Investigation on the Solidification Behavior of Bi Droplets Embedded in Cu Matrix by Nanocalorimetry

Background: The solidification of metals is usually initiated by heterogeneous nucleation, which, however, is less controllable due to the unexpected impurities. In this study, the nucleation behavior of micro- and nano-sized Bi droplets embedded in the Cu matrix, where well-identified catalyst interfaces were introduced, was studied. Objective: This paper demonstrated the nucleation behavior of the nano-sized Bi droplets embedded in the Cu matrix to determine the nucleation sites and contact angle. Methods: Cu-20Bi (wt. %) melt-spun ribbons consisting of numerous micro- and nano-sized Bi droplets embedded in the Cu matrix were prepared. Based on the structure characterization by transmission electron microscopy (TEM), the nucleation behavior of these embedded Bi droplets was demonstrated by nanocalorimetry. Results: The orientation relationship between the nano-sized Bi droplets and the Cu matrix was determined. The undercooling of the nano-sized Bi droplets with the cooling rate was more stable than that of the micro-sized Bi droplets, and a nucleation contact angle of 72° was obtained. Conclusion: The undercooling increases with the reduction of droplet size. The nucleation of the Bi droplets is significantly affected by their interfacial structure with the Cu matrix. Compared with hexagonal Zn, the Cu matrix with face-centered cubic structure has a less catalytic effect on the nucleation of Bi.