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

摘要

Cyclic superheating and cooling were carried out for the undercooled hypereutectic Co_(80)B_(20), eutectic Co_(81.5)B_(18.5,) and hypoeutectic Co_(83)B_(17) 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.