Metastable coupled-growth kinetics between primary and peritectic phases of undercooled hypoperitectic Fe_54.5Ti_45.5 alloy

The metastable coupled-growth kinetics between the primary Fe₂Ti and peritectic FeTi phases of undercooled Fe_54.5Ti_45.5 alloy was systematically investigated by both electromagnetic levitation and drop tube techniques. Employing a high-speed camera, the rapid crystallization processes of levitated bulk alloy were recorded in the undercooling range of 34–187 K. In small undercooling regime below 143 K, peritectic solidification proceeded and the dependence of primary Fe₂Ti dendritic growth velocity V on the bulk undercooling ΔT satisfied a power relation of V = 2.43 × 10⁻¹⁴× ΔT^7.72 (mm s⁻¹). Once liquid undercooling increased beyond 143 K, the metastable coupled-growth was induced and the microstructure was characterized by the Fe₂Ti rods embedded in FeTi phase. Furthermore, the coupled-growth velocity decreased linearly with the rise in undercooling according to V = 1.47 × 10³-7.44ΔT (mm s⁻¹). In drop tube experiment, peritectic solidification characteristics of small alloy droplets disappeared and the primary and peritectic phases directly nucleated from undercooled liquid and grew cooperatively to form spherical coupled-growth cells if droplet diameter decreased below 481 μm.