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Sankt Stefan ob Leoben, Austria

Li J.H.,University of Leoben | Schumacher P.,Austrian Foundry Research Institute
TMS Light Metals | Year: 2016

The addition of CrB2 and Sr into Al-10Si-0.3Mg (wt.%) based alloys not only modify the eutectic Si, but also refine the eutectic grains. CrB2 particles were found to nucleate the eutectic grains, despite the A1P particles being partly removed or depleted by the interaction between Sr and A1P. Furthermore, P level was found to be one of the dominant factors that affect the nucleation of eutectic grains. CrB2 addition with a low P level results in a refined eutectic grain size, despite less A1P particles being present to nucleate the eutectic grains. Porosity was found to be mainly located at the boundaries of eutectic grains, highlighting the importance of the refinement of eutectic grains to reduce the size and optimize the distribution of porosity. This investigation demonstrates a novel approach to refine eutectic grains and modify eutectic Si simultaneously into Al-10Si-0.3Mg (wt.%) based alloys, and thereby suggests possible industrial applications. Source


Li J.H.,University of Leoben | Zarif M.Z.,University of Leoben | Albu M.,University of Graz | McKay B.J.,University of Leoben | And 3 more authors.
Acta Materialia | Year: 2014

A series of high-purity Al-5 wt.% Si alloys with trace additions of Sr, Fe and P were prepared by using arc-melting and subsequent melt-spinning. The nucleation phenomenon incorporating the free growth criterion of eutectic Si was investigated by using the entrained droplet technique, atomic resolution scanning transmission electron microscopy and differential scanning calorimetry. It was found that Sr addition exerts no positive effect on the nucleation process; instead, an increased undercooling was observed. A combined addition of Sr and Fe further increased the undercooling, as compared with the addition of Sr only. Only trace P addition has a profound effect on the nucleation of Si by a proposed formation of AlP patches on primary Al. The estimated AlP patch size was found to be sufficient for the free growth of Si to occur inside the eutectic droplet. Nucleation kinetics was discussed on the basis of classical nucleation theory and the free growth model. For the first time, realistic and physically meaningful nucleation site values were obtained. The interactions between Sr and P were also highlighted. This investigation demonstrates strong experimental supports for the free growth nucleation kinetics and the well-accepted impurity-induced twinning growth mechanism, as well as the poisoning of the twin plane re-entrant edge growth mechanism. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Source


Li J.H.,University of Leoben | Albu M.,University of Graz | Hofer F.,University of Graz | Schumacher P.,University of Leoben | Schumacher P.,Austrian Foundry Research Institute
Acta Materialia | Year: 2015

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. © 2014 Acta Materialia Inc. Source


Li J.H.,University of Leoben | Schumacher P.,University of Leoben | Schumacher P.,Austrian Foundry Research Institute
IOP Conference Series: Materials Science and Engineering | Year: 2016

The nucleation and growth of primary Al3Sc phase has been investigated in high purity Al alloys and commercial purity Al alloys, with a special focus on the impurity effects. In the case of high purity Al alloys, most primary Al3Sc phases were pushed to grain boundaries during moving solidification front. Such type of primary Al3Sc phase does not contribute to the heterogeneous nucleation and thereby no significant grain refinement of Al alloys was observed, although some Al3Sc particles remain the same orientation with the Al matrix. In the case of commercial purity Al alloy, the presence of impurities, e.g. Ti, Fe and Si, enhances the heterogeneous nucleation of primary Al3Sc phase. Most primary Al3Sc phases are located within the Al matrix, and keep the same orientation with the Al matrix. Furthermore, the presence of impurities also changes the growth mode of primary Al3Sc phase. In the case of commercial purity Al alloy, a layer by layer growth was observed. This investigation demonstrates that impurities have important effects on the nucleation and growth of primary Al3Sc phases in Al based alloys. Source


Li J.H.,University of Leoben | Schumacher P.,University of Leoben | Schumacher P.,Austrian Foundry Research Institute
IOP Conference Series: Materials Science and Engineering | Year: 2011

The solidification and age hardening behaviour of an Mg-6Zn-2Gd (wt. %) alloy has been investigated. It was found that the microstructure of the as-cast samples was composed of equiaxed α-Mg grains surrounded by some eutectic compound both at triple points and along grain boundary. The eutectic compound was composed of MgZn 2, Mg 5Gd phases and α-Mg matrix. After a solution treatment at 500°C for 18 h, the eutectic compound almost dissolved, but some discontinuous second phase particles still survived at grain boundaries. The Mg 3Gd 2Zn 3 phase also formed during subsequent ageing. The addition of Gd not only improved the thermal stability of the second phase formed during solidification, but also postponed the overaging during ageing at 200°C up to 100 h. The precipitates with three different morphologies: [0001] α rods/lath, (0001) α plates and blocky particles, were observed in the α-Mg matrix. The solute elements of Zn and Gd were found to significantly partition into the precipitates, especially for blocky particles. This significant partition can be correlated directly to improved mechanical properties at elevated temperature. Source

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