Bismuth impurity effects on copper: examining embrittlement in immiscible binary metallic systems
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Immiscible binary metallic systems impart a special scientific intrigue for researchers in thermodynamics and condensed matter. At equilibrium one might expect that the interfaces between each of the metallic components to be sharp and well-defined. This resembles a picture painted in undergraduate chemistry courses examining aqueous/organic phase separations. In binary metallic melts involving immiscible components much of the same is expected, especially in systems like Bi-Cu where the enthalpies of mixing are very endothermic, approaching 7000 J/mol. Solidus interactions are believed to be moot, but recent evidence has shown that Bi intercalates into the Cu microstructure inducing cracking after significant exposure. The objective of this thesis is to present the results of a study examining the effect of Bi and temperature on the fracture response of a Cu specimen. Both laboratory experiments and finite element modeling were used to better understand this phenomenon. Prior work has demonstrated the embrittlement of copper by the introduction of bismuth, even in dilute quantities. Furthermore it is commonly accepted that the embrittlement is a result of a faceting that occurs at the grain boundary. Although this phenomenon has been extensively observed it is not well understood. Duscher et al. studied this system and submitted that the embrittlement stems not only from the strain of a large impurity at the grain boundary, but also from a reduced hybridization between the d and s states of the copper atoms that surround the bismuth impurity. Additionally, it has been suggested by others that bismuth will intercalate into the grain boundaries of copper under certain conditions. This thesis presents a comprehensive parametric model of the effects of temperature and the presence of Bi on the fracture response of the sample. The experimental data provide an extensive matrix of conditions within a decreased ductility trough for Cu. Preferential faceting, surface science, and mechanical strength data are keys to understanding this bimetallic system.
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