Development of a metastable phase diagram to describe solidification in undercooled Fe–Co melts

Droplets of Fe–0.3Co and Fe–0.5Co (at. fraction) alloys were undercooled using the electromagnetic levitation technique. Temperature–time profiles revealed that above a critical undercooling, a metastable phase formed on solidification which transformed back to the equilibrium phase during recalescence. A metastable phase diagram for Fe–Co was calculated by extending the liquidus and solidus lines of the δδ-ferrite (bcc) phase beyond the equilibrium composition range. Two parameters in the thermodynamic database were then altered so that the metastable solidus line fitted well to the metastable solidus temperature measured from the levitation experiments. The adjusted database was shown to have good agreement with both equilibrium and metastable experimental data and the parameters for bcc Co were shown to be reasonable. The activation energy for formation of a spherical nucleus was calculated using values of free energy and entropy from the adjusted database. The results showed that the metastable bcc phase has a lower activation energy for nucleation than the stable fcc phase below a critical level of undercooling. The experimentally measured critical undercooling was in good agreement with the calculated value.