Full length articleSolidification velocity of undercooled Fe-Co alloys

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 ΔHf/Tm2 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¯γδ, 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.

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