is the migration of atoms from a region of high concentration
to a region of low concentration. In a homogeneous material, atoms
are routinely moving around but the movement is random (i.e. there
is always an equal number of atoms moving in all directions).
In an inhomogeneous material, all the atoms are moving near randomly,
but there is a migration of atoms to areas where their concentrations
are lower. In other words, there is a net diffusion.
Atom diffusion can occur by the motion of host or substitutional
atoms to vacancies (vacancy diffusion), or interstitial impurities
atoms to different interstitial positions (interstitial diffusion).
In order to move, an atom must overcome the bond energy due to
nearby atoms. This is more easily achieved at high temperatures
when the atoms are vibrating strongly. Carburizing, which will
be discussed later, is an example of diffusion is used.