Christine Borchers

Abstract:

Through severe plastic deformation of metals, defects like vacancies, dislocations, and grain boundaries are generated. These defects are stabilized in metals by segregation of hydrogen and carbon to these defects. Excess vacancies and their clusters are determined by positron annihilation spectroscopy and indirectly by orders of magnitude enhanced diffusion coefficients. Transmission Electron Microscopy (TEM) and Atom-Probe-Tomography (APT) reveal the nanocrystalline microstructure and segregation to grain boundaries. The excess solute reduces defect formation energies and leads to an increased defect density during plastic deformation. This is quantitatively described in the defactant concept [1,2]. According to a theory originating from Gibbs [3], an excess of a different substance at boundaries can reduce their energy, leading to a reduced driving force for their annihilation, the most prominent example of which being the reduction of surface energy by surfactants like foaming agents. This was extended to grain boundaries containing segregated alloying elements, and to general defects like vacancies and dislocations [1,2]. It was proposed that these alloying elements lead to reduced defect formation energies which in turn lead to increased defect densities, the limiting case of which is an amorphization of the structure.