Diffusion is the mass flow process in which atoms change their positions relative to their neighbours in a phase under the influence of thermal energy and a concentration gradient.
Interstitial diffusion
The diffusing atoms jumps from one interstitial to neighboring interstitial site in executing a unit step. The neighbouring site is usually vacant in dilute solutions. The activation energy is simply the energy barrier along the path from one interstitial to next.
Substitutional diffusion
This occurs with the aid of vacancies. A substitutional atom jumps from a regular site into a neighbouring vacant site in executing the unit step. In addition to the probability of an atom crossing the activation barrier along the path, the probability of the neighbouring site being vacant has to be considered.
Hence, substitutional diffusion is generally 4 – 6 orders of magnitude slower than interstitial diffusion.