  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.
