Diffusion is a process where material is transported by atomic motion. One of the simplest forms of diffusion , "diffusion bonding", occurs when two materials come in contact with each other. There are two basic mechanisms for diffusion: Vacancy diffusion and Interstitial diffusion.

The reason for the two types of diffusion stems from the relationship between their relative atomic sizes. Vacancy diffusion occurs primarily when the diffusing atoms are of a similar size, or substitutional atoms. The movement of a substitutional atom requires a vacancy in the lattice for it to move into. Interstitial diffusion occurs when the diffusing atom is small enough to move between the atoms in the lattice. This type of diffusion requires no vacancy defects in order to operate.

Diffusion depends on five main variables. These variables are: initial concentration, surface concentration, diffusivity, time, and temperature. The initial concentration of the material that is diffusing into the base ("solid") material is often referred to as the "solute" or impurity which can be but is not always zero. The surface concentration is the amount, weight %, of solute near the surface of the "solid", base, material. Diffusivity is defined as the rate at which the solute or impurity penetrates into the the solid, base, material. The time and temperature are fairly self-explanatory, though it should be noted that the temperature increases the rate of diffusion with increasing temperature.

The following list of topics provide a more comprehensive desription of diffusion:

The Diffusion section of the Class Notebook was a production of the Class of 1997.

Materials Science & Engineering 2094 Class Project, Spring 1995

Any comments and suggestions about the material presented above, please contact Ron Kriz at

Ronald D. Kriz
Virginia Tech
College of Engineering