Heterogeneous nucleation of embedded droplets in the Zn-15Bi immiscible alloy studied by nanocalorimetry

Abstract

In this study, micro-sized Bi droplets embedded in the Zn matrix were prepared by melt-spinning technique, and their heterogeneous nucleation behaviors were investigated by nanocalorimetry at cooling rates ranging from 500 to 10,000 K/s. Upon rapid heating, a melt layer between the embedded Bi and Zn matrix formed during eutectic melting, which was then followed by the melting of the residual solid Bi phase. On cooling, both eutectic reaction and solidification of Bi droplets occurred with different nucleation kinetics. At a higher cooling rate, the undercooling for the eutectic reaction increased and then reached a maximum value while the undercooling for the solidification of Bi was nearly independent on the cooling rate. With the classical nucleation theory (CNT), the underlying nucleation mechanism for eutectic reaction and solidification of Bi droplets was addressed, revealing that the evolution of active nucleation sites with the cooling rate in different interfaces was responsible for this discrepancy.