Abstract: Bismuth (Bi), as an impurity element in copper and copper-based alloys, usually has a strong tendency of grain boundary (GB) segregation, which depends on the GB characters. However, the influence of such a segregation on the properties of ultrahigh-purity copper has been rarely reported and the exact structural arrangements of Bi atoms at different GBs remain largely unclear. In this study, we investigated the influence of trace amounts of Bi (50-300 wt ppm) on the ductility of an ultrahigh-purity copper (99.99999%) in the range of room temperature to 900 °C. The tensile results show that the addition of Bi seriously damages the ductility of the ultrahigh-purity copper at temperatures of 450-900 °C, which is due to the GB segregation of Bi. On this basis, such a segregation behavior at different types of GBs, including high and low angle GBs (HAGBs/LAGBs), and twin boundaries (TBs), via the scanning electron microscope-electron backscattered diffraction (SEM-EBSD) and aberration-corrected scanning transmission electron microscope (AC-STEM) investigations, combined with the first-principles calculations were systematically studied. The atomistic characterizations demonstrate an anisotropic Bi segregation, where severe enrichment of Bi atoms typically occurs at the HAGBs, while the absence of Bi adsorption prevails at LAGBs or TBs. In particular, the segregated Bi at random HAGBs exhibited the directional bilayer adsorption, while the special symmetrical Σ7 HAGB presented a unique Bi-rich cluster superstructure. Our findings provide a comprehensive experimental and computational understanding on the atomic-scale segregation of impurities in metallic materials.

Key words: Grain boundary character, Embrittlement, Bi segregation, Atomic configuration, Ultrahigh-purity copper