# Diffusion of carbon from ferrite to cementite in the eutectoid transformation

Why does the carbon of an alloy diffuse from ferrite (0.022% C) to cementite (6.70% C) during eutectoid transformations? I know that the diffusion of a generic species (e.g. of atom A) carries A from regions where the amount of A is higher to regions where the amount of A is lower.

I found this sentence on page 337 of Callister (9th ed.).

• The rule you mention does not apply if different phases are involved. Moreover, cementite is iron carbide $\ce{Fe3C}$ that crystallizes during the transformation. The 6,7% C come from its stoichiometry. Feb 8 '17 at 20:52