X-Ray Videomicroscopy Studies of Eutectic Al-Si Solidification in Al-Si-Cu
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-BASE alloys constitute more than 50 pct of the commercial market for non-ferrous casting alloys, with hypo- to hypereutectic variants from the Al-Si system having a dominant share. Alloying with Si has a profound effect on the castability of Al, promoting fluidity and feeding, and improved resistance toward casting defects such as porosity and hot tearing. Si alloying also contributes to reducing the specific weight and thermal expansion. Commercial Al-Si casting alloys contain a substantial fraction of eutectic. The Al-Si eutectic is an archetype of a so-called irregular eutectic, where the fcc Al phase, with a relatively modest crystalline anisotropy and a low melting entropy, grows nonfaceted, whereas Si, which bonds covalently in a strongly anisotropic tetrahedral arrangement, is associated with a higher melting entropy and grows faceted along specific crystallographic directions.[1] Despite the commercial importance of irregular eutectics, such as Al-Si and Fe-C, the literature available R.H. MATHIESEN, Associate Professor, Department of Physics, and L. ARNBERG, Professor, Department of Material Science, are with NTNU, N-7491 Trondheim, Norway. Contact e-mail: ragnvald. [email protected] Y. LI, Research Scientist, is with SINTEF Materials & Chemistry, N-7465 Trondheim, Norway. V. MEIER, formerly Student, Department of Physics, NTNU, is Postdoctoral Student, with the Institut fur Strukturphysik, Technical University Dresden, D-01060 Dresden, Germany. P.L. SCHAFFER, Research Scientist, is with Norsk Hydro Sunndalsora, N-6600 Sunndalsora, Norway. I. SNIGIREVA and A. SNIGIREV, Reserach Scientists, are with the Experiments Division, ESRF, F-38043 Grenoble, France. A.K. DAHLE, Professor, is with the Department of Materials Engineering, University of Queensland, Brisbane QLD 4072, Australia. Manuscript submitted January 14, 2010. Article published online October 19, 2010 170—VOLUME 42A, JANUARY 2011
on their solidification microstructure formation is limited compared to the vast amount available for regular eutectics. For the latter, constitutive relations exist to relate the solidification microstructures of regular lamellar and rodlike eutectics to experimental parameters. Within the operation limits of quasi-planar nearisothermal interface propagation, the pattern selection that defines the regular eutectic growth morphologies is fairly well described by the Jackson–Hunt model,[2,3] and more recent extensions to this, e.g., by phase field simulations and experiments with transparent analogues, for growth behavior beyond the basic state interface stability limits.[4–6] The growth mechanisms of irregular eutectics are inherently more complex. Generally, the faceted phase has restricted branching ability, and consequently, progression in three dimensions is considerably more cumbersome than for a nonfaceted component. The difference in branching ability, or solid-liquid interface stiffness, also implies that the two eutectic phases have different capabilities to adapt to varying growth conditions, and generally the faceted phas
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