Invited Lecture
Diffusion-related phenomenon in ultrafine-grained materials

Studies of the last decade have provided convincing evidence that severe plastic deformation by high pressure torsion or by equal channel angular pressing is a very effective approach to produce ultrafine-grained (UFG) metallic materials with extremely high fraction of grain boundaries (GBs). It is small surprise that GB diffusion controls various properties in the UFG materials. This report presents the results of recent studies of some diffusion-related phenomena, such as corrosion, nitriding and superplasticity, in a number of UFG materials produced by SPD processing. Depending on the SPD processing regimes (temperature, rate and degree of strain), the UFG materials are shown to possess different types of GBs: low- and high-angle, special and random, equilibrium and non-equilibrium ones with strain-distorted structure, and GBs may also include the segregations of alloying elements. These structural features of GBs control the mechanisms and kinetics of GB diffusion and are sure to define the properties of UFG materials. On the basis of the obtained results, the nature of GB diffusion in these materials is discussed and the ways of improving their physical and mechanical properties of interest are considered.