Effects of trace elements (Y and Ca) on the eutectic Ge in Al-Ge based alloys

Effects of trace elements (0.2Y and 0.2Ca (wt%) on the eutectic Ge in high purity Al-20Ge (wt%) alloys were investigated by multi-scale microstructure characterization techniques. Particularly, the distribution of trace elements (Y and Ca) within the eutectic Ge and/or at the interface between eutectic Ge and eutectic Al was investigated by atomic resolution high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) imaging and atom probe tomography (APT). The combined investigations indicate Al-Y and Al-Ca co-segregations. Such co-segregations change significant morphology and growth of the eutectic Ge. In addition, large Al2Ge2Y and Al2Ge2Ca phases were also measured. The modification of eutectic Ge is discussed in terms of previously postulated modification mechanisms: twin plane re-entrant edge growth mechanism, impurity-induced twinning, and growth restriction of eutectic Ge. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.