We found that the fluidity of samples with 0.4 wt.% Sc was not significantly
higher than the fluidity of samples with 0.2 wt.% Sc, or
0.2 wt.% Sc and 0.2 wt.% Zr. However, the samples with no Sc addition
(with the largest grain sizes) did have the lowest fluidity. The
diminishing difference in fluidity between the base alloy and the
grain refined alloy at high pouring temperature can be explained
by the small difference in grain size at the flow tip of both alloys.
The few large dendrites in the flow tip of the base alloy can be
broken and multiplied when the alloy flows a long distance. This
phenomenon can make grain sizes at the flow tip almost as small as
they are in the grain refined alloy. Based on grain size, we still cannot
explain why samples with 0.4 wt.% Sc did not have the highest
fluidity. Thus, we investigated the sample microstructures further,
as shown in Fig. 4.