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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.).

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  • $\begingroup$ 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. $\endgroup$
    – aventurin
    Commented Feb 8, 2017 at 20:52

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Diffusion is perhaps more complicated than initially taught. Yes, all things being equal you will get diffusion from regions of high concentration to regions of low concentration.

But, in the case you mention, all things are not equal - there is a phase change occurring, and that means that there are chemical potential gradients as well as concentration gradients. Now you have an additional driving force for motion of impurities as they prefer to be in the phase with a lower Gibbs free energy.

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