Solute adsorption and entrapment during eutectic Si growth in A-Si-based alloys

The solute adsorption and/or segregation as well as the solute entrapment of Sr, Na and Yb atoms during eutectic Si growth in a series of high-purity Al-5 wt.% Si alloys was investigated by multi-scale microstructure characterization techniques, including high-resolution transmission electron microscopy and atomic-resolution scanning transmission electron microscopy. The adsorption of Sr atoms was directly observed along the [1 1 2](si) growth direction of Si and/or at the intersection of multiple Si twins, which can be used to interpret the poisoning of the twin plane re-entrant edge and impurity induced twinning modification mechanisms, respectively. In contrast, Yb shows a different mechanism compared to the adsorption of Sr atoms. No significant Yb-rich cluster was observed at the intersection of Si twins. However, considerable Yb-rich segregation lines were observed along the [1 1 2](si) direction, which can be attributed to the solute entrapment caused by a few Si twins through the natural twin plane re-entrant edge and growth mechanism. Active poisoning of the twin plane re-entrant edge and impurity induced twinning growth mechanisms cannot be observed due to the absence of Yb atoms within eutectic Si. Furthermore, the solute entrapment of modifying elements (X, Sr or Yb) together with Al and Si was proposed to interpret the formation of Al2Si2X phases or X-rich clusters within eutectic Si. Such types of Al2Si2X phases or X-rich clusters were further proposed to be an artefact caused by the solute entrapment during eutectic Si growth, rather than an active factor affecting the modification. The observed solute adsorption and entrapment can be used to interpret the different observations in the cases of different modifying elements, including impurity effects and so-called quenching modification, thereby elucidating the modification of eutectic Si in Al-Si alloys. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.