Softening Effect on Fracture Stress of Pure Copper Processed by Asynchronous Foil Rolling

Abstract

In order to study the size effect on the mechanical property of micro-scale metal, pure copper strips with thicknesses in the range of 20 µm to 600 µm were obtained through the asynchronous foil rolling technology. Progressive mechanical property tests indicated that the pure copper experiences softening effect at a micro-scale when the thickness is below 80 µm, which is contrary to the traditional work hardening theory. The related mechanisms were analyzed and discussed through the observation of microstructure and fracture morphology. The decrease of fracture stress with the decrease of thickness can be attributed to the decreased interfacial energy and dislocation density, which contributes to the release of the cumulative distortion energy and the tendency to soften. In addition, the distribution of misorientation angle and changed Taylor factor with the decrease of thickness are other important factors. The fracture morphology indicated a reduction in the number of micro-voids and the nature of fracture transformed from dimpled pattern to knife edge rupture with thickness. The traditional Hall-Petch relationship is no longer applicable due to the softening effect. A modified Hall-Petch relation considering the distribution of misorientation angle and Taylor factor was established, which provided a better relationship between flow stress and grain size.

Analysis of Mechanical Parameters of Asymmetrical Rolling Dealing with Three Region Percentages in Deformation Zones

“…Analysis of the relationship between the deformation zone configuration and rolling parameters shows that the minimum thickness can be reached when the forward-slip disappears, and the rolling parameters keep the deformation zone configuration as a cross-shear zone and backward-slip zone (C + B), or an all-cross-shear zone (AC) [ 22 ]. The softening phenomenon [ 23 ] and size effect [ 24 ] in asymmetrical rolling of pure copper foil were studied. An analytical model based on the slab method, which considers the percentage of three regions in the plastic deformation zone, was used to study the effect of rolling parameters on the deformation zone configuration, and provided an accurate calculation of roll force and roll torque [ 25 , 26 ].…”

Mechanical Properties of Rolled Copper Foils in Cryogenic and Room‐Temperature Environments

“…[3] It is well known that the strength of copper foil increases after rolling due to work hardening, but studies have found that when the copper foil is thinned to a certain degree, the strength decreases, which is softening phenomenon. [4] This is attributed to the reduction of interface energy and dislocation density, leading to the release of cumulative deformation energy, [5] resulting in a softening trend. Cryogenic processing has attracted wide attention in recent years as it can improve the mechanical properties of materials.…”