Structure tailoring and nucleation behavior of Bi droplets embedded in a Zn matrix

Abstract

Well-characterized nucleants are important for heterogeneous nucleation because of their catalyst effect. In this study, micro- and nano-sized Bi droplets embedded in the Zn matrix were prepared by tailoring the solidification process of the Zn–xBi (x = 5, 10, 15, 20, 25, 30 wt%) alloys. With the help of nanocalorimetry, the nucleation behavior of the embedded Bi droplets at cooling rates ranging from 1000 K/s to 10,000 K/s was demonstrated based on the classical nucleation theory. Under this condition, the evolution of undercooling with scanning rate suggests a single interface heterogeneous nucleation both in the micro- and nano-sized droplets. The nucleation of the micro-sized droplets is explored by repeated undercooling measurements, producing a contact angle of 37° ± 1.4°. With droplet size down to nanoscale, a transition from rhombohedral to cubic structure is found in the Bi droplet. Based on Cantor's model, the nucleation kinesics of these nano-sized droplets is elaborated, where a contact angle of 63° ± 2.5° is obtained. The discrepancy highlights the dominant role of the interfacial structure in determining the nucleation behavior.