Three-dimensional visualization of the microstructure development of Sr-modified Al–15Si casting alloy using FIB-EsB tomography M. Timpel a,⇑ , N. Wanderka a , B.S. Murty b , J. Banhart a a Helmholtz-Zentrum Berlin fu ¨ r Materialien und Energie GmbH, Hahn-Meitner-Platz 1, D-14109 Berlin, Germany b Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600 036, India Received 7 May 2010; received in revised form 16 August 2010; accepted 17 August 2010 Available online 27 September 2010 Abstract Unmodified and Sr-modified (after 5 and 120 min of melt holding) Al–15Si alloys were investigated by transmission electron micros- copy and focused ion beam tomography using energy-selective backscattered electrons for imaging. The three-dimensional visualization of the microstructure provided not only the true morphology of the Al–Si eutectic and Fe-rich intermetallics, but also allowed us to esti- mate their volume fractions. The evolution of Fe-rich a-phase morphology in the unmodified alloy proceeded during eutectic growth according to a model proposed. In the unmodified alloy, only Fe-rich a-phase was found, whereas in the Sr-modified alloy after 5 min of melt holding, two morphologies of Fe-rich phases were observed, namely a Fe-rich a-phase and a Fe-rich d-phase. Both phases segregated mainly at modified eutectic grain boundaries. After 120 min of melt holding, the eutectic microstructure is similar to the unmodified structure again and only the Fe-rich a-phase could be observed. Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: FIB-EsB tomography; Al–Si alloys; Sr modification; Intermetallic compounds; Microstructure 1. Introduction The eutectic modification of Al–Si alloys is very impor- tant, primarily to improve the mechanical properties of cast materials [1]. Additions of Sr, Na or various rare earth elements to Al–Si alloys result in a transformation of the eutectic Si from coarse plate-like morphology to a fine fibrous structure [2–4]. Modification not only affects the morphology of the eutectic Si but also the development of intermetallic phases formed by impurities during solidi- fication. In Al–Si foundry alloys the most common impu- rity element is Fe. Different Fe-rich intermetallics, including a-Al 8 Fe 2 Si or -Al 15 (Fe, Mn) 3 Si 2 , b-Al 5 FeSi, d- Al 4 FeSi 2 and p-Al 8 Mg 3 FeSi 6 , have been identified in Al– Si casting alloys, strongly depending on the composition of the alloy [5–7]. However, the effect of the addition of Sr on the formation of Fe-rich intermetallics in foundry alloys is still not well understood [8–12]. Fe-rich intermetallics appear in a number of morpholo- gies such as “polyhedral”, so-called “Chinese script” or “needle-shaped” in two-dimensional (2-D) micrographs that are projections of their true shape. In order to obtain the true shape of eutectic or intermetallic phases, a three- dimensional (3-D) reconstruction based on many 2-D images is required. Recently, a qualitative tomographic analysis of eutectic morphologies based on metallographic 2-D optical images with a resolution of about 2 lm in the slicing direction has been reported [13,14]. A characteriza- tion of eutectic Si with a much higher resolution of about 60 nm in the image plane and 300 nm in depth was recently achieved using serial sectioning with a focused ion beam (FIB) combined with energy-dispersive X-ray spectroscopy (EDX) [15]. Nevertheless, only a few tomographic investi- gations have been performed on the Al–Si eutectic [15,13,14]. A 3-D visualization of the eutectic microstruc- ture at even smaller length scales is still urgently needed 1359-6454/$36.00 Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2010.08.021 ⇑ Corresponding author. E-mail address: melanie.timpel@helmholtz-berlin.de (M. Timpel). www.elsevier.com/locate/actamat Available online at www.sciencedirect.com Acta Materialia 58 (2010) 6600–6608