Liquid–liquid structure transition and nucleation in undercooled Co-B eutectic alloys

Cyclic superheating and cooling were carried out for the undercooled hypereutectic Co₈₀B₂₀, eutectic Co_81.5B_18.5, and hypoeutectic Co₈₃B₁₇ alloys. For each alloy, there is a critical overheating temperature T_c ^° at which there is a sharp increase of the mean undercooling, i.e., below (above) T_c ^°, and the mean undercooling is about 80 °C (200 °C). DSC measurements show that there is a thermal absorption peak in the heating process, the peak temperature of which is nearly equal to the critical overheating temperature, indicating that the temperature-induced liquid–liquid structure transition does occur and should relate highly to nucleation in the undercooled Co-B eutectic melts. The effect of the liquid–liquid structure transition on nucleation was interpreted by the recent nucleation theory that considers the structures of overheated melts, and the composition-dependent overheating temperature was ascribed to the change of local favored structures. The present work provides further evidences for the liquid–liquid structure transition and is helpful for understanding solidification in undercooled melts.