Kinematics of damage governed by severe plastic deformation

SPD techniques are known as a tool to produce fine-grained materials, which can exhibit superplastic behaviour in a certain range of thermo-mechanical parameters. However, damage introduced during SPD processing can be a reason for further cavitation in superplastic forming. The development and recovery of damage in continuously cast aluminum alloy 6061 is investigated using the processes of Equal Channel Angular Extrusion and Drawing. 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. The kinematic equation for damage evolution is proposed and its coefficients are defined.