In situ Investigation of the Heterogeneous Nucleation Sequence in Al-15 Weight Percent Cu Alloy Inoculated by Al-Ti-B

The sequence of heterogeneous nucleation events is investigated in situ via synchrotron-based X-ray radiography on Al-15 wt pct Cu alloy. In addition to the normal wave-like nucleation, which is predominant ahead of the solidification front when the temperature gradient moves across the field of view, two other types of nucleation are observed: nucleation among the growing grains and nucleation at one site. The nucleation among the growing grains occurs on the less potent TiB₂ particles and accounts for ~ 20 pct regardless of the cooling rates. The third type is the nucleation at one site at cooling rates below 8 K/min and gradually weakens with the increasing cooling rate, which is probably due to the agglomeration of inoculant particles. The latter two types of nucleation are believed to be important for increasing the efficiency of nucleant particles.