382 Prakt. Met. Sonderband 46 (2014) 
3.3. Al-5Si ALLOY WITH Yb ADDITION along 
with a 
The addition of Yb (up to 6100 ppm) into Al-5Si alloy has no significant effect on Si interse 
twinning, when compared to the additions of Sr (up to 200 ppm) (Fig. 1) and Na (up to 160 
ppm) (Fig. 2). Fig. 3 shows a faceted Si particle tilted to the <011> zone axis along the 
grain boundary in Al-5Si-6100 ppm Yb alloy. Unexpectedly, no significant Si twinning was 
observed, despite the favourable atom size ratio according IIT mechanism. This 
observation is in contrast to the prediction of IT mechanism. Furthermore, no significant 
Yb-rich cluster was observed along the <112>g growth direction of Si and at the 
intersection of Si twins (Fig. 3d). No Al,Si,Yb particle was observed within eutectic Si 
because no significant solute adsorption and / or entrainment of Yb atoms occur during 
eutectic Si growth. Instead, most Al,Si,Yb particles were observed adjacent the Si phase 
(Fig. 3a, b), strongly indicating that most Yb atoms segregate out of eutectic Si. 
_ Junr “ene 
Fig. ¢ 
dir 
en 
solidi 
Siar 
other : 
mu 
ai nm The ac 
Fig. 3: (a), (c) Dark field images, (b), (d) STEM-HAADF images taken from Al-5Si-6100 directi 
ppm Yb alloy, (c), (d) are enlarged from (a), (b), showing no significant Yb-rich cluster and entran 
thereby Si twinning within Si. B // [011]s. this c: 
enrich 
Finally 
4. DISCUSSIONS subse 
the sc 
Fig. 4 shows a schematic representation of the Sr solute adsorption and / or segregation compc 
as well as the possible solute entrainment ahead of solidification front of eutectic Si in Al-Si enoug 
alloys. In the liquid state, the alloying elements (i.e. Al, Sr and Si) are randomly distributed, The pr 
although solute clustering (i.e. Si clustering) may occur. During eutectic Si growth (Fig. Si-Sr-r 
4a), Sr and Al solute will segregate ahead of the solidification interface (ka < 1 and ks, < obser 
1). A solute redistribution of Sr and Al, and thus an enrichment of Sr and Al ahead of the than 
solidification front may occur. During continuous Si growth, the adsorption of Sr atoms on Cluster 
{111}; plane occurs along the <112>g; growth direction of Si and / or at the intersection of eutect 
two {111}s; twins (Fig. 4b). According to the poisoning of the TPRE. the adsorption of Sr of two