Investigation of fatigue cracks in an Al-based alloy by means of pulsed positron (micro-)beams

Positron lifetime studies have been undertaken with the munich pulsed beam and the scanning positron microscope to investigate areas close to fatigue cracks of a precipitation-hardened AlMgSi (6013) alloy. These cracks were initiated by cyclic loading under full automatic stress and crack length control. Close to the surface of fatigue cracks, always three lifetimes of about 245 ps, 300 - 500 ps and 1200 - 2500 ps are observed which are attributed to annihilations at dislocations, vacancy clusters and voids. These defects are believed to indicate remnants of dislocation interactions within the cyclic plastic zone in front of the fatigue crack tip. (orig.)