Yield Point Phenomenon Induced by Twinning Accompanied by Abnormal Grain Growth in Fine-Grained Copper

In this study, an equiaxed fine-grained copper specimen was fabricated by powder injection molding, followed by hot isostatic pressing. The specimen showed a yield point phenomenon with a Luders-type deformation during a uniaxial tensile test. This study aimed to identify the cause of the yield point phenomenon in the fine-grained copper specimen. First, the Luders band was designed to partially propagate within the gauge section through an interrupted uniaxial tensile test equipped with a digital image correlation system. The microstructures of the Luders and non-Luders band regions were compared by electron backscatter diffraction. It was observed that self-annealing was accelerated by the applied stress in the Luders band region. Consequently, the microstructure of the Luders band region transformed into a bimodal structure composed of fine grains and abnormally coarse grains containing numerous large twins. Identification of active twin variants in the abnormally coarse grains suggested that a significant tensile strain can be accommodated by this twinning. Based on these results, it was confirmed that the strain burst generated by the sudden occurrence of numerous large twins induces the yield point phenomenon in the fine-grained copper.