Vacancy clusters close to a fatigue crack observed with the München scanning positron microscope

The accumulation of plastic strain by low cycle fatigue has been expected for a long time to induce vacancy clusters, particularly in the most damaged region, i.e. close to a fatigue crack. For the first time, the München scanning positron microscope (SPM) enables the detection of the expected vacancy clusters by positron lifetime studies of a fatigue crack with micrometer resolution. A fatigue crack with a length of about 8 mm was created in a single edge-notched specimen of cold-rolled technical copper. The fatigue crack propagation test was performed under K-controlled conditions with a constant stress intensity factor of 9 MPa m1/2. Lifetime images from a region of about 200×400 μm2 around the crack tip were obtained at about 5 μm spatial resolution for 5, 8 and 16 keV positron implantation energies. Independent of position, a positron lifetime of about 190 ps is observed, indicating annihilation of positrons at dislocations. Within about 40 μm from the crack path, however, a second lifetime in the range of 360–420 ps is observed at all positron implantation energies and with an intensity up to 25%. Therefore in this region there must be large vacancy clusters with a trapping rate comparable to one of the dislocations.