A Model of Extrusions and Intrusions in Fatigued Materials. Pt. 1. Point-Defect Production and the Growth of Extrusions

A model of the surface profile of persistent slip bands (PSB's) is proposed. An essential feature is the prediction of vacancies by the annihilation of edge dislocations which, combined with irreversible slip processes, leads to an elongation of the PSB parallel to the active slip vector. A net extrusion forms rather rapidly. The growth processes and their dependence on the temperature are discussed. It is proposed that cracks are initiated at the surface steps at the PSB/matrix interface and later also at the notch/peak profile from the random slip processes. The possibility of void formation is noted. Using HVEM, Ogura and Karashima found 100 nm diam. voids along the active slip plane in fatigued Al, and this appears consistent with a more recent study by Yamamoto, Saka and Imura. It is unclear why such large voids have not been observed by conventional TEM.81 refs.--EPAA/AF.