Eutectic nucleation in Al–Si alloys

doi:10.1016/j.actamat.2004.05.043

Acta Materialia

Volume 52, Issue 14, 16 August 2004, Pages 4273-4280

https://doi.org/10.1016/j.actamat.2004.05.043 Get rights and content

Abstract

In addition to a change in silicon morphology, modification of aluminium–silicon alloys with strontium or sodium increases the size of the eutectic grains. To determine the mechanism responsible, eutectic solidification in commercial purity and ultra-high purity aluminium–silicon alloys, with and without strontium additions, was examined by a quenching technique. In the commercial unmodified alloy, nucleation was prolific while in the high-purity unmodified alloy few eutectic grains nucleated. The addition of strontium to the commercial alloy reduced the number of eutectic grains that nucleated. Addition of strontium to the high-purity alloy did not significantly alter nucleation. It is concluded that commercial purity alloys contain a large number of potent nuclei that are susceptible to poisoning by impurity modification. The flake-to-fibre transition that occurs with impurity modification is shown to be independent of any change in eutectic nucleation mode and frequency.

Introduction

Recent research on eutectic solidification mechanisms in hypoeutectic aluminium–silicon alloys has identified large differences in the nucleation and growth modes of unmodified and impurity modified alloys [1], [2]. Eutectic nucleation in unmodified alloys occurs by the nucleation of a large number of eutectic grains,¹ at or near the primary aluminium dendrite–liquid interface. Relatively few eutectic grains nucleate in strontium modified alloys and nucleation occurs independent of the surrounding dendrites. The difference in the number and size of eutectic grains is quite dramatic, as shown in the quenched samples of Fig. 1. The reason for reduced nucleation remains unclear, but it has been proposed that strontium and sodium poison phosphorus-based nucleants that are active in the unmodified alloy [1].

During eutectic growth, the eutectic interface growth rate will vary inversely with the total eutectic solid–liquid surface area of the system [3]. Because fewer grains nucleate in modified alloys, the solid–liquid eutectic interface is much smaller and consequently advances at a higher growth rate. This increase in growth rate will contribute to refining the eutectic in modified aluminium–silicon alloys [1], [4] and it is difficult to separate the contributions of refinement occurring as a result of the increased growth rate and that resulting from impurity modification. In fact, it has been suggested that impurity modification occurs solely because of the increased growth rate that arises due to altered nucleation frequency and solid/liquid interfacial area [3].

The hypothesis that strontium additions cause modification by increasing the growth rate of the eutectic has been tested by directionally solidifying unmodified and strontium modified alloys [5]. It was found that alloys modified with strontium have a smaller eutectic spacing than unmodified alloys, even when both are grown at identical velocities. The difference in spacing was not as large as occurs in a typical sand casting and it could be concluded that nucleation still must have a significant effect on the refinement of the eutectic.

If the eutectic nucleation frequency can affect the eutectic spacing, then a variety of techniques could be used to control the microstructure. Techniques that increase nucleation would be expected to coarsen the eutectic, by decreasing the growth rate, and those that decrease nucleation would be expected to refine the eutectic, by increasing the growth rate. This may be why vibration during solidification, which is expected to increase nucleation, results in a coarsening of the eutectic phases [6], [7]. In contrast, superheating, which has been proposed to dissolve potential nuclei, results in a refinement of the eutectic phases [8], [9]. The addition of phosphorus results in coarsening of the eutectic phases [10], [11], which supports the theory that phosphorus-based compounds are effective as eutectic nucleation sites.

From the above analysis, there is reason to suspect changes in the nucleation and growth of eutectic grains will occur with changes in alloy purity. This is certainly the case in directional solidification, where increasing the impurity level in the melt results in a columnar-to-equiaxed transition in the eutectic growth mode [12]. Despite this, to the best of our knowledge, the effect of alloy purity on the nucleation and growth of eutectic grains in non-directionally solidified castings has not been investigated. The current research was therefore designed to compare eutectic solidification in hypoeutectic aluminium–silicon alloys of commercial purity and ultra-high purity. The objective was to examine the influence of impurities on the nucleation of eutectic grains in aluminium–silicon alloys and on the resulting eutectic microstructure, particularly the size and morphology of the eutectic silicon phase.

Section snippets

Experimental method

Alloys of commercial purity and ultra-high purity of nominal composition aluminium–10 wt% silicon were used for experimentation. The commercial purity alloys were produced by placing the correct proportions of commercial purity aluminium (major impurities 0.08 wt% Fe, 0.03 wt% Si) and silicon (major impurities 0.18 wt% Fe, 0.012 wt% Ti and 0.04wt% Ca) in a clay-graphite crucible and melting in an electric resistance furnace to a melt temperature of 760 °C. The phosphorus content of the

Results

The chemical analysis of the four alloys used in experimentation is shown in Table 1. The level of phosphorus was below the lower limit of detections for the analysis techniques used. Assuming that no phosphorus contamination occurred during melting and pouring, the level should be less than 10 ppm for the commercial purity alloy and less than 1 ppm for the high-purity alloy.

The cooling curves obtained during solidification of the commercial purity and high-purity alloy are shown in Fig. 3,

Discussion

Microstructure evolution during solidification of commercial hypoeutectic aluminium–silicon alloys can be considered as being comprised of two main stages. The first stage involves the nucleation and growth of aluminium dendrites and the second stage is the nucleation and growth of the aluminium–silicon eutectic. At the time when eutectic nucleation occurs, the conditions in the melt will in part be imposed by the characteristics of the previous reaction. In particular, the dendritic growth

Conclusions

Eutectic solidification was examined using a quenching technique in both high-purity and commercial purity hypoeutectic aluminium–silicon alloys with and without strontium additions. A large number of eutectic grains nucleated in the commercial purity unmodified alloy, each growing with an asymmetrical grain shape and coarse interphase spacing. In the unmodified high-purity alloy, on the other hand, very few grains nucleated and each grain grew with a flatter, spherical interface. The eutectic

Acknowledgements

The authors like to thank Dr. Karl Forwald and Mr. Torfinn Buseth of Elkem Solar for the generous supply of the high-purity silicon used in these experiments and Dr. Gustav Heiberg of Det Norske Veritas for the supply of the high-purity aluminium and sintered alumina crucibles.

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Eutectic nucleation in Al–Si alloys

Abstract

Introduction

Section snippets

Experimental method

Results

Discussion

Conclusions

Acknowledgements

Mater. Lett.

Acta Mater.

Acta Mater.

Metall. Mater. Trans. A

Metal Science

J. I. Met.

Metall. Trans. A

J. Mater. Sci.