The key role of grain boundary state in deformation-induced softening effect in Al processed by high pressure torsion

Abstract

The influence of type and amount of additional severe plastic deformation (SPD) on the magnitude of the deformation-induced softening (DIS) effect in ultrafine-grained (UFG) aluminum processed by high-pressure torsion (HPT) has been studied for the first time. Drastic DIS effect (the increase of plasticity from ∼1% to ∼23%), while maintaining high strength (ultimate tensile strength ∼ 180 MPa) is observed after a small additional deformation by HPT to 0.25 turns in UFG aluminum annealed at 150 °C for 1 h. An increase of additional HPT deformation to 0.75 turns does not lead to an increase of the DIS effect. The change of the type of additional deformation from HPT to cold rolling does not lead to any DIS effect, but slight hardening occurs and plasticity remains at low level (1–3%). The analysis of the evolution of the microstructure in correlation with mechanical behavior after annealing and subsequent deformations of both types indicates the key role of the state of the grain boundaries for the DIS effect in pure aluminum processed by HPT.