Diffusion Mechanisms in Various Materials

Diffusion of relatively small atoms that normally occupy interstitial sites in the solvent crystal generally occurs by the interstitial mechanism. For example, hydrogen atoms are small and migrate interstitially through most crystalline materials. Carbon is small compared to Fe and occupies the interstitial sites in b.c.c. Fe illustrated in Fig. 8.8 and migrates between neighboring interstitial sites. 

Migration of atoms that occupy substitutional sites may occur through a range of mechanisms involving either vacancy- or interstitial-type defects. In f.c.c., b.c.c., and hexagonal close-packed (h.c.p.) metals, self-diffusion occurs predominantly by the vacancy mechanism [4, 5]. However, in some cases self-diffusion by the 

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