Nucleation behaviour and anomalous eutectic formation in highly undercooled Fe₂O₃-La₂O₃ eutectic melts

Employing an aero-acoustic levitator, the Fe₂O₃-16.5 mol% La₂O₃ eutectic alloy was levitated, melted, undercooled and then solidified under a containerless condition when a continuous laser beam heating system was incorporated. By revealing the surface and cross-sectional microstructures, copious nucleation is confirmed to take place in the undercooled melts solidified by either an external seeding or spontaneous crystallization. The nucleation behaviour of eutectic alloys is summarized, indicating that copious nucleation may be an intrinsic attribute of a eutectic system with the exact eutectic composition in unconstrained solidification. Considering the complexity level of crystal structures of eutectic oxide phases in the Fe₂O₃-La₂O₃ alloy, the linear kinetic constants are approximately estimated and the growth kinetics are discussed. The sluggish growth kinetics of the perovskite-type phase, that is LaFeO₃ with a higher complexity level in a unit cell leads to a decoupled growth within a single eutectic colony when the melt is undercooled to exceed the critical undercooling ΔT = 125 K. It is the decoupled growth that results in the formation of anomalous eutectics. The present concept based on the kinetic constant of different phases can also be applicable to account for the growth behaviour of other oxide eutectics when considering the stable and metastable eutectic reactions in different solidification conditions. The nucleation behaviour and growth modes in some oxide eutectic systems have been predicted in the free solidification from an undercooled state.