Primary phase growth within highly undercooled Cu-Ge eutectic alloys

A denucleating agent, Cu-36wt%Ge hypoeutectic and Cu-45wt%Ge hypereutectic alloys were undercooled by up to 189K(0.2T_L) by using B₂O₃. Microstructural observations revealed that significant grain refinement of primary phase takes place in both alloys at large undercoolings. Furthermore, a 'plate-shaped to granular-shaped' structural transition of primary (Ge) phase was observed in Cu-45wt%Ge hypereutectic alloy while the undercooling level exceeds 200 K. It was suggested that such a growth morphology change of primary Ge phase could be related to the depression of anisotropy of (Ge) growth and/or the remelting of plate (Ge) caused by a faster growth velocity at large undercoolings.