Austenite and Ferrite

H. K. D. H. Bhadeshia

Information on the crystallography, electron and X-ray diffraction from austenite and ferrite.

Quicktime Movies

• Stucture of austenite
• Structure of ferrite

• Location of carbon atom in octahedral interstice of austenite.
• Location of carbon atom in another equivalent octahedral interstice of austenite.
• Location of carbon atom in tetrahedral interstice of austenite. Unlike the tetrahedral interstice in ferrite, that in austenite is regular.
• Location of carbon atom in octahedral interstice of ferrite.
• Location of carbon atom in an irregular tetrahedral interstice of ferrite. Only three of the tetrahedral bonds are illustrated.
• Location of carbon atom in tetrahedral interstice of ferrite. There are eight unit cells plotted and the iron atoms forming the tetrahedron are coloured red. The carbon atom has been omitted for clarity.

Images

• Stucture of austenite.
• X-ray diffraction pattern of austenite.
• Electron diffraction from austenite.
• Data corresponding to X-ray diffraction pattern of austenite.
• Structure of ferrite.
• Electron diffraction from ferrite.
• X-ray diffraction pattern of ferrite.
• Data corresponding to X-ray diffraction pattern of ferrite.

• Location of carbon atom in octahedral interstice of austenite.
• Location of carbon atom in another equivalent octahedral interstice of austenite.
• Location of carbon atom in tetrahedral interstice of austenite. Unlike the tetrahedral interstice in ferrite, that in austenite is regular.
• Location of carbon atom in octahedral interstice of ferrite.
• Location of carbon atom in tetrahedral interstice of ferrite. There are eight unit cells plotted and the iron atoms forming the tetrahedron are coloured red. The carbon atom has been omitted for clarity.

Atom Models of Austenite and Ferrite

The following images have kindly been provided by Andrew Fairbank who created them for teaching purposes. They are reproduced with permission.

Face-centred cubic, body-centred cubic and body-centred tetragonal arrangements of iron atoms.	Face-centred cubic, body-centred cubic and body-centred tetragonal arrangements of iron atoms.
Body-centred cubic and face-centred cubic (alternatively, cubic close-packed) arrangements of iron atoms.	Carbon atom in an octahedral interstice in austenite.
Carbon atom in an octahedral interstice in austenite, with the face-centering iron atom replaced into position.	Carbon atom in an octahedral interstice in austenite.
Carbon atom in an octahedral interstice in ferrite.	Carbon atom in an octahedral interstice in ferrite.
Carbon atom in an octahedral interstice in ferrite.
Possible position of carbon atom in a tetrahedral interstice in ferrite. Carbon prefers the octahedral interstices in ferrite.	Possible position of carbon atom in a tetrahedral interstice in ferrite.
Possible position of carbon atom in a tetrahedral interstice in ferrite. The strain energy is greater when carbon is in a tetrahedral interstice, because the expansion is isotropic, unlike the octahedral case where the strain is tetragonal.	Possible position of carbon atom in a tetrahedral interstice in ferrite.
Chromium atom substituted into ferrite.	Chromium atom substituted into ferrite.
Silicon atom substituted into ferrite.	Atomic radii. It has been assumed in the preceding figures that iron has an atomic radius of 124 pm for all the crystal structures.

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