The diffusion phenomena which were observed in various materials of this type were classified, and illustrated by means of experimental examples. These categories were: materials with a concentration gradient of one component that was different from zero (tracer diffusion, spinodal decomposition, up-hill diffusion), those with a steady-state non-equilibrium structure and a tracer gradient (irradiated, amorphous and nanocrystalline materials), and those which had a non-equilibrium structure which was far from equilibrium (solid-state amorphization, feedback effects of stress fields and pattern formation).

D.L.Beke: Key Engineering Materials, 1995, 103, 51-78