Eutectic grains in unmodified and strontium-modified hypoeutectic aluminum-silicon alloys
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27/4/04
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Eutectic Grains in Unmodified and Strontium-Modified Hypoeutectic Aluminum-Silicon Alloys STUART D. McDONALD, ARNE K. DAHLE, JOHN A. TAYLOR, and DAVID H. StJOHN Additions of strontium to hypoeutectic aluminum-silicon alloys modify the morphology of the eutectic silicon phase from a coarse platelike structure to a fine fibrous structure. Thermal analysis, interrupted solidification, and microstructural examination of sand castings in this work revealed that, in addition to a change in silicon morphology, modification with strontium also causes an increase in the size of eutectic grains. The eutectic grain size increases because fewer grains nucleate, possibly due to poisoning of the phosphorus-based nucleants, that are active in the unmodified alloy. A simple growth model is developed to estimate the interface velocity during solidification of a eutectic grain. The model confirms, independent of microstructural observations, that the addition of 100 ppm strontium increases the eutectic grain size by at least an order of magnitude compared with the equivalent unmodified alloy. The model predicts that the growth velocity varies significantly during eutectic growth. At low strontium levels, these variations may be sufficient to cause transitions between flake and fibrous silicon morphologies depending on the casting conditions. The model can be used to rationally interpret the eutectic grain structure and silicon morphology of fully solidified aluminum-silicon castings and, when coupled with reliable thermal data, can be used to estimate the eutectic grain size.
I.
INTRODUCTION
DESPITE the fact that the aluminum-silicon eutectic occupies a large volume fraction of most commercial foundry alloys, the size and distribution of eutectic grains* in *For the purpose of this article, a eutectic grain refers to connected regions of aluminum-silicon eutectic that have grown from a common source. This definition implies nothing about crystallography and either phase may be polycrystalline or monocrystalline depending on the growth mechanisms that operate.
unmodified and modified alloys are rarely characterized. This is probably related to the difficulties associated with the visualization of eutectic grains. In contrast to the primary aluminum grain size, which can be characterized by a variety of relatively simple techniques, eutectic grains are difficult to observe without the use of specialized techniques. These techniques include the addition of segregating elements,[1,2] draining liquid from partially solidified castings,[3,4] chemical or thermal etching,[1,2] and, most commonly, interrupting solidification by quenching.[2,5–11] In unmodified alloys of eutectic composition, a solid shell of eutectic often forms adjacent to the mold wall early in solidification.[5,7–9] Equiaxed eutectic grains may be observed ahead of this interface, although it has been suggested that all of the eutectic is actually connected in three dimensions.[5,7] Alternatively, it has been proposed that unmodified eutecti
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