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.