Crystal Structure All Metals and their alloys have crystalline structure. Within a grain the atoms are arranged in a regular pattern which repeats in all three directions

At the Grain boundary the atoms do not follow the above pattern

Crystal StructureThe atoms in every grain of iron form a regular network, having a cubic structure.

According to temperature, pure iron exist in three crystalline forms: (alpha) iron - which has a body centered cubic structure. (gamma) iron - which has a face centered cubic structure. (delta) iron - which has a body centered cubic structure.

Body Centered Cubic Structure

When the temperature of pure iron is below 910C , the structure of iron takes the shape of a cube having one atom at every corner and one more at the center. Hence this is called Body Centered Cubic - BCC structure. 9 atoms (Fe)

One of the charecteristic of iron is its Magnetism.

But when it is heated between 723 and 910 C, the iron retains its alpha structure but looses its magnetism gradually.

Face Centered Cubic StructureWhen the temperature of iron is between 910 and 1390 C , its structure transforms to another form of cube having one atom at every corner (8) plus one more atom at the center of each face (6).

This structure is called Face Centered Cubic FCC structure. 14 atoms (Fe)

Body Centered Cubic Structure

Finally, between 1390 C and the melting point, 1535 C , the iron takes another form called delta (), which is also having a Body Centered Cubic structure. 9 atoms (Fe)

Allotropy of IronIt must be noted that the iron is present in three crystalline forms: , and .It is not necessary that a metal is in one single crystalline structure system. Depending on the temperature, pressure, composition, etc, it can adopt different crystalline structures. Allotropy.

Allotropy of IronThe allotropy of iron is of great importance to industry:

The transformation of is the one that allows refinement of the grain and tempering of steels. LiquidSpecific VolumeTemperature C.91014011537

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Transformation from alpha to gamma during heating

Transformation from gamma to delta during heating

Transformation from delta to liquid during heating

Structure of iron as a function of temperatureIron alpha Iron gammaIron deltaFerriteAustniteBCCFCCBCC

C% C mass00Line A1Line AcmLine A3723910Line eutectoid 0.8% carbonIRON-C DiagramIdentification of different lines

C% C by mass0072190600721906 Liquid

Hypo-eutectoid steel

Eutectoid steel

Hyper-eutectoid steel

only onlyA)B)C)D)E)F)G) + Fe3C( + Fe3C) (pearlite)Fe3C + + + Fe3C(pearlite) + + Fe3C(pearlite) onlyEutectoidcomposition0.8% carbon

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