Phase Transformations in Metals Stimulated by a Pulsed High-Energy Electromagnetic Field

Abstract

Processes occurring in metals with microdefects when metallic specimens are treated by short high-density electric current pulses are considered. The variations in the electric and temperature fields in the material and their influence on the phase transformations and the stress–strain state in the vicinity of microdefects in the form of plane cracks with linear sizes of the order of 10 μm are studied. A mathematical model of the effect of an electromagnetic field on a predamaged thermoelastoplastic material with an ordered system of defects is proposed. The model takes into account melting and evaporation of the material and the dependence of all of its physical and mechanical properties on temperature. The solution of the resulting system of equations is sought by the finite element method on moving grids with the use of the combined Euler–Lagrange method. The dependence of the processes on the boundary conditions of the model is considered. We estimate the error that occurs when solving the problem for one representative cell rather than for the whole sample with an ordered system of defects. The influence of the distance between the cracks on the deformation and healing of microdefects is investigated. Numerical modeling has shown that a high-density current with large field gradients arises in the vicinity of microdefects, which leads to intensive local heating accompanied by thermal expansion and melting of the metal on the tips of the microcracks. This results in high compressive stresses near microcracks, intensive plastic flow of the material and, as a consequence, clamping of microcrack sides, decrease in microcrack length, and ejection of the molten material into the crack. As a result, the microcrack is completely healed. The numerical results obtained by the proposed model agree with experiments. Computations showed that if the distance between microcracks is equal to or greater than ten of their lengths, then the time for complete defect healing weakly depends on the distance between defects and the micro-defect interaction can be neglected. The interaction between microcracks in the metal significantly affects their healing process if the distance between them is reduced to about 5÷6 of microcracks lengths. With further decrease in the distance between the defects up to one microcrack length, the healing process does not change qualitatively, but slows down significantly: the ejection of molten material into the crack still happens, but the crack size reduction, especially in the transverse direction, is substantially smaller.