Thermal stability of ultra-fine grained (UFG) nickel (mean grain size 114 nm) prepared by high pressure torsion was studied by means of positron-lifetime spectroscopy (PLS) combined with TEM. The experimental results obtained by PLS are interpreted using the diffusion trapping model, which allows for determination of important physical parameters characterizing the specimens. The microstructure of the material studied is strongly inhomogeneous. The grain interiors with low dislocation density are separated by distorted regions with high numbers of dislocations. We have found that positrons are trapped at dislocations inside the distorted regions and in the microvoids situated inside the grains. Structure evolution with increasing temperature was studied in details using isochronal annealing of the specimen. We have found that recovery of the UFG structure involves the abnormal grain growth followed by further recrystallization in the whole volume of samples. It was shown that PLS is sensitive to structure changes, caused by the magnetostriction phenomenon.

Positron-lifetime (PL) and electrical resistivity studies of polycrystalline aluminium deformed by tensile stress at T 77 K were performed. It was found that the conventional trapping model (CTM) fails in interpretation of measured PL spectra due to strongly non-uniform spatial distribution of dislocations in the deformed samples. Therefore, a modified trapping model (MTM) was used in this study. Strongly non-uniform dislocation density was directly taken into account in the development of MTM. Comparison of the results obtained by MTM and CTM is given in the present work. Mean cell size in the deformed samples obtained from MTM analyses of PL spectra are compared with those measured by TEM. 440 J. C Ï i Âz Ïek et al. J. C Ï i Âz Ïek et al.