Effects of electric current and field on the behavior of metallic materials.

摘要

The effects of electric current and field on the behavior of metallic materials has been studied in terms of the drop in flow stress caused by an electric current pulse and the changes in superplastic behavior and phase transformation kinetics resulting from applying an external electric field. The experimental procedure employed previously by Conrad et al to investigate the electroplastic effect was modified to facilitate the elimination of side effects from the observed drop in flow stress due to a current pulse, to determine the electron wind force on dislocations and to establish the effect of electric current on each of the thermal activation parameters in the Arrhenius strain rate equation. The experimental values of the electron wind force were in general accord with those predicted by available theories for FCC metals, but a much higher value was obtained for BCC Nb. The measured electron wind force decreased with increasing stacking fault energy and in turn with decreasing width of extended dislocations in FCC metals. To fully evaluate the experimental values of the electron wind force, further improvement of present theories is necessary which take into account the band structure and Fermi surface geometry of individual metals. The electric current reduced the activation volume and free energy, most likely by changing the force-distance curve of the thermal activation process. It is concluded that the observed increase in the pre-exponential term produced by current pulse arises partly from the increase in the density of mobile dislocations and the area swept out per successful thermal fluctation, but mostly results from the difference between the static and dynamic responses of the test system to the pulsed load drop.; An external electric field reduced the flow stress and work hardening, significantly suppressed the cavitation and retarded strain-enhanced grain growth of 7475 Al during superplastic deformation. In addition, the application of an external electric field during the heat treatment of two steels accelerated austenization, increased hardenability and retarded the dissociation of quenched martensite during tempering. Possible mechanism for these effects are discussed.