Investigation of Grain Boundary Segregation and Embrittlement Mechanisms of the Cu-Bi System by Analytical Electron Microscopy

Grain boundary (GB) segregation and embrittlement of copper (Cu) by small amounts of bismuth (Bi) has been investigated on 6°, 13°, and 33° Cu twist bicrystals. The results from micro-mechanical double edge notched testing showed no embrittlement effects in the 6° GB. The 33° GB has been shown to be significantly embrittled by the introduction of Bi. Single edge notch testing of the 13° GB also showed a reduction in fracture toughness. These mechanical results have been interpreted through the use of analytical electron microscopy (AEM) studying the GB geometry, the atomic structure, the electronic structure, and the chemical compositions of the GBs. The 6° and 33° GBs were found to be close to pure twist boundaries but with more accurate twist angles of 4.3° and 38.0°, respectively. The electronic structure of the GBs was not found to be a good indication of the presence of Bi, which was confirmed on the 13° and 33° GBs. The Bi GB coverage was confirmed via quantitative XEDS on the 33° GB to correspond to 0.12 ± 0.03 monolayers of Bi and through 3-dimensional scanning transmission electron microscope (STEM) through focus imaging to be 0.02 – 0.09 monolayers of Bi. The presence of edge dislocations along the 33° GB was confirmed with Bi segregating to edge dislocation cores. The Bi atoms on the dislocation cores embrittle the GB by increasing the energy required to move a dislocation in response to an applied stress resulting in reduced plasticity at the crack tip which promotes GB cleavage