SENSITIVITY OF POSITRON-ANNIHILATION TO PLASTIC-DEFORMATION

In situ measurements of the positron lifetime in copper single crystal s during tensile tests and fatigue experiments have been performed. In tensile tests the mean positron lifetime starts to increase after a t hreshold value for the resolved plastic shear stress of tau approximat e to 10 MPa has been exceeded. This stress, regarded as being a sensit ivity threshold of positron annihilation to homogeneously distributed dislocations, belongs to a critical dislocation density of about 3 X 1 0(12) m(-2) and a mean dislocation spacing of about 0.5 mu m Since thi s spacing equals twice the mean diffusion length of positrons in coppe r (L dagger approximate to 0.25 mu m), positron annihilation is sensit ive to dislocations when each positron has the chance of reaching a di slocation on its random walk. For deformations up to the threshold str ess, positron trapping in vacancies produced by plastic deformation ca n be neglected. In fatigue, the positron lifetime starts to increase w hen a shear stress amplitude of <(tau)over cap>approximate to 8 MPa is exceeded. The corresponding inhomogeneous microstructure has been tre ated tentatively as a two-phase system consisting of dislocation-free (perfect crystal lattice) and densely populated (saturation trapping) areas.