Q. Define the polymorphism.
Ans. POLYMORPHISM: It is defined as the change in atomic structure which occurs at a definite transformation temperature. The cooling diagram of iron. Iron exists more than one atomic structure depending upon temperature.
The melting point of iron is 1539 degree C. When molten iron solidifies from the liquid state at 1539 degree C and just below 1539 degree C, then the iron has B.C.C. structure.
This structure is called delta iron. When it is further cooled at 1401 degree C. The structure now converts into F.C.C. structure.
This is called γ iron, it is non-magnetic on further drop in temperature i.e. at 910 degree C it again converts in two B.C.C. structure. This is called α-iron. The α-iron is non magnetic up to 768 degree C.
When further drop in temperature from 768 degree C it becomes magnetic but atomic structure remains unchanged.