Fig.1 shows the naked-eye observation of the
different refining effects of Sc and Ti additions on
aluminum. The grain sizes of Al-Ti and Al-Sc against the
Sc or Ti content are displayed in Fig.2. When the solute
content is low (<0.6%, mass fraction), Ti has a better
refining effect on aluminum than Sc, and an unrefined
large grain size is measured in Al-Sc alloy. When the
content of Sc is increased beyond a certain value (around
0.5%), a dramatic reduction in the grain size is observed
in Al-Sc, thus Sc performs a much stronger refining
effect on aluminum than Ti when the contents of Sc and
Ti are larger than 0.7%.
Fig.3 presents metallographic images of the grain
structures of Al-Ti and Al-Sc alloys. Equiaxed grains
with an average size of 220 μm are achieved in Al-0.2Ti,
while coarse columnar grains with the size order of
millimetre still dominate in Al-0.2Sc. When the solute
content is increased up to 1.2%, the grain size of Al-Ti
alloy is around 90 μm without large difference compared
with Al-0.2Ti. While for Al-1.2Sc, there is a total
absence of dendritic sub-structure, the grain size is
sharply decreased to around 25 μm.
In the backscatter electronic images (Fig.4), it is
obvious that Al3
Ti in Al-1.2Ti is needle-like in 2D, with a
dimension of 80−120 μm, while Al3
Sc is cubic with a
size of 5−10 μm.