In situ investigation of unidirectional solidification in Sn-0.7Cu and Sn-0.7Cu-0.06Ni

The dynamics of off-eutectic solidification are studied in two Pb-free solders using synchrotron X-ray radiography and post mortem scanning electron microscopy. The early stages of growth revealed the onset of coupled growth and phase competition. In Sn-0.7 wt.% Cu, the βSn-Cu₆Sn₅ eutectic then grew with Cu₆Sn₅ rods leading the βSn phase, with a range of rod spacings and local interface curvatures. However, these nonfaceted-faceted eutectic features were exhibited much more weakly than in the Al-Si and Fe-C eutectics, despite primary Cu₆Sn₅ crystals exhibiting marked faceting. In Sn-0.7 wt.% Cu-0.06 wt.% Ni, primary intermetallic laths grew ahead of a rod-like βSn-(Cu,Ni)₆Sn ₅ univariant eutectic front. The univariant eutectic had a narrow freezing range and a similar spacing to the invariant eutectic at the same velocity. Composition measurements suggest that the univariant eutectic groove descends in the direction of the Sn + Cu₆Sn₅ binary eutectic point.