  By comparing the grain sizes under different nucleation conditions, the
different nucleation mechanisms were investigated. The primitive nuclei origin
at some specific interface, and subsequently disperse into the bulk melt with
melt flow. The survival probability of nuclei decides the nuclei density in
bulk melt, and the growth of survived nuclei finally constitutes the solidified
grains. The nucleation process is highly dynamic, evolving and variable with
actual experimental condition and production condition.
