Positron lifetime measurements of single crystals, during tensile tests and fatigue experiments, were performed  in situ. In the case of tensile tests, the mean positron lifetime began to increase when a threshold value (about 10MPa) of the resolved plastic shear stress was exceeded. This stress was regarded as being a threshold of sensitivity of positron annihilation, to homogeneously distributed dislocations, and was associated with a critical dislocation density of about 3 x 1012/m2 and a mean dislocation spacing of about 0.5µ. As this spacing was twice the mean diffusion length (about 0.25µ) of positrons in Cu, the positron annihilation was sensitive to dislocations when each positron had a chance of reaching a dislocation on its random walk. For deformations up to the threshold stress, positron trapping in the vacancies which were produced by plastic deformation could be neglected. During fatigue, the positron lifetime began to increase when a shear stress amplitude of about 8MPa was exceeded. The corresponding inhomogeneous microstructure was treated here as being a 2-phase system that consisted of dislocation-free (perfect crystal lattice) and densely populated (saturation-trapping) areas.

Sensitivity of Positron Annihilation to Plastic Deformation T.Wider, S.Hansen, U.Holzwarth, K.Maier: Physical Review B, 1998, 57[9], 5126-39