GRAIN SIZE IN CASTING AND WROUGHT Al-ALLOYS BY POSITRON LIFETIME TECHNIQUE

Positron behavior at grain boundaries is characterized by their bulk diffusibility D, their bulk free lifetime τf and their trapped lifetime τt(τt > τf). AlSi11.35Mg0.23, AlSi10.9Mg0.17Sr0.06, 7075 and 2024 alloys have been homogenized at 673 K for 12 hours. The mean lifetime, as (indirectly) measured at various strains (degrees of deformation), show a good fitting with a FORTRAN program. τf and τt for the above materials are shown as: (187.3 ± 0.10, 229.7 ± 0.10), (198.0 ± 0.10, 230.3 ± 0.10), (205.4 ± 0.05, 213.4 ± 0.06) and (197.1 ± 0.55, 215.2 ± 0.57) Ps respectively. The mean lifetime was found to vary exponentially with the degree of deformation.