Lattice Distortion
Kinetic acceleration phenomena where mechanical stress fields alter the migration rate of atoms through a crystal lattice provide a link between mechanical state and chemical transport. Activation of strain assisted diffusion occurs because the deformation of the lattice changes the energy barriers that an atom must overcome to move between sites. Tensile strain generally increases the free volume and lowers these barriers while compressive strain can have the opposite effect on the jump frequency.
Activation Energy
Changes in the distance between neighboring atoms modify the electronic environment and the potential energy landscape. This distortion affects both the formation energy of vacancies and the migration energy of the diffusing species. In thin films, the mismatch with the substrate provides a constant source of strain that can be used to tune material properties.
Flux Enhancement
Stress gradients create an additional driving force for diffusion beyond the simple concentration gradient. Atoms tend to move toward regions where their presence reduces the total elastic energy of the system. This behavior leads to the segregation of solutes at crack tips or grain boundaries where the stress is most intense.
Mechanical Limit
High levels of strain may eventually trigger plastic deformation through the movement of dislocations. Once dislocations are present, they provide high speed pipes for transport. The transition between these two modes depends on the temperature and the yield strength of the material.
