Abstract
A high density of lattice defects is introduced to materials by severe plastic deformation (SPD). Numerous experimental techniques, in particular electron microscopy, X-ray, electron and neutron diffraction, etc. are employed to characterize the evolution of microstructure and defects with strain introduced to the material by SPD.
These techniques concentrate mainly on the investigation of planar (grain boundaries) and line defects (dislocations). On the other hand, point defects, namely vacancies and their agglomerates are investigated in less detail.
Positron annihilation spectroscopy (PAS) proved to be an effective method for the investigation of point defects and dislocations in ultra-fine grained (UFG) materials. This study summarizes the results of the investigation of lattice defects in UFG metals with fcc (Al, Ni, Cu), bcc (Fe, Nb, W) and hcp (Mg, Ti) structure prepared by high pressure torsion (HPT).
Two techniques of PAS were employed (i) positron lifetime spectroscopy (LT) allowing to characterize the type and concentration ratio of lattice defects in the severely deformed material and (ii) Doppler broadening (DB) of annihilation radiation providing analysis of the homogeneity of the UFG structure and spatial distribution of defects. The latter technique was complemented by mapping of microhardness distribution throughout the surface of the HPT specimens.
The LT studies revealed that HPT straining at room temperature introduced not only dislocations but also a high concentration of vacancies. A significant fraction of deformation-induced vacancies disappeared by diffusion to sinks at grain boundaries.
Remaining vacancies agglomerated into vacancy clusters. The average size of vacancy clusters differs in various metals and is affected by the activation energy for migration of vacancies in the given material.
The analysis of DB of positron annihilation radiation and its correlation with microhardness distribution indicated that dislocation density tends to saturate with strain. On the other hand, the spatial (lateral) distribution of vacancy clusters remains non-uniform even in samples subjected to a high number of HPT revolutions.
The average size of vacancy clusters increases with radial distance from the centre of the sample due to the increasing production rate of vacancies.