The solubility of the alloying elements in the matrix must also be considered. If too much material is added it will be unable to all be in solution in the matrix and second phases will form. In the iron-carbon system one of these phases is the chemical compound Iron Carbide, Fe₃ C. The balance of the carbon can be in solid solution in the iron matrix. The structure of this two phase binary alloy, Pearlite, is shown below.

In the photograph the white sections are the interstitial solid solution of carbon in iron (alpha-iron or Ferrite) and the black regions are the iron carbide (Fe₃ C ). The lamellar structure of this solid alloy makes deformation hard, and this material requires more work to deform than ferrite. The boundaries between the phases hamper the shear-slip processes between crystal planes that permit plastic deformation to occur. These slip processes do not involve complete crystal planes, but local slip on these planes controlled by "dislocation" motion

From: Callister, "Materials Science and Engineering, 4th Edition"
Wiley (1997)