Simulation of high-density pulse current-induced stress relaxation

Abstract

The phenomenological equation relating pulse current-induced relaxation of elastic tensile stresses to current density, initial stresses, and temperature is proposed. The equation allows approximation of experimental data on positive and negative tensile stress relaxation with the above parameters.Keywords: pulse current, positive and negative stress relaxation, phenomenological equation. Introduction.High-density pulse current-based technology is widely used for shaping difficult-to-form metals [1,2]. Its application is determined by several factors. First of all, it is a known thermal current effect in the conducting material involving its uniform heating through energy spent by the current source for charge transfer: collision of charge carriers between themselves and crystal lattice elements results in the transfer of current source work to electrons and the crystal lattice as kinetic energy of chaotic motion or, according to the molecular-kinetic theory, as heat [1]. Moreover, numerous experimental data point to a specific high-density pulse current effect on the metal, which results in a significant decrease in deformation resistance and stress relaxation [2,3]. Such a decrease cannot be explained by an increase in average temperature of the metal since, as a rule, it changes inconsiderably. In the literature this phenomenon was termed the electroplastic effect [2,3].The nature of this effect is as yet little understood. Judging by known data, it may be defined as a specific pulse current effect leading to nonuniform changes in the material at the microlevel because of localized heat release on structural defects [4,5], micrononuniform material properties, deformations [6], and direct interaction of electrons with defects of the crystal lattice [7][8][9][10]. A nonthermal pulse current effect decreases with an increase in current pulse duration or a decrease in microvolume sizes: at a sufficient heating time, temperature in microvolumes equalizes and approaches the average one, which explains only a thermal low-density current effect.Evaluation of a nonthermal pulse current effect should account for the nonuniformity of physicomechanical material properties determining the micrononuniformity of heating and deformation. Pulse current-induced temperature arising in a micrononuniform material may be evaluated in terms of the thermal effect on electrical resistance, heat capacity, and heat conduction of the metal only by numerical methods. The absence of data on material properties necessary for such evaluation is evidence that the development of phenomenological equations for calculating deformation kinetics of the metal with the account of thermal and nonthermal pulse current effects is currently central.Phenomenological equations used in analysis of positive relaxation, when loads decrease under the action of pulse current, and negative relaxation, when loads are growing rather than decreasing after loading, partial unloading, and further pulse current action, are proposed below [11]. Similar behavior of the...