Dislocation structures near a propagating fatigue crack in an [formula omitted] alloy

Abstract

Notched polycrystalline specimens of an Al 1 2% Mg alloy were fatigue-cracked at four different stress levels to a crack length of approximately 1 mm. After removal from the fatigue machine, the samples were cut using a spark machine to produce disc specimens containing the crack tip. These specimens were electropolished to prepare the material in the vicinity of the crack as thin foil suitable for examination in the transmission electron microscope. All observations relate to specimens cut from bulk material. Progressive modification of the elongated dislocation loop structure observed in regions remote from the crack takes place as the crack is approached. The dislocation loops shorten, and the dislocation density increases. Closer to the crack, a cellular structure is formed out of the pre-existing clusters of dislocation loops, which is steadily refined in size with decreasing distance from the crack tip. A different type of cell structure is observed at the tip and the sides of the crack itself. The shapes and sizes of the zones of deformation have been described for the four applied stresses. The application of the observations to mechanisms of crack extension is briefly discussed.