Polymorphism and Allotropy

Polymorphism is a physical phenomenon where a material may have more than one crystal structure. A material that shows polymorphism exists in more than one type of space lattice in the solid state. If the change in structure is reversible, then the polymorphic change is known as allotropy. The prevailing crystal structure depends on both the temperature and the external pressure.

One familiar example is found in carbon: graphite is the stable polymorph at ambient conditions, whereas diamond is formed at extremely high pressures.

The best known example for allotropy is iron. When iron crystallizes at 2800 oF it is B.C.C. (d -iron), at 2554 oF the structure changes to F.C.C. (g -iron or austenite), and at 1670 oF it again becomes B.C.C. (a -iron or ferrite).

Figure 1. Cooling curve for pure iron. (Allotropic behavior of pure iron)

 

a -iron (alpha) :

Figure 2. Alpha iron (B.C.C) unit cell

The other name for a -iron is ferrite. This crystal has body centered cubic structure. The unit cell and the micrograph of the crystal are shown in Figures (2) and (3).

Figure 3. Ferrite crystals.

 

g -iron (Gamma):

Figure 4. Face centered cubic crystal unit cell.

The other name for g -iron is austenite. This crystal has face centered cubic (F.C.C) structure. The unit cell and the micrograph of the crystal are shown in Figures (4) and (5).

Figure 5. Austenite crystals.

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Last Update: October 16, 1999

By: Serdar Z. Elgun