Effect of pulse current and its application direction on the size effect of nanocrystalline nickel foil

Abstract

In order to gain a deeper understanding of the effect of pulsed current on the mechanical properties and size effect of nanocrystalline Ni foils, nanocrystalline Ni foils with different grain thickness-to-grain size ratios (λ) were prepared using pulsed electrodeposition in this paper and unidirectional tensile experiments were carried out at room temperature with different currents and their applied directions. The experimental results show that the nanocrystalline Ni foil produces an obvious electroplasticity effect after applying the current field, and when 300< λ <1100, the current weakens the size effect of nanocrystalline Ni foils to a certain extent, and the angle between the current direction and the deformation direction also affects the mechanical response of nanocrystalline Ni foils, and when the angle between the current direction and the deformation direction is 0°, electroplasticity effect is the best, and the current has the most significant effect of abating the size effect of the material. The mechanism of unidirectional tensile deformation of nanocrystalline Ni foils under the effect of pulsed current was analyzed using TEM and TKD. It was found that the applied pulse current increased the activity of the nanocrystalline boundaries, promoted the movement of dislocations, and reduced the tendency of dislocation entanglement. The higher the peak current density and the smaller the angle between the direction of the current and the direction of deformation, the smaller the grain boundary orientation difference, the more dispersed the grain orientation, and the lower the density of geometrically necessary dislocations (GND) in the deformed nanocrystalline foil, the more significant the effect on material plasticity improvement.

1. Introduction

With the rapid development of the electronics industry and precision machinery, the miniaturization of parts has become a major development trend, so the forming and processing of miniature parts is becoming more and more important, especially in the field of micro-manufacturing such as communications, electronics, microsystems technology (MST) and micro-electro-mechanical systems (MEMS) [1]. Nanocrystalline metal foils are widely used in micro-manufacturing due to their unique mechanical properties. Due to the miniaturization of the grain size of nano-metal foils, their mechanical properties are significantly different from those of conventional grain size metallic materials [2, 3]. Despite the high yield strength and tensile strength of nanocrystalline metal foils at room temperature, their high brittleness and size effect make them very susceptible to machining defects such as brittle fracture during the process of machining and deformation, which leads to low microdeformation capacity and poor formability of nanocrystalline metal foils at room temperature, thus affecting the expected material properties and manufacturing accuracy [4, 5]. Therefore, improving the processing defects of nanometallic materials, reducing and controlling the size effect on the microforming of nanocrystalline foils have become the research focus of a large number of research scholars in recent years.

A pulsed electric field, i.e., pulsed electric-assisted (EA) microforming, is applied during the material microforming process. The technology has shown general advantages in enhancing the microplastic forming ability and improving the formability of nano-metal foils. In addition, the electric current increases the plasticity and decreases the deformation resistance of metallic materials, which is known as the electroplasticity effect [6–8]. The combined effect of electroplasticity and dimensional effects in EA forming of metallic materials has attracted the attention of some scholars. SIOPIS et al [9] investigated the dimensional effects of EA compression annealing of pure copper, and the experimental results showed that the effect of current-induced reduction of the flow stress of the specimen diminishes with the increase of the grain size of the specimen. LIU et al [10] conducted EA uniaxial tensile experiments on nickel-based high-temperature alloy plates with different grain sizes of the same thickness.