Mechanisms of eutectic lamellar destabilization upon rapid solidification of an undercooled Ag-39.9 at.% Cu eutectic alloy

The eutectic Ag-Cu alloys exhibiting fine Ag-Cu lamellar eutectic structure formed upon rapid solidification have great potentials being used in various engineering fields. However, the desired fine primary lamellar eutectic structure (PLES) is usually replaced by a coarse anomalous eutectic structure (AES) when the undercooling prior to solidification exceeds a certain value. The forming mechanism of AES in the undercooled eutectic Ag-Cu alloy has been a controversial issue. In this work, the undercooled Ag-39.9 at.% Cu eutectic alloy is solidified under different cooling conditions by using techniques of melt fluxing and copper mold casting. The results show that the coupled eutectic growth of this alloy undergoes a transition from a slow eutectic-cellular growth (ECG) to a rapid eutectic-dendritic growth (EDG) above a undercooling of 72 K, accompanying with an abrupt change of the distribution and amount of AES in as-solidified microstructures. Two kinds of primary lamellar eutectic structures are formed by ECG and EDG during recalescence, respectively. The destabilization of PLES that causes the formation of AES is ascribed to two different mechanisms based on the microstructural examination and theoretical calculations. Below 72 K, the destabilization of PLES formed by slow ECG is caused by the mechanism of “termination migration” driven by interfacial energy. While above 72 K, the destabilization of PLES formed by rapid EDG is attributed to the unstable perturbation of interface driven by interfacial energy and solute supersaturation.