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.
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Ceramics and Composites
- Polymers and Plastics
- Metals and Alloys