Crystal nucleation in Au49Ag5.5Pd2.3Cu26.9Si16.3 glass and undercooled melt

Crystal nucleation is an indispensable step in the crystallization of metallic glasses. With the increase of undercooling, different nucleation kinetics from that predicted by the classical nucleation theory (CNT) could be revealed. A prerequisite to addressing the nucleation kinetics is building a well-identified amorphous structure, which is yet arduous. Moreover, due to the limited temporal and spatial resolution of the conventional experimental methods, nucleation during the crystallization of metallic glass cannot be well recognized. In this study, nanocalorimetry with ultrahigh sensitivity and ultrafast scanning rate is used to address the nucleation kinetics in glass and undercooled melt. With the increase in cooling rate, crystallization, homogeneous nucleation, and heterogeneous nucleation are successively distinguished. Accordingly, the critical cooling rates suppressing crystallization and nucleation are estimated, based on which a well-identified amorphous phase for nucleation studies is produced. By applying Tammann’s two-stage crystal nuclei development method, nucleation in glass and undercooled melt is investigated. According to the evolution of crystallization heat and overall heat on the reheating after the annealing, the underlying kinetic mechanism is revealed with the Johnson-Mehl-Avrami (JMA) method. The nucleation half-time indicates that homogeneous nucleation is dominant for the crystallization below Tg. For the annealing at and above Tg, crystallization completes by the inevitable heterogeneous nucleation followed by crystal growth.