  In the last decades severe plastic deformation techniques have gained
increasing interest as they allow the production of bulk nanostructured
materials with superior mechanical and functional properties. However, because
of mechanically induced grain boundary migration, the achievable grain size
reduction is not indefinite but tends to stagnate once sufficient strain has
been applied. Consequently, addition of solute elements or second phase
particles offers the possibility to access the true nanocrystalline regime. Due
to their low solubility and high mobility, interstitial elements are extremely
effective at subduing boundary migration. Herein the effect of carbon on grain
refinement and the resulting mechanical properties are summarized. As carbon
may not only be added as graphite but could also be introduced in other forms
or as allotropes such as nanotubes, nanodiamonds, or carbides, the respective
advantages and problems associated with it are the center of discussion.
Independent of the strategy used, strength levels hardly achievable with other
alloying elements can be obtained. Moreover, as carbon does not have a negative
effect on grain boundary cohesion, despite the enormous strength levels even
ductility and toughness can be widely maintained.
