Revealing the nucleation and growth modes upon rapid solidification of undercooled Co-24 at.% Sn eutectic alloy by the crystallographic orientation relations

In the realm of undercooled eutectic alloys, the nucleation (i.e., copious vs. single) and the growth (i.e., coupled vs. decouple) modes, particularly the formation of anomalous eutectics, remain subjects of intense debate. This study delves into the crystallographic orientation relationships (ORs) during rapid solidification of an undercooled Co-24 at.% Sn eutectic alloy, where the high-temperature eutectic phases (β-Co₃Sn₂ and α-Co) persist to room temperature. A significant finding is the consistent observance of the same eutectic OR, i.e., 112̅0_β−Co3Sn2 // 11̅1_α−Co and 0001_β−Co3Sn2 // 110_α−Co, across various undercooling conditions from low to high and eutectic morphologies from lamellar to anomalous. This consistency underscores a unified growth mode of coupled eutectic growth. Furthermore, the observance of twin OR, i.e., 112̅1<11̅26>, 112̅2<1123̅> or 112̅4<2243̅>, between β-Co₃Sn₂ phases in diverse eutectic colonies, spanning from low to intermediate undercooling, and would also be the case of high undercooling, reinforces a single nucleation mode except under exceedingly low undercooling. Building on these insights, the anomalous eutectic colonies are suggested to be formed from a single origin where coupled eutectic growth of eutectic seaweeds or eutectic dendrites instead of dual origins. This study not only illuminates the underlying mechanisms of rapid solidification in undercooled eutectic alloys but also paves the way for innovative microstructure modulation strategies.