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 a t.% Co alloy. Applying the regular solution model, the equilibrium Fe-Co al loy 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 Delta G* and the ste ady-state nucleation rate I, have been calculated in terms of stable face-c entered cubic (f.c.c.) (gamma) and metastable b.c.c. (delta) phases. The cr ystal growth velocities of b.c.c. and f.c.c. phases as a function of underc oolings Delta T and tip radius R were also given according to Bottinger, Co riell and Trivedi (BCT model). The critical undercooling for the formation of metastable b.c.c. (delta) phase in Fe-30 at.% Co alloy was 204 K. The tr ansmission electron microscopy results indicated that the microstructure of the primarily nucleated b.c.c. phase exhibited fine dendritic structure. T he composition analysis showed that the b.c.c. dendrite structure was enric hed in cobalt. (C) 1999 Elsevier Science S.A. All rights reserved.