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

Abstract

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–Cu 6Sn 5 eutectic then grew with Cu 6Sn 5 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 Cu 6Sn 5 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 + Cu 6Sn 5 binary eutectic point.