Bernd Oberdorfer

Dilatometry: a powerful tool for the study of defects in ultrafine-grained metals

Vacancies, dislocations, and interfaces are structural defects that are deliberately introduced into solids during grain refinement processes based on severe plastic deformation (SPD). Specific combinations of these defects determine the improved mechanical properties of the obtained ultrafine-grained materials. High-precision, non-equilibrium dilatometry, i.e., measurement of the irreversible macroscopic length change upon defect annealing, provides a powerful technique for the characterization and the study of the kinetics of these defects. It is applied to determine absolute concentrations of vacancies, to characterize dislocation processes, and to assess grain boundary excess volume in pure, FCC and BCC ultrafine-grained metals processed by SPD.

Direct measurement of vacancy relaxation by dilatometry

A model is proposed for directly determining the volume of lattice vacancies by means of dilatometric measurements of the anisotropic irreversible length change which occurs during annealing of lattice vacancies at grain boundaries of shape-anisotropic crystallites. The model is tested using nanocrystalline Ni after the high-pressure torsion deformation which exhibits excess concentration of lattice vacancies and elongated crystallite shape. Different length changes upon annealing parallel and perpendicular to the elongation axis occur from which a vacancy volume can be derived.

Comprehensive defect characterization of different iron samples after severe plastic deformation

Bulk nanostructured materials produced by means of severe plastic deformation such as high-pressure torsion (HPT) show a high strength in combination with good ductility. These materials exhibit a microstructure down to grain sizes of approximately 100 nm. The defects and microstructure produced by HPT are severely affected by the presence of alloying components. Two iron disks with a purity of about 99.8% (ARMCO) and 99.98+% were deformed by HPT conditioning. Slices of the deformed material were then investigated and subjected to thermal treatment. Upon isochronal annealing at increasing temperatures the decrease of the deformation induced defects was monitored. The defect structure and its annealing behavior are investigated by positron lifetime spectroscopy and Doppler broadening spectroscopy. In addition specimens were analyzed by high precision difference dilatometry, while the evolution of the microstructure is monitored by electron microscopy. In addition, in situ Doppler broadening measurements upon annealing were performed at the high intensity positron beam at FRM II. The lifetime data in dependence of the annealing temperature are fitted to a model including positron trapping at grain boundaries and vacancy-type defects.