Diffusion in Solids

Why solid-state motion matters

Atoms move through solids slowly but decisively. Diffusion enables carburising, precipitation, sintering, homogenisation, oxidation and many phase transformations.

Two descriptions

Fick’s first law describes steady-state flux driven by a concentration gradient. Fick’s second law describes how concentration changes with time under non-steady conditions.

Temperature dominates

Diffusivity follows an Arrhenius relation. A modest temperature increase can produce orders-of-magnitude faster diffusion. Interstitial atoms generally move faster than substitutional atoms because they do not require the same vacancy exchanges.

Fast paths

Grain boundaries, dislocation cores and surfaces provide rapid diffusion routes. Fine-grained materials may therefore react, sinter or creep differently from coarse-grained versions of the same composition.

Takeaway

Whenever a process requires atoms to rearrange, estimate the diffusion distance and ask whether the available temperature and time are sufficient.