Bulk 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.