Mechanisms for the electroplastic effect in metals

The effects of high-density d.c. current pulses (1,000 A/sq mm for 60 microseconds) on the flow stress of a number of polycrystalline metals (Al, Cu, Ni, Fe, Nb, W and Ti) tested in uniaxial tension at 300 K were investigated with the objective of determining the mechanisms responsible for the concurrent load drops. Both reversible and irreversible (plastic) strains contributed to the load drops. The major component of the reversible strain was the thermal expansion due to Joule heating; skin, pinch, and magnetostrictive effects were of less importance. An analysis of the plastic-strain contribution to the load drops suggested that it resulted from the enhancement of dislocation mobility due to the action of drift electrons. Employing the thermally activated plastic flow concepts, the electron wind push coefficent B/sub ew/ could be determined for Al and Cu and was found to be of the order of 10,000 dyn-s/sq cm in accord with Roschupkin et al.'s theory. This value is in accord with the dislocation damping constant B determine by other techniques at approx. 300 K; it is, however, about an order of magnitude larger than is normally expected for the electron drag coefficient B/sub e/. In addition to the force exertedmore » on dislocations by an electron wind, the anlaysis indicated that drift electrons also have a significant effect on one or more of the other parameters of the thermally-activated rate equation. Because of test-machine inertial effects, oscillations in the load occurred as it dropped in response to a current pulse.« less