Estimation of Dislocation Concentration in Plastically Deformed Al–Li Based Alloy by Positron Annihilation

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Abstract
Measurements of positron annihilation mean lifetime τ have been performed on eight different specimens of Al–Li based alloy plastically deformed at room temperature up to 40% thickness reduction. This measurement shows clearly positron trapping by dislocations. The positron lifetime τ exhibits a saturation for deformations larger than (15%) thickness reduction. The fitted lifetime varies from (183 ±2 ps) for annealed sample to (205 ±2 ps) for the dislocation saturated value. Using a trapping model, the data yield the values of µ= 3.83×10-8 cm3 s-1 for the specific trapping rate and σ= 3.58×10-15 cm2 for the trapping cross section, some what lower than those for plastically deformed Al single crystals. The value obtained for Δτ, the increase in lifetime of positrons trapped at dislocations in plastically deformed Al–Li based alloy sample over annihilation in the annealed sample, is 22 ps. This is about 40% of the lifetime increase for the case of positrons trapped at dislocations in plastically deformed Al single crystals. Dislocation densities at different thickness reduction have been estimated.