Diffusion in solids occurs by which mechanisms?

• A. Vacancy diffusion, interstitial diffusion, and exchange mechanisms.
• B. Dislocation glide, climb, and climb-assisted diffusion.
• C. Grain boundary diffusion only.
• D. Spinodal decomposition.

Answer: (A)

At the heart of diffusion in solids is how atoms move through a crystal lattice. Atoms migrate by discrete hops that change their local surroundings. The main pathways are: vacancy diffusion, where an atom jumps into a neighboring empty lattice site and leaves behind a vacancy; interstitial diffusion, where small atoms move through the gaps between lattice atoms by hopping from one interstitial site to another; and exchange diffusion, where an atom swaps places with a neighboring atom, effectively moving without creating a separate vacancy or interstitial. These mechanisms together describe how atoms transport through the solid bulk.

In real materials, diffusion can be further enhanced along defects such as grain boundaries or dislocations, giving routes like pipe diffusion along dislocations or boundary diffusion along grain boundaries, but these are defect-assisted paths rather than the fundamental bulk mechanisms. Spinodal decomposition, while driven by diffusion, is a phase-separation process rather than a diffusion mechanism by itself.