Anomalous twinning in AZ 31 magnesium alloy during electrically assisted forming

The electro plastic effect (EPE) occurs in materials exposed to high electric currents, on the order of 102 to 104 A mm-2, during elastic or plastic deformation. Current pulses with durations of about 10-3 s are usually used to limit resistive heating of the sample. As a result, a reduction the macroscopic of flow stress and enhanced ductility is observed, The EPE may therefore be exploited to support the deformation of inherently brittle materials. The underlying microscopic mechanisms enabling the flow stress reduction and increase in ductility are still unresolved. Besides the obvious contribution of Joule heating, various mechanisms of electron -dislocation interactions, resulting in increased dislocation mobility or changed dislocation density, have been proposed. In the present study, the EPE was investigated using samples of extruded pure magnesium and AZ 31 Mg alloy, which were subjected to one or ten current pulses with a current density of 700 A mm-2 and 1 ms duration while subjected to constant compressive strain below the yield point. During the experiments the mechanical response of the sample to the current impulse, a drop of stress, the occurrence of residual plastic strain and hardening of the sample, was observed. The magnitude of the observed reduction in stress depends on the relative orientations of texture and current direction. In the case of multiple pulses, the first current pulse led to a significantly larger drop than the subsequent pulses. Reference experiments using hot air and inductive heating were conducted, in which samples were subjected to identical strains and similar temperature profiles. A similar softening could not be observed. The subsequent optical and EBSD microstructural observations, using appropriate metallographical preparation techniques, revealed unusual twinning in the samples subjected to current pulses. Please click Additional Files below to see the full abstract