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Solidification of metastable δ -phase (bcc) and subsequent transformation into the stable γ -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 v and the morphology of the dendritic network of the primary metastable phase, which both depend on undercooling Δ T . Using the relation v ( Δ 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.

The goal of this work was to investigate intra-alloy relationships, as they pertain to rapid solidification, which can be applied to computational materials modeling. Those relationships can be utilized to improve the accuracy of predictive modeling by leveraging previous experimental results. With that in mind, Fe–Co samples were prepared with 30–50 at.% cobalt and they were processed via electrostatic levitation (ESL) or electromagnetic levitation (EML). The samples were levitated, melted, and allowed to cool and solidify in a vacuum for ESL testing and under He gas for EML testing. If sufficient undercooling was achieved, the sample solidified via double recalescence. In that event, the metastable δ–phase would grow into the undercooled liquid, and then the stable γ–phase would grow into a combination of the metastable phase and remaining undercooled liquid, or mushy zone. The velocities of the solid phases growing into undercooled liquid were analyzed with current dendrite growth theories. The purpose of the growth velocity analyses was two-fold: 1) Assess the validity of current dendrite theory as it applies to the Fe–Co system. 2) Evaluate the kinetic growth coefficient, μ, assuming a constant kinetic rate parameter, Vo. The results of the analyses indicate that it is reasonable to assume that the kinetic rate parameter, Vo, is constant for a given phase within an alloy system if Δ H f / T m 2 does not vary significantly within the system, or within the composition range of interest. The average growth velocities of the stable phase into the mushy zone, V ¯ γ δ , for the Fe–30, 40, and 50 at.% Co compositions are 1.6, 2.4, and 4.9 m/s, respectively, which scale with the thermal driving forces of the transformations, ΔTγδ, which are 10 K, 24 K, and 40 K, respectively.