Investigation of Bi Segregation of Cu Bicrystal Boundaries Using Aberration-Corrected STEM Depth Sectioning

The Cu-Bi system exhibits nearly no solubility of Bi in Cu along with the highest solute boundary enrichment factor among metal systems making the Cu-Bi system ideal for segregation studies [1]. Cu-Bi also shows a high degree of segregation induced grain boundary embrittlement. The precise cause of this embrittlement has been attributed to Bi size effects along the Cu boundary or modification of the electronic states at the boundary resulting in differences in Cu-Cu bonds, though the exact cause is not agreed upon [2,3]. High-angle annular dark-field Scanning Transmission Electron Microscopy (HAADF-STEM) imaging has reviled the presence of possible Bi bilayer formation along Cu grain boundaries. Segregation behavior similar to that present in the Cu-Bi system has been observed in the Ni-Bi system and HAADF-STEM imaging of these Ni boundaries was interpreted as Bi bilayer formation along the grain interfaces [4]. To further investigate the cause of this embrittlement and to verify Bi bilayer formation on Cu grain boundaries, an accurate 3-dimensional representation of the boundary and its atomic structure must be characterized. In this study the atomic structure of the boundary has been investigated using a JEOL JEM-ARM200CF aberration-corrected STEM. This instrument was used to perform atomic resolution depth sectioning of a Bi-doped Cu bicrystal boundary to reveal the position of Bi atoms through the thickness of the specimen.