ON THE MECHANISM OF MODIFICATION OF THE
ALUMINUM-SILICON EUTECTIC BY STRONTIUM: THE ROLE OF NUCLEATION M. Makhlouf
Worcester Polytechnic Institute, Worcester, MA, USA Copyright © 2010 American Foundry Society Abstract
The purpose of this contribution is to demonstrate that the presence of strontium in hypoeutectic Al-Si alloys significantly influences the kinetics of nucleation of eutectic silicon; and therefore, contrary to current belief, a complete theory for the mechanism of modification of the eutectic phases in hypoeutectic
Introduction
Formation of eutectic silicon in the later stages of solidi- fication of hypoeutectic Al-Si alloys requires two pro- cesses; namely, Nucleation and Growth. The process of Nucleation involves the formation of very small silicon particles (nuclei), and the process of Growth involves the increase in size of these nuclei by further addition of sili- con atoms from the eutectic liquid. The purpose of this correspondence is to demonstrate that the presence of strontium in hypoeutectic Al-Si alloys significantly influ- ences the kinetics of nucleation of eutectic silicon; and
therefore, contrary to current belief,1 a complete theory for the mechanism of modification of the eutectic phases in hypoeutectic Al-Si alloys by strontium must address not only the Growth aspects of the process, but also its Nucleation aspects.
Al-Si alloys by strontium must address not only the Growth aspects of the process, but also its Nucleation aspects.
Keywords: eutectic phases, aluminum-silicon alloys, nucleation kinetics
Nucleation of a solid phase in a liquid, such as the nucleation of eutectic silicon in molten Al-Si alloys, occurs due to ther- mal and/or compositional fluctuations within the bulk of the liquid that bring into existence clusters of silicon atoms with the right configuration. In the case of un-modified Al-Si al- loys, nucleation of the eutectic silicon particles begins when two groups, each made up of a few silicon atoms arranged in a tetrahedron, combine to form what is typically referred to as
an embryo.2
The optimum shape of this embryo is dictated by surface energy considerations and an embryo bound by low energy facets of {111} planes is the most thermodynamically stable.3
Calculation of the surface energy of various polyhe-
drons that can form from tetrahedrons show that a decahedron formed from five tetrahedrons in a twin relationship has the least surface energy.4
Hence the most stable silicon nucleus in
un-modified hypoeutectic Al-Si alloys is a decahedron bound by {111} facets as shown schematically in Fig. 1.
(a)
(b)
(c)
Figure 1. Schematic representations of (a) two tetrahedrons with the {111} mirror plane (highlighted) act as a silicon embryo, (b) side view of the silicon nucleus, and (c) top view of the silicon nucleus showing the five tetrahedrons that make up the decahedron.
International Journal of Metalcasting/Winter 10
47
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