In-situ three-dimensional microstructural investigation of solidification of an Al-Cu alloy by ultrafast x-ray microtomo
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I. INTRODUCTION
ALUMINUM alloys are used extensively to produce parts of various shapes by many forming processes. These processes can be divided into three classes: solid processing (such as extrusion, rolling, etc.), liquid processing (casting, injection molding, etc.), and semisolid processing (such as rheocasting, thixocasting, etc.). In solidification processes, the alloy always consists of a solid-liquid mixture before complete solidification. The presence of two different phases (solid and liquid) is an origin for the possible macrosegregation in the casting (local variations in volume fractions of solid and liquid), porosity, or cracks (hot-tearing phenomenon) that form at the end of solidification.[1] The origins of these phenomena are still not yet well understood, partly because experimental observations are difficult to perform. It is known that the microstructure of the mushy zone and especially the morphology of the liquid phase are important factors influencing these phenomena and that the permeability of the solid network (ability of the liquid to flow through the solid phase) also plays a key role.[2,3] If possible, in-situ three-dimensional (3-D) observations of the microstructure during solidification of aluminum alloys will give significant insights into these phenomena. To study solidification, most in-situ experiments were performed with specific organic alloys, particularly succinonitrileacetone mixtures.[4] These mixtures were used also to investigate the hot-tearing mechanism,[5,6] but in all cases, two-dimensional (2-D) observations are carried out under an optical microscope. More recently, 2-D observations of solidification have been performed on Sn-Pb and Al alloys using OLIVIER LUDWIG, Postdoctor, is with the Ecole Polytechnique Federale de Lausanne, CH-1015 Lausanne, Switzerland. MARCO DIMICHIEL, Beam-Line Operator Manager, is with the European Synchrotron Radiation Facility, ESRF, 38000. Grenoble, France. LUC SALVO, Assistant Professor, and MICHEL SUÉRY, CNRS Research Professor, are with the Institut National Polytechnique de Grenoble, 38409 Saint-Martin d’He`res, France. Contact e-mail: [email protected] PETER FALUS, Postdoctor, is with Argonne National Laboratory, Argonne, IL, 60439. Manuscript submitted July 20, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
radiography.[7,8] The 3-D observations of the microstructure can be carried out using two different techniques. (1) Serial sectioning and polishing, with subsequent stacking of 2-D micrographs, allows for reconstruction of the 3-D microstructure. It was first used by Ito et al. on an Al-Si alloy with a 40 m resolution.[9] Since then, several studies have used this technique with various alloys, namely, Al-Cu and Sn-Pb,[10,11,12] and it was optimized by Alkemper et al.[13,14,15] The method allows taking 20 sections per hour, with a spatial resolution in the range of 1 to 20 m between sections. The method is, therefore, time consuming, but has the advantage of using common laboratory equipment. (2) The X-ray microto
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