This is the microstructure of a grey cast iron. This is an alloy of iron (Fe) with 4% carbon (C) by weight.
The microstructure has two main constituents. The long pale regions are flakes of graphite. They have a shape similar to the cornflake breakfast cereal. The background or matrix of the alloy is pearlite. This is a fine mixture of ferrite and iron carbide.
You should compare this micro- structure with the microstructure of high carbon steel which has a micro- structure of pearlite.
This alloy has a lower melting point than steel, due to the high carbon content. This makes it very appropriate and economical for cast products, hence its name.
The graphite also improves the wear resistance of the iron by providing lubrication. However, it causes a big reduction in toughness and ductility since the brittle graphite acts as a source of cracks. Cast irons are much less tough than steels.
You can see similar alloys and images of this microstructure at other magnifications in Categories: - Metals