Abstract
An electronic fatigue machine has been constructed and used to fatigue stable and metastable beta brass in reverse bending beneath the lens of a microscope provided with stroboscopic illumination. This enabled the progress of fatigue to be continously observed during tests. A study has been made of resulting fatigue damage using X-ray and various metallographic techniques such as taper sectioning and the removal of surface layers by electro-polishing. The fatigue behaviour of stable beta brass is shown to be in some respects unusual and to exhibit certain features previously unreported. One of the most important observations has been the initiation of fatigue cracks and the formation of extrusions in short slip markings adjacent to grain boundaries. Subsequent removal of fatigue damage by electro-polishing has revealed 'persistent markings' which indicate that these cracks originate in the layers adjoining grain boundaries and not in the grain boundaries. These cracks afterwards propagated into and across grain boundaries. Fatigue cracks have also been seen to be initiated at triple point intersections of grain boundaries. Intense and 'permanent' slip markings excluding those described occurring adjacent to grain boundaries have not been observed to form. Extrusions were found to be rare and comparatively small. Fatigue debris considered to be an attrition product consisting of oxidised fragments of metal has been observed. An endeavour has been made to explain these phenomena in terms of the nature of dislocations and their movements in the superlattice. It is thought that cross slip is unlikely to occur and that this inhibits the formation of intense slip markings and extrusions. A disordered grain boundary layer had been proposed to account for the initiation of cracks adjacent to grain boundaries. A fatigue induced precipitate has been described in metastable beta and an explanation has been given in terms of enhanced diffusion due to the generation of vacancies by moving dislocations during fatigue.