Investigation of lattice defects in the early stage of fatigue in iron by positron annihilation techniques

Positron annihilation lifetime (PAL) and Doppler broadening (DB) techniques have been performed to identify defects, such as vacancy clusters and dislocations, and to study their distributions on the sample in the early stage of fatigue cycle (0-10% of cycles to failure) in pure Fe. From two-lifetime component analysis, the component of the longer lifetime of about 130 ps, corresponding to a mono-vacancy on edge dislocation, has been observed from 0.01% Nf fatigue. However, the long lifetime component of about 200 ps appears in 10% Nf fatigue at a certain position of the sample. Vacancy clusters have possibly been formed to some extent at this fatigue life. The results suggest that the generation of micro-cracks during fatigue test is related to the formation of three-dimensional vacancy clusters.