By severe plastic deformation of metals defects like vacancies, dislocations and grain boundaries are generated. It will be shown by a variety of experimental techniques that these defects are stabilized in iron by segregation of carbon to these defects. Excess vacancies and their clusters are determined by positron annihilation spectroscopy and indirectly by orders of magnitude enhanced diffusion of nickel. Transmission Electron Microscopy (TEM) and Atom-Probe-Tomography (APT) reveal the nanocrystalline microstructure and carbon segregation to grain boundaries. The excess carbon reduces defect formation energies and leads to an increased defect density during plastic deformation. This is quantitatively described in the defactant concept (Kirchheim 2007a).
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