Composition: Al-14%Si-0.5%Fe-0.10%Mn.

Processing: Casting.

Etching: 10-30s immersion in 0.5% HF.

This is a hypereutectic Al-Si alloy consisting of large primary Si crystals and a eutectic aggregate (α-Al + Si). The interface between primary Si and the aluminium matrix is incoherent. This facilitates the nucleation and propagation of cracks, and more so when Si has an acicular morphology. The second micrograph shows that primary Si is surrounded by Al-rich regions (white). Eutectic Si is unmodified (acicular shape) and is randomly dispersed with an irregular orientation. Additionally, two different intermetallic compounds are visible: α-AlFeSi with a chinese script morphology and β-AlFeSi with a plate-like morphology (needle shape in 2D). Both can be recognized at high magnification by their lighter tonality in comparison with Si crystals. These intermetallics are formed due to the presence of Fe impurity and are undesireable most of the times due to their negative effect on the mechanical and corrosion properties.

Al-Si diagram

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