Transformation kinetics of the metastable bcc phase during rapid solidification of undercooled Fe-Co alloy melts

Solidification of metastable delta-phase (bcc) and subsequent transformation into the stable gamma-phase (fcc) in undercooled Fe-Co alloy melts have been investigated on electromagnetically levitated samples. Rapid solidification is monitored by a high-speed video camera, which enables the direct observation of the solidification front. Metastable phase formation is revealed by a two-step recalescence event showing primary solidification of metastable bcc phase and subsequent formation of stable fcc phase within the mushy-zone of primary bcc phase. A decisive parameter describing the kinetics of phase transformation is the delay time between nucleation of the metastable and the stable phase. The delay time obtained from high-speed video recordings strongly decreases with rising undercooling prior to solidification. A model is proposed, which yields an explanation for the origin of the undercooling dependency. It is based on the classical nucleation theory combined with an estimation of the evolution of the number of heterogeneous nucleation sites within the expanding mushy-zone during dendritic growth of the primary phase. Thus, the delay time is not only determined by nucleation kinetics but it also depends on both the growth velocity nu and the morphology of the dendritic network of the primary metastable phase, which both depend on undercooling Delta T. Using the relation nu(Delta T) for the metastable bcc phase calculated by current models for dendritic growth and verified by measured growth velocities, the experimental data as a function of undercooling are well described by the calculated delay times in a wide range of compositions between 30 and 65 at.% Co.