Positron annihilation lifetime and Doppler broadening measurements were used to study the early stages of fatigue of type-304 stainless steel at room temperature. It was found that the average positron lifetime and the S-parameter increased with increasing number of fatigue cycles for each point of the specimen. Two-lifetime component analyses showed that a longer-lifetime component, of about 170ps, corresponded to single or divacancies which were observed from 0.01% of Nf. It remained stable up to 10% of Nf. However, the lifetime increased rapidly to about 230ps on approaching a fatigue life of about 20% of Nf at a certain point of the sample. Vacancy clusters were thought to have formed to some extent at this fatigue life. Fatigue failure occurred at almost the same point at which the longer positron lifetime was observed. The results suggested that the generation of micro-cracks during fatigue test was related to the formation of 3-dimensional vacancy clusters. Detailed measurements of the positron lifetime and Doppler broadening were also carried out on small areas of fractured samples.

Positron Annihilation Study of the Early Stage of Fatigue in Type 304 Stainless Steel. F.Hori, R.Oshima: Physica Status Solidi A, 2002, 191[2], 409-17