Micrograph 712 and full record

Micrograph no

712

Brief description

As-cast wrought-grade aluminium alloy (Al-Mg-Fe-Si containing < 1wt.% of each solute). Addition of TiB₂ particles facilitates the formation of a fine, equiaxed grain structure (grain refinement).

Keywords

alloy

, aluminium

, anodising

, dendrite

, equiaxed

, iron, magnesium, metal, silicon

Categories

Metal or alloy

System

Al-Mg-Fe-Si

Composition

Not specified

Standard codes

Reaction

Processing

As-cast

Applications

Sample preparation

Electrolytically etched using Barker's reagent.

Technique

Cross-polarised light microscopy

Length bar

400 μm

Further information

This micrograph illustrates one of the possible growth morphologies that a solidifying metal can adopt (c.f. micrograph 711). The grains in this structure exhibit no dendritic branching, i.e. the solid-liquid interfaces of the growing grains are smooth. When a solid grain is growing from the liquid phase, it will initially exhibit a smooth interface; such an interface can become unstable as the grain becomes larger due to rejection of solute into the liquid phase. Hence, smooth grain boundaries are associated with fine grain structures and low solute contents.

The Barker's etch and applied electrical field produce a thick oxide layer on the grains of aluminium (anodising). When viewed in cross-polarised light, interference in the oxide layer produces colours which depend on grain orientation; hence the grain structure is imaged.

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