diffusion mechanisms

If atoms in a solid move by hopping, the natural question is: hopping where, exactly? There are two main answers, two distinct mechanisms, and which one applies depends entirely on whether the moving atom is about the same size as the host or much smaller. Getting this distinction straight explains why some elements diffuse through a metal thousands of times faster than others at the same temperature.

The vacancy mechanism handles atoms that sit on regular lattice sites — the host's own atoms (self-diffusion) and any similar-sized substitutional solute. An atom can only move if there happens to be an empty site (a vacancy) right next to it; then it hops in, and the vacancy effectively moves the other way. Since the atom must wait for a vacancy to arrive and must have the energy to make the jump, this mechanism is comparatively slow, and its rate is tied directly to how many vacancies exist — which, recall, climbs exponentially with temperature. The interstitial mechanism handles small atoms — carbon, nitrogen, hydrogen — that live in the gaps between host atoms. Such an atom simply hops from one interstitial gap to an adjacent empty one. It does not need to wait for a vacancy at all, because interstitial sites are plentiful and mostly empty, so this mechanism is far faster.

The practical upshot is a big speed gap. At a given temperature, carbon (interstitial) diffuses through iron orders of magnitude faster than nickel (substitutional) does, precisely because carbon never has to wait for a vacancy and squeezes through a smaller barrier. This is why carburizing (an interstitial process) can harden a steel surface in hours, while homogenizing a substitutional alloy can take much longer. When you see a diffusion activation energy quoted, it reflects which mechanism is at work: interstitial diffusion has a lower activation energy, substitutional (vacancy) diffusion a higher one.