Damage evolution under severe plastic deformation

The development and recovery of damage in continuously cast aluminium alloy 6061 due to plastic deformation is investigated for different stress histories. The processes of Equal Channel Angular Extrusion and Equal Channel Angular Drawing are used to introduce damage into the specimen for a specified stress history. The amount of plastic deformation is determined by the angle between the two intersecting channels, while the stress history is varied by applying different back-pressures. The damage is related to the density, measured using Archimedes' principle. The development of damage was observed to increase proportionally with the extent of accumulated plastic shear strain. The influence of stress history, characterised by a stress index, was found to be twofold. First, the stress index defines the intensity of the porosity development, which increases with the stress index as it changes from negative to positive values. Second, the stress index, when in the negative value region, governs the recovery process. A superimposition of high compressive stresses on the plastic shear deformation leads to a recovery of damage and an associated density increase. The kinematic equation for damage evolution is proposed and its coefficients are defined.