Prakt. Met. Sonderband 46 (2014) 379 
igh vertical ATOMIC OBSERVATION ON THE SOLUTE ADSORPTION AND 
ems of the ENTRAINMENT DURING Si GROWTH IN AI-Si BASED ALLOYS 
oad loos JH. Li", M. Albu?, F. Hofer?, P. Schumacher? 
ray > > Chair of Casting Research, Montanuniversität Leoben, Austria 
Lo Institute for Electron Microscopy and Fine Structure Research (FELMI), Graz University 
r lead times : . 
of Technology, Center for Electron Microscopy, Austria 
® Austrian Foundry Research Institute. Leoben. Austria 
ABSTRACT 
1 the field of 
nt forms of The solute adsorption and entrainment of Sr, Na and Yb during Si growth in a series of 
\anics. The high purity melt spun Al-5 wt.% Si alloys was investigated by atomic resolution high 
rial surfaces angular annular dark filed imaging in an aberration corrected scanning transmission 
uidelines to electron microscope. The adsorption of Sr or Na atoms was observed at the twin plane re- 
are used for entrant edge and at the intersection of Si twins, which can be used to interpret the well- 
jata and 3D known poisoning of twin plane re-entrant edge and impurity induced twinning modification 
levelopment mechanisms. In contrast, the segregation of Yb atoms was distinguished to be different 
from the adsorption of Sr or Na atoms along the {111}s; growth planes. The absence of Yb 
ith complex within Si, in particular at the twin re-entrant edges or at the intersection of Si twins, was 
n are steep related to a refinement, rather than a modification of eutectic Si in Al-Si alloys. 
“an also be 
1. INTRODUCTION 
Modification of eutectic Si in Al-Si alloys can be dated back to 1920 [1], since the first 
modification phenomenon was discovered by Pacz. Recent technological developments of 
ice texture: electron microscopy, e.g. high resolution transmission electron microscopy (HRTEM), high 
resolution scanning transmission electron microscopy (HRSTEM) and atom probe 
1IsSensors. tomography (APT), make it possible to investigate the modification mechanisms at an 
3), Vision 9, atomic scale. To date, it is generally accepted that impurity induced twinning (IIT) [2] and 
twin plane re-entrant edge (TPRE) growth mechanism [3, 4], as well as poisoning of the 
TPRE [5] are valid under certain conditions. However, neither IIT, TPRE, nor poisoning of 
the TPRE can be used to interpret all the previous modification observations [6-9]. For 
example, Yb addition into Al-Si alloys has been reported to only refine, rather than modify, 
the eutectic Si [6], even though Yb atom has an exactly suitable radius ratio (ry, / rsi = 
1.646) according to the IIT mechanism. The observed disagreements strongly indicate that 
the well-accepted IIT mechanism, based on the atomic radius alone, is not capable of 
explaining the modification of eutectic Si, and additional mechanisms are still expected to 
be active. Therefore, a revision of the solute adsorption and entrainment of modifiers 
during modification of eutectic Si is thus of great necessity to elucidate this important melt 
treatment for Al-Si based alloys. 
In this paper, the solute adsorption and entrainment during eutectic Si growth in a series of 
high purity Al-5 wt.% Si alloys with Sr, Na and Yb additions were investigated, with a 
special focus on the distribution of Sr, Na and Yb atoms within eutectic Si, in particular 
along the <112>¢; growth direction of Si and at the intersection of multiple Si twins.