Grain boundaries in ultrafine grained materials processed by severe plastic deformation and related phenomena

Highlights: ► UFG materials produced by SPD do exhibit specific GBs defined earlier as "non-equilibrium GB". ► Such boundaries differ from regular GBs on atomic structure and appearance of long-range stresses. ► The apparent thickness of "non-equilibrium" GB is typically in a range of 1 to 2 nm. ► GBs in SPD materials may promote segregations and mechanical mixing that affect the properties. ► "Non-equilibrium" GBs do significantly enhance the atomic mobility in SPD materials. - Abstract: Grain boundaries in ultrafine grained (UFG) materials processed by severe plastic deformation (SPD) are often called "non-equilibrium" grain boundaries. Such boundaries are characterized by excess grain boundary energy, presence of long range elastic stresses and enhanced free volumes. These features and related phenomena (diffusion, segregation, etc.) have been the object of intense studies and the obtained results provide convincing evidence of the importance of a non-equilibrium state of high angle grain boundaries for UFG materials with unusual properties. The aims of the present paper are first to give a short overview of this research field and then to consider tangled, yet unclear issues and outline the ways of oncoming studies. A special emphasis is given on the specific structure of grain boundaries in ultrafine grained materials processed by SPD, on grain boundary segregation, on interfacial mixing linked to heterophase boundaries and on grain boundary diffusion. The connection between these unique features and the mechanical properties or the thermal stability of the ultrafine grained alloys is also discussed.