Calculation of Free Volume in Computer Generated Grain Boundaries

The reduction of the grain size down to the nm regime has opened new and fascinating avenues for research in several aspects of materials science, including mechanical properties. In recent years large efforts, both experimental and computational, have been made to understand the deformation mechanisms at the nanoscale. Computer simulations suggest both grain boundary accommodation mechanisms and intra granular slip involving dislocation emission and absorption at grain boundaries, as possible deformation mechanisms. In these simulations the structure of the grain boundaries plays a crucial role. In the past 15 years positron annihilation lifetime spectroscopy has been used extensively to investigate the defect structure at grain boundaries in nanocrystalline (nc) materials. However, grain boundaries are often composed of very complex defects structures that, in principle, cannot be described by one or two single positron lifetimes components. Recently we have developed a method to calculate the positron signal in very large simulated nanostructures, which enables us to gain a better understanding of the free volume distribution in grain boundaries [1–4].