Diffusion can simply be thought of as the rearrangement of the atoms inside the crystal (lattice) structure of the metal. Diffusion is controlled by the rate at which these atoms change position and increases exponentially when temperature is applied. The mechanisms involved are:

  • Vacancy diffusion (Fig. 1) is the predominant diffusion mechanism in metals due to the low energy required to move atoms into atomic vacancies that form during heating. The vacancy diffusion process occurs when an atom on a normal lattice site jumps into an adjacent unoccupied (vacant) site. It turns out that only adjacent atoms move into a vacancy. The rate depends on concentration of vacancies in the lattice.
  • Chemical diffusion (aka inter-diffusion or impurity diffusion; Fig. 2) is the exchange of atoms when two metals or alloys are placed in contact and migration across the contact boundary occurs by atom exchange. Diffusion of atoms is typically from high concentration to low concentration.
  • Interstitial diffusion (Fig. 3) occurs if a solute atom is sufficiently small and moves to a position between larger solvent atoms in an energy-favorable configuration. Atoms move from an interstitial position to another interstitial site nearby. Migrating atoms are typically small in size (e.g., N, C, H and O). Interstitial diffusion occurs much more rapidly than diffusion by the vacancy mode since interstitial atoms are smaller and more mobile. This movement (or jump) involves a considerable distortion of the lattice.
  • Self-diffusion (Fig. 4) occurs when the atoms of a metal exchange positions within that metal.
  • Grain boundary (Fig. 5), or dislocation core, diffusion occurs along these defects in the crystal structure due to high interfacial energy and relatively weak bonding.


As one would suspect, diffusion rates are slower in solids than liquids. In metals, atomic movements are restricted to so-called equilibrium positions (due to bonding considerations). If energy (heat) is introduced, the resulting thermal vibrations of the atoms about their equilibrium position and, at sufficiently high energy levels, may be strong enough to break the bonding and make the atom move.
 

Application Examples

Processes that rely on diffusion mechanisms include case hardening such as boronizing (the addition of boron), carbonitriding (the addition of carbon and nitrogen), carburizing (the addition of carbon) and nitrocarburizing (the addition of nitrogen and carbon). Other examples of applications that utilize diffusion are:

  • Case hardening of steel (e.g., carbon diffusion in steel)
  • Oxidation of metals
  • Sintering (fusion of powder particles at solid state)
  • Doping (e.g., electronic semiconductors)


Summary

Understanding the diffusion mechanism in ferrous alloys is necessary to help us predict how irons and steels will respond to heat treatment. Next time we will talk about nonferrous alloys.
 

References
1.    Herring, Daniel H., Atmosphere Heat Treatment, Volume I, BNP Media, 2014.
2.    Diffusion in Solids, IE-114 Materials Science and General Chemistry, Lecture 5, Çankaya University
3.    Faculty of Engineering, Kiel University, Germany (www.tf.uni-kiel.de/en)