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.
