Dislocation Density Changes in Ultrafine-grain Aluminum during Tensile Deformation

Abstract

It has been reported that the tensile strength of UFG aluminum is several times higher than that of coarse-grain aluminum, and that UFG aluminum exhibits unique mechanical phenomena under tensile deformation. This implies that dislocation behavior in UFG aluminum differs from that in coarse-grain aluminum. In this research, changes in dislocation density during tensile deformation were examined by in-situ X-ray diffraction, and the effect of grain size on dislocation multiplication behavior was investigated. In samples with smaller grains, the increase in dislocation density during tensile deformation was larger and the decrease in dislocation density after unloading via fracture was also larger. Introduction Recently, it has become very important to reduce CO 2 discharge by improving the fuel efficiency of automobiles while reducing environmental loading by improving their recyclability. Therefore, recyclable structural metallic materials of light weight and high strength must be developed. Until now, strengthening by precipitation hardening and solution strengthening by alloying have been used. Unfortunately, there are limitations on strengthening by these techniques, and deterioration of recyclability by the addition of large alloying elements is a significant problem. For structural metallic materials, polycrystalline materials are commonly used, with a minimum conventional grain size of about 10 μm. However, recently, the severe plastic deformation (SPD) process has made it possible to fabricate ultra-fine grained (UFG) materials with a nano-order grain size 1-3. For example, it was reported that the strength of pure aluminum could be increased by four times using ultra-refinement, even without an alloying element. 1 Therefore, UFG aluminum may offer a lightweight high strength structural material with high recyclability. However, UFG materials show various unique mechanical characteristics, which conventional coarse-grained (CG) materials do not show. Hence, in order to understand the expression mechanism of these phenomena, more detailed research is urgently needed to enable practical applications. Two reported examples of unique phenomena in UFG materials are yield point reduction in FCC metals 1 and hardening by annealing / softening by deformation 4. Furthermore, it is well known that the yield strength generally increases according to the Hall-Petch law by grain refinement. However, it has also been reported that the yield strength of UFG materials with a grain size of <1 μm is much higher than that of the corresponding CG materials. This is called the extra-hardening phenomenon 5. These unique phenomena are considered to occur in order that the behavior of the dislocation, which bears a deformation, is significantly affected by