Sensitivity of positron annihilation to plastic deformation

In situ measurements of the positron lifetime in copper single crystals during tensile tests and fatigue experiments have been performed. In tensile tests the mean positron lifetime starts to increase after a threshold value for the resolved plastic shear stress of τ≈10 MPa has been exceeded. This stress, regarded as being a sensitivity threshold of positron annihilation to homogeneously distributed dislocations, belongs to a critical dislocation density of about 3×10¹²m^-2 and a mean dislocation spacing of about 0.5 μm. Since this spacing equals twice the mean diffusion length of positrons in copper (L₊≈0.25μm), positron annihilation is sensitive to dislocations when each positron has the chance of reaching a dislocation on its random walk. For deformations up to the threshold stress, positron trapping in vacancies produced by plastic deformation can be neglected. In fatigue, the positron lifetime starts to increase when a shear stress amplitude of τ̂≈8MPa is exceeded. The corresponding inhomogeneous microstructure has been treated tentatively as a two-phase system consisting of dislocation-free (perfect crystal lattice) and densely populated (saturation trapping) areas.