A study of the electroplastic effect in metals

Abstract : The effects of high density, direct current pulses (100 to 1000 A/sq. mm for periods of 55-120 microsecs) on the flow stress determined in an Instron tensile testing machine were studied in a number of polycrystalline metals representing various crystal structures (Pb, Sn, Fe and Ti), with most attention being focused on Ti. The current pulses produced a decrease in the flow stress delta (o sub p) in all four metals. The work on Ti indicated that delta (o sub p) contains a contribution in addition to heating and pinch effects. This contribution (the electroplastic effect) increased significantly with current density, interstitial content and temperature (77-300K), to a lesser degree with strain, but no change was detected for a 25 fold increase in strain rate. Computer calculations of the dynamic stress changes associated with the approximately sinusoidal current-time pulses employed here yielded a contribution due to the electron-dislocation interaction (for a current density of 5000 A/sq mm at 300K) which was 32% of delta (o sub p). A model is proposed for the electroplastic effect which is based on theoretical considerations that the current flow exerts a force on the dislocations which adds to the applied force pushing a dislocation against obstacles. Complementary studies were performed on: (a) the decremental unloading method for determining the internal stress in metals, (b) the effects of temperature, gain size and interstitial content on the flow stress of Ti and (c) plastic instability in Ti sheet. (Author)