DIFFUSION MECHANISMS

from an atomic perspective,diffusion is just the stepwise migration of atoms from lattice site to lattice site.in fact,the atoms in solid materials are in constant motion,rapidly changing positions.for an atom to make such a move,two conditions must be met :(1) there must be an empty adjacent site,and(2) the atom must have sufficient energy to break bounds with its neighbor atoms and then cause some lattice distortion during the displacement.this energy is vibrational in nature (section 4.7) . at a specific temperature some small fraction of the total number of atoms are capable of diffusive motion,by virtue of the magnitudes of their vibrational energies . this fraction increases with rising teperature.

            several different models for this atomic motion have been proposed ;of these possibilities,two dominate for metallic diffusion.

VACANCY DIFFUSION

one mechanism involves the interchange of an atom from a normal lattice position to an adjacent vacant lattice site or vacancy,as represented schematically in figure 5.3a .this mechanism is aptly termed vacancy diffusion.of course,of course ,this process necessitates the presence of vacancies,and the extent to which vacancy diffusion can occur is a function of the number of these defect that are present ; significant concentrations of vacancies may exist in metals at elevated temperatures (section 4.2).since diffusing atoms and vacancies exchange position ,the difusion of atoms in one direction corresponds to the motion of vacancies in the opposite direction .both self-diffusion and inter diffusion occur by this mechanism; for the latter,the impurity atoms must substitute for host atoms.