The accumulation of plastic strain by low-cycle fatigue had long been expected to produce vacancy clusters; particularly in the most damaged regions, such as those close to a fatigue crack. Here, for the first time, scanning positron microscopy permitted detection of the expected vacancy clusters via positron lifetime studies of a fatigue crack at micron-scale resolution. A fatigue crack with a length of about 8mm was created in a single edge-notched specimen of cold-rolled technical-grade Cu. Fatigue crack propagation tests were performed, under K-controlled conditions, at a constant stress intensity factor of 9MPam1/2. Lifetime images from a region of about 200 x 400μm2 around the crack tip were obtained, at a spatial resolution of about 5μm, for 5, 8 and 16keV positron implantation energies. Regardless of the position, a positron lifetime of about 190ps was observed; indicating annihilation of the positrons at dislocations. However, within about 40μm of the crack path, a second lifetime of 360 to 420ps was observed at all positron implantation energies; with an intensity of up to 25%. Therefore, in this region there were suggested to exist large vacancy clusters with a trapping-rate which was comparable to that of the dislocations.

Vacancy Clusters Close to a Fatigue Crack Observed with the München Scanning Positron Microscope. W. Egger, G.Kögela, P.Sperr, W.Triftshäuser, S.Rödling, J.Bär, H.J.Gudladt: Applied Surface Science, 2002, 194[1-4], 214-7