Electropulsing suppresses extrusion growth in polycrystalline copper subjected to fatigue loading

Abstract

The effect of electropulsing on the extrusion due to persistent slip bands (PSBs) and the defects in PSBs was investigated. To extend the service life and improve structural safety, technology is needed to repair fatigue damage such as cracks and dislocations directly. It has been shown that electropulsing of damaged metallic materials delays fatigue crack initiation and extends fatigue life, but the detailed mechanism of this has not been explained. When cyclic strain is localized into a PSB, the PSB forms lamella, thin walls that contain rich dislocation, and thick channels that contain poor dislocation. The cyclic plastic strain within a PSB causes extrusions and intrusions, called persistent slip markings, to appear on the surface. Cracking is initiated at the tip of the intrusion. Fatigue tests were performed using an electromagnetic force fatigue testing system while applying electropulsing every 2000 cycles to polycrystalline copper. The extrusion height before and after electropulsing was observed through atomic force microscopy. The resulting extrusion growth was slowed by the electropulsing. Polák’s model was applied to examine the effect of electropulsing on the extrusion growth. When the vacancy concentration reaches a steady state in the channel and electropulsing is performed, the vacancies migrate and are assumed to be annihilated at the matrix or walls. Therefore, the vacancy concentration in the PSB decreases with electropulsing. The decrease in vacancy concentration in the PSB is thought to suppress the extrusion growth until the vacancy concentration reaches a steady state again.