Dissemination of IT for the Promotion of Materials Science (DoITPoMS)

DoITPoMS Micrograph Library Full Record for Micrograph 59

Full Record for Micrograph 59

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Micrograph no
Brief description
Al 88, Si 12 (wt%), eutectic alloy
alloy Link to MATTER Glossary entry for alloy, aluminium Link to MATTER Glossary entry for aluminium, eutectic Link to MATTER Glossary entry for eutectic, metal, modification Link to MATTER Glossary entry for modification, silicon Link to MATTER Glossary entry for silicon
Metal or alloy
Al-Si   Link to phase diagram
Al 88, Si 12 (wt%)
Standard codes
As-cast (metal mould)
Sample preparation
Reflected light microscopy
Length bar
80 μm
Further information
This sample is a casting alloy of eutectic composition. From the melt a eutectic is formed between aluminium solid solution and virtually pure silicon. Slow solidification produces a very coarse microstructure. The eutectic comprises large plates of silicon in the aluminium matrix. This microstructure displays poor ductility due to the brittleness of large silicon plates. The microstructure is normally refined through either rapid solidification, which lets the silicon phase assume a fibrous form, or by a process known as modification.

It may be noted that primary aluminium dendrites can be seen, although the composition is very close to the eutectic point and an entirely eutectic microstructure might thus have been expected. This effect is a consequence of the strongly skewed nature of the eutectic "coupled zone" in the Al-Si system. The coupled zone represents the combinations of melt composition and interfacial undercooling (or growth velocity) for which (coupled) eutectic growth can occur. It can be plotted on the phase diagram by extending the two liquidus lines below the eutectic temperature. The Si liquidus line curves sharply back towards higher Si contents as the undercooling is increased. (This is associated with the facetted growth mode of the Si phase.) Thus, depending on growth conditions, a nominally eutectic alloy may solidify initially outside the coupled zone, leading to primary aluminium dendrite formation (before the melt composition rises sufficiently for the coupled zone to be entered and eutectic growth to occur). For more details, see, for example, Acta Mater. vol. 40 (1992) p.1637-1644.
Prof T W Clyne
Department of Materials Science and Metallurgy, University of Cambridge
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Attribution-Non-Commercial-Share Alike 2.0 UK: England & Wales
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