Growth of slip bands during fatigue of 6061-T6 aluminum

Abstract

The growth of persistent slip bands (psb), the initial surface manifestation of metal fatigue, was measured with a photoelectron microscope equipped with a fatigue stage. Once the psb had been identified and located, the specimen was subjected to a final detailed examination by scanning electron microscopy. In 6061-T6 aluminum psb initiated within a grain, usually at the site of an inclusion. The psb appeared as a small extrusion and elongated across the grain by the sequential addition of new extrusions. As the psb elongated, the initial extrusions became more pronounced and eventually microcracks developed. Under constant amplitude loading, the rate of elongation of a psb in polycrystalline material varied inversely as the length, whereas in a large grain specimen the rate remained constant. This difference is attributed to the constraints imposed upon a small grain by the surrounding material. The growth laws can be accounted for in terms of a simple two phase model in which the psb has a much lower yield stress than the matrix of the grain.