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)6Sn5 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.