384 Prakt. Met. Sonderband 46 (2014) 
for modification of eutectic Si in Al-Si alloys. While, the observed Al-Si-Sr-rich clusters with 
different sizes and compositions are caused by the solute segregation and thereby the 
solute entrainment of Sr atoms, which were formed during eutectic Si growth, rather than 
the first and dominant step to lead to poisoning of the TPRE and lIT growth mechanisms. 
In contrast to the cases of Sr addition, no significant multiple Si twins was observed in the 
case of Yb addition. Because of the absence of Yb atoms along the <112>g; growth 
direction of Si and at the intersection of two {111}s; twins, Yb addition (up to 6100 ppm) 
appears not to promote the formation of the parallel or multiple Si twinning (Fig. 3). If any, 
only single Si twinning can be observed, which is similar to the case of Al-5Si based alloy 
caused by natural TPRE. Furthermore, no Al;Si;Yb particle was observed within eutectic 
Si because no significant solute adsorption and / or entrainment of Yb atoms occurs during 
eutectic Si growth. Instead, most Al;Si;Yb particles were observed adjacent the Si phase 
(Fig. 3), strongly indicating that most Yb atoms segregate out of eutectic Si. 
5. CONCLUSION 
(1) The solute adsorption of Sr atoms along the <112>s; growth direction of Si and at 
the intersection of Si twins during Si growth was observed, which can be used to 
interpret the well-known poisoning of the TPRE and IIT mechanisms, respectively. 
(2) In contrast, the segregation of Yb atoms is distinctly different from the adsorption of 
Sr along the {111}s; growth planes. No significant Yb-rich cluster was observed at 
the intersection of Si twins. 
(3) The solute entrainment of modifying elements (X) was proposed to interpret the 
formation of Al;Si;X (X, Sr or Yb) phase or X-rich clusters within eutectic Si. Such 
types of Al,SioX phases or X-rich clusters were further proposed to be an “artefact” 
caused by the solute entrainment during eutectic Si growth, rather than a “true” 
modification mechanism. 
(4) The proposed solute adsorption and entrainment can be used to well interpret the 
different observations in the cases of different modifying elements, thus elucidating 
the modification of eutectic Si in Al-Si alloys with respect to growth. 
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