Positron annihilation investigation for an estimation of the dislocation density and vacancy concentration of plastically deformed polycrystalline Ni of different purity

From the angular correlation measurements of the positron annihilation radiation on cold rolled, polycrystalline Ni of purity 99.99% and 99.84% the curve-shape parameters S are determined for treating the as-deformed and annealed samples up to a thickness reduction of 90%. The trapping rate of the positrons can be determined for a thickness reduction up to 15 or 18% for the dislocations and for the vacancies as well. Using known relations for the hardening within range 2 the specific trapping rates are estimated for the dislocations, μd = 2.9 × 10−15 s−1, and for the vacancies, μv = 2.2 × 10−15 s−1. Estimated dislocation densities and vacancy concentrations vary between 108 and 1011 cm/cm3 and 1016 and 1019 cm−3, respectively. Both of them show a remarkable dependence on the purity of the samples. Ni of low purity exhibits formation of vacancy clusters after annealing.