No kinetic aspect in materials is as fundamental and important as diffusion! Diffu­sion can be defined as the effective movement of atoms/molecules relative to their neighbors under the influence of a gradient. The process is assisted by the intrinsic thermal or kinetic energy of atoms. The driving force or the gradient can be of vari­ous types. It can be chemical potentials arising from the concentration gradient or gradients in electrical field, mechanical stresses, or even gravitational field. The movement of atoms could be over a large number of interatomic distances (i. e., long-range diffusion) or over one or two interatomic distances (i. e., short-range dif­fusion). Although diffusion in liquid and gaseous states is easier to visualize, diffu­sion of atoms in solids is not so. Diffusion is regarded as one of the most important mechanisms of mass transport in materials. It is sometimes difficult to assume that atoms remain “diffusible” until the temperature of the solid is brought down to the absolute zero (still a hypothetical situation though)! It is again amazing to know how many well-known materials phenomena are influenced by diffusion. Here are some examples: phase transformations, precipitation, high-temperature

creep, high-temperature oxidation of metals, metal joining by diffusion bonding, impurity transistors, grain growth, and radiation damage defects and their migration.

There are two general ways by which diffusion can be categorized. If one consid­ers diffusion of atoms in a pure metal, the diffusion happens basically between its own lattice atoms. This diffusion is called self-diffusion. On the other hand, diffusion of alloying elements or impurities may well be occurring in the parent lattice and then the diffusion is termed as heterodiffusion. In the following section, the macro­scopic diffusion theories are first dealt with and then the topics of atomic diffusion.

2.3.1