Microstructure evolution in undercooled Ag-50at. %Cu hypereutectic alloy

Ag-Cu alloys, as one type of typical eutectic solders, have been widely used in microelectronic packaging. The welding performance of Ag-Cu solders is closely associated with their microstructures. However, the microstructure evolution of Ag-Cu alloys during solidification is still poorly understood. In this work, a new melt fluxing technique was proposed to investigate the microstructure evolution of undercooled Ag-50at.%Cu hypereutectic alloy. The maximum undercooling of Ag-50at.%Cu alloy can reach up to 135 K. The obtained results show that the microstructures consist of primary dendrite and lamellar eutectic at low undercooling (ΔT ∼ 10 K). With the increase of undercooling, dendrites undergo refining (ΔT < 65 K) and then disappear completely (ΔT = 78 K) and appear again (ΔT = 119 K). It is also discovered that cellular lamellar eutectics formed with ΔT within range of 65 K ∼ 78 K. Unexpectedly, irregular microstructures are observed when ΔT > 135 K. Theoretical calculation results indicate that the formation of irregular eutectic is associated with the dendritic fragmentation and eutectic coarsening under larger undercooling (ΔT = 135 K). These results not only elucidate the relationship between the solidification microstructure and undercooling of Ag-50at. %Cu hypereutectic alloys but also has positive significance for other hyper-eutectic systems.