Dissemination of IT for the Promotion of Materials Science (DoITPoMS)

DoITPoMS Micrograph Library Micrograph 14 and full record

Micrograph 14 and full record

Link to image file for micrograph 14
Micrograph no
Brief description
Fe, C 0.8 (wt%), eutectoid transformation - pearlite (coarse)
alloy Link to MATTER Glossary entry for alloy, austenite Link to MATTER Glossary entry for austenite, carbon, cementite Link to MATTER Glossary entry for cementite, eutectoid reaction Link to MATTER Glossary entry for eutectoid reaction, ferrite Link to MATTER Glossary entry for ferrite, iron, lamella Link to MATTER Glossary entry for lamella, metal, pearlite Link to MATTER Glossary entry for pearlite, steel Link to MATTER Glossary entry for steel, supercooling Link to MATTER Glossary entry for supercooling
Metal or alloy
Fe-C-X   Link to phase diagram
Fe, C 0.8 (wt%)
Standard codes
Slow cooled
Sample preparation
Reflected light microscopy
Length bar
200 μm
Further information
This steel is of the eutectoid composition. Once the temperature is lowered below the eutectoid temperature the steel becomes simultaneously supersaturated with both ferrite and cementite. A eutectoid transformation results (g to a + Fe3C). The resultant microstructure, known as pearlite, comprises lamellae of cementite (dark) embedded in ferrite (white). The platelets are parallel to each other and do not follow a specific crystallographic direction.

Each pearlite colony is made up of a number of subgrains. Thus each pearlite colony consists of two interpenetrating single crystals having an orientation relationship with respect to each other and with respect to the austenite grain they grow from, but not with respect to the austenite grain they have grown into. Changes in the apparent interlamellar spacing from colony to colony in the photograph are due to differences in the lamellae spacing with respect to the polished surface.

The coarseness of the pearlite is determined by the interlamellar spacing. This spacing is inversely proportional to the undercooling. This is primarily because of the increased rate of carbide nucleation with increased undercooling.

The pearlite in this sample is coarse due to it being slowly cooled. The undercooling is low so the lamellae spacing is relatively large resulting in a coarse microstructure.
Prof T W Clyne
Department of Materials Science and Metallurgy, University of Cambridge
Licence for re-use
Attribution-NonCommercial-ShareAlike 4.0 International
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