Slip-band damage and extrusion

Slip-band extrusion was first observed in a fatigued aluminium + 4% copper alloy, but has now been found to occur in many other materials. It takes the form of a thin ribbon or scroll of the crystal material projecting as much as 20 $\mu $ from the slip band. The thickness of the ribbon varies with different materials and fatigue conditions, that observed on the above alloy being less than 0$\cdot $1 $\mu $. The process of reverse slip which causes slip-band ridges and grooves on the surface of fatigued materials is thought to be the mechanism by which the extrusion occurs. However, the confinement of the slip to a few active planes as occurs in most aluminium alloys is due to some localized softening process. The local softening processes can be identified in some materials as recrystallization, ageing and a combination of both effects. In pure aluminium and some aluminium alloys holes may also be observed along the slip bands. By using silver chloride it has been revealed that both extrusion and crevices may form by what seems to be the same process of reverse slip. In this material, cracks may propagate by the formation of voids ahead of the root of the slip crevice.