Micrograph 59 and full record

Micrograph no

Brief description

Al 88, Si 12 (wt%), eutectic alloy

Keywords

alloy

, aluminium

, eutectic

, metal, modification

, silicon

Categories

Metal or alloy

System

Al-Si

Composition

Al 88, Si 12 (wt%)

Standard codes

Reaction

Processing

As-cast (metal mould)

Applications

Sample preparation

Technique

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.

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