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.