Mumbai University > Mechanical Engineering > Sem 4 > Material Technology

Marks: 5M

Year: May 2014

Allotropy or allotropism is the property of some chemical elements to exist in two or more different forms, known as allotropes of these elements. Allotropes are different structural modifications of an element; the atoms of the element are bonded together in a different manner.

Iron has three allotropic forms of crystal at different temperature. It undergoes all the allotropic forms when it is heated from normal temperature to high temperature (molten state).

1. Alpha Iron. It occurs from normal temperature to 910 degree centigrade and has got body-centered cubic (BCC) lattice crystals.
2. Ferromagnetic alpha iron which occurs from normal temperature to 770 degree centigrade.
3. Paramagnetic alpha iron which occurs from 770 degree centigrade to 910 degree centigrade.
4. Gamma Iron. This occurs from 910 degree centigrade to 1400 degree centigrade, and has got crystal structure of face-centered cubic (FCC) lattice.
5. Delta Iron. This occurs from 1400 degree centigrade to 1539 degree centigrade (molten state), and has got crystal structure of body-centered lattice.

Pure Iron is soft and has got silvery white colour. It is strongly magnetic in presence of a magnetic field or electric current. When inducing field is removed the induced magnetism is not retained by pure iron. This power of retentively of magnetism of pure iron is improved by the addition of other elements such as carbon, cobalt, or nickel. Iron loses its magnetic properties when heated to 770 degree centigrade.