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Severe plastic deformation (SPD) processes have attracted considerable attention due to their potential for fabricating large quantities of material with an overall small grain size. [1,2] In the last decade, various SPD processes have been proposed for ultragrain refinement, such as high pressure torsion (HPT), [1] equal channel angular pressing (ECAP) [3] accumulative roll bonding (ARB) [4] or-for achieving the smallest grain sizes for pure metals through SPD-repeated cold rolling (RCR) [5,6] as attractive routes for fabricating bulk nanoand submicron-grained materials. With these approaches, high densities of lattice defects are introduced into the material, which, according to the general view, can then rearrange to attain a minimum energy configuration by forming a submicron cell/sub-grain structure that can evolve to a fine grained microstructure with large fractions of high angle grain boundaries (HAGBs) upon continued straining. It has been observed that materials with fine grain sizes that had been synthesized by such a severe deformation route, exhibit spectacular properties and property combinations, such as, e.g., a very high yield strength and high ductility at the same time, enhanced hydrogen storage capacity and enhanced hydrogen permeation velocity or combinations of high mechanical strength and high electrical conductivity, see, e.g., the recent overview in ref. [7] Along with the modification of the grain structure towards finer grains, the high number of lattice defects that are created led to the postulation of modifications of the grain boundary structure to explain the unusual (mechanical) properties that were observed. In the simplest description, high numberBulk nanostructured-or ultrafine-grained materials are often fabricated by severe plastic deformation to break down the grain size by dislocation accumulation. Underlying the often spectacular property enhancement that forms the basis for a wide range of potential applications is a modification of the volume fraction of the grain boundaries. Yet, along with the property enhancements, several important questions arise concerning the accommodation of external stresses if dislocation-based processes are not longer dominant at small grain sizes. One question concerns so-called ''non-equilibrium'' grain boundaries that have been postulated to form during severe deformation and that might be of importance not only for the property enhancement known already today, but also for spectacular applications in the context of, e.g., gas permeation or fast matter transport for self-repairing structures. This contribution addresses the underlying issues by combining quantitative microstructure analysis at high resolution with grain boundary diffusion measurements.758

Severe plastic deformation is nowadays used to produce sizable amounts of bulk nanocrystalline materials, which render them suitable for innovative applications ranging from biomedical implants to off-shore or aerospace structures, owing to favorable combinations of high mechanical strength and enhanced ductility they offer. Enhanced atom diffusion along internal interfaces is largely responsible for the resulting property combinations. Severe plastic deformation processing of metals is demonstrated to create bulk nanostructured materials with a hierarchy of internal interfaces. On top of that, specific diffusion channels providing pathways for ultrafast transport of atoms have been identified. The defects that represent the constituents of the fast diffusion network were visualized by means of the focused ion beam technique. Nonequilibrium grain boundaries, nonequilibrium triple junctions, and microvoids/microcracks compose the percolating network of ultrafast diffusion channels, which represent an important and newly recognized feature of severely deformed materials.

Radiotracer experiments on diffusion of 63Ni and 86Rb in severely deformed commercially pure copper (8 passes of equal channel angular pressing) reveal unambiguously the existence of ultrafast transport paths. A fraction of these paths remains in the material even after complete recrystallization. Scanning electron microscopy and focused ion beam techniques are applied. Deep grooves are found which are related to original high-energy interfaces. In-depth sectioning near corresponding triple junctions reveals clearly multiple microvoids or microcracks caused by the severe deformation. Long-range tracer penetration over tens of micrometers proves that these submicrometer-large defects are connected by highly diffusive paths and that they appear with significant frequency.