable phase formation in undercooled Fe-30 at.% Co melt

The microstructure evolution and phase selections of Fe-30 at.% Co alloy at various undercoolings were investigated in this paper. The metastable body-centered cubic (b.c.c.) phase was detected in a highly undercooled Fe-30 at.% Co alloy. Applying the regular solution model, the equilibrium Fe-Co alloy phase diagram was evaluated and its metastable extension was taken into account to explain the formation of the metastable b.c.c. phase. Based on the classical nucleation theory, the activation energy ΔG^* and the steady-state nucleation rate I_hs have been calculated in terms of stable face-centered cubic (f.c.c.) (γ) and metastable b.c.c. (δ) phases. The crystal growth velocities of b.c.c. and f.c.c. phases as a function of undercoolings ΔT and tip radius R were also given according to Bottinger, Coriell and Trivedi (BCT model). The critical undercooling for the formation of metastable b.c.c. (δ) phase in Fe-30 at.% Co alloy was 204 K. The transmission electron microscopy results indicated that the microstructure of the primarily nucleated b.c.c. phase exhibited fine dendritic structure. The composition analysis showed that the b.c.c. dendrite structure was enriched in cobalt.