Abstract
Ultra-fine-grained (UFG) aluminum with a grain size of 260 nm was fabricated by annealing a severely plastically deformed A1100 alloy. The resulting UFG aluminum exhibited a 0.2% proof stress (σ0.2) that was four times larger than that predicted by the conventional Hall-Petch relation. In this study, the UFG aluminum, the fine-grained aluminum with a grain size of 960 nm and the coarse-grained aluminum with a grain size of 4.47 µm were prepared. The change in the dislocation density, ρ was investigated during tensile deformation using in-situ X-ray diffraction measurements at SPring-8. It was found that as the strain increased, the ρ changed in four distinct stages. The first stage was characterized by elastic deformation, and little change in the ρ occurred. For the coarse-grained aluminum, this stage was almost absent. In the second stage, the ρ rapidly increased until the stress reaches σII in which the plastic deformation begins to occur at a constant strain rate. In the third stage, only a moderate change in the ρ occurred. Finally, in the fourth stage, the ρ rapidly decreased as the test pieces underwent fracture. Additionally, it was found that the σ0.2-σI was followed by the conventional Hall-Petch relation in all grain size range.
