Measurements of defect structures of a cyclically deformed Al–Mg–Si alloy by positron annihilation techniques

Defect distributions close to cracks created by monotonic and fatigue tests in the Al–Mg–Si alloy Al 6013 have been investigated by positron lifetime studies. The defect structure close to the crack surface was studied at sub-μm resolution with a pulsed positron beam, whereas the lateral defect distribution in the crack-tip near region was determined using a scanning positron microscope with a few μm resolution. A high concentration of large vacancy clusters of up to 30 vacancies and voids close to the fatigue crack, as well as a high dislocation density in a region with an extension of ∼100 μm around the crack tip, were detected in the fatigued samples. Moreover, the generation of vacancy clusters found in the fatigued samples seems to be independent of the loading history. In monotonically fractured samples, no vacancy clusters have been found. If this difference holds as a general rule, positron lifetime spectroscopy would be suitable to distinguish quantitatively monotonic from fatigue fracture without any further fractographic studies.