Annihilation of positrons in F.C.C. cold-worked polycrystals

Positron annihilation techniques have been used to estimate dislocation densities induced in cold rolled f.c.c. polycrystals by plastic deformation. The positrons are assumed to be trapped by defects in the crystal lattice, thus causing the narrowing of the annihilation lineshapes obtained in a Ge(Li) spectrometer. A trapping cross section is derived from the experimental results, using a theoretical model. In plastically deformed polycrystals of Ni, Cu, Ag, and Au a linear strain hardening range is observed up to 15 to 20% deformation. In that range, dislocation densities vary from about 108 to 1010 cm−2 for Ni and Cu, and from about 107 to 109 cm−2 for Ag and Au. The trapping cross section of positrons by dislocations were found to be 6 × 10−16 cm2 for Ni, 9 × 10−16 cm2 for Cu, 5 × 10−15 cm2 for Ag, and 2 × 10−15 cm2 for Au.