Image icon  As-cast wrought-grade aluminium alloy

 
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Description: The micrograph shows Al-Mg-Fe-Si containing < 1wt.% of each solute. Addition of TiB2 particles facilitates the formation of a fine, equiaxed grain structure (grain refinement). 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.
Keywords: alloy • aluminium • anodising • dendrite • equiaxed • iron • magnesium • metal • silicon
Categories: Science approaches > Microstructure
Materials > Metals & alloys > Aluminium & alloys
Processes > Shaping > Casting
Testing, analysis & experimentation > Metallography
Scale > Micro
Created by: T Quested, Department of Materials Science and Metallurgy, University of Cambridge
DoITPoMS, University of Cambridge
Published by: DoITPoMS, University of Cambridge
License: This resource is released under the Creative Commons Attribution Non-Commercial Share Alike license (2.0 UK: England & Wales).
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Date created: 20 January 2003
Date added: 21 August 2009
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Resource ID: 1741