It is desirable to use nanostructured composites to produce high-strength, high-conductive materials for magnet development. In this research, severe plastic deformation (e ≥ 2.0) is used to produce nanostructured Cu-Ag composites in an off-eutectic (Cu-16at%Ag) and an eutectic (Cu-60at%Ag) composite. These two compositions are chosen in order to obtain a better understanding of the off-eutectic microstructure which is comprised of both proeutectic and eutectic regions. The mechanical and conductive properties of Cu-Ag material make it ideal for use in magnetic applications. The scanning electron microscopy images demonstrate that the as-cast off-eutectic material has large proeutectic Cu-rich dendrite embedded within nano-sized lamellar eutectic Cu + Ag. On the other hand, the eutectic material has a homogeneous distribution of alternating nano-sized Cu + Ag lamellae. The evolution of the microstructure and mechanical properties is dependent on the processing method and the composition of the materials. Both drawing and rolling processes resulted in an increase in mechanical strength, Vickers hardness, electrical resistivity, texture strengthening, alignment and refinement of the Cu-Ag lamellae in both materials. The enhancement of the mechanical strength and hardness is attributed to the alignment and texturing in the nanostructured fiber composites. The fabrication methods, flux-melt-casting (EF), flux-melt-casting + Equal Channel Angular Extrusion (ECAE to e = 2.0) (EF_E), and flux-melt-directional-solidification (EFS) produces lamellae thicknesses of 210+/-33 nm, 199+/-36 nm, and 161+/-21 nm, respectively. In spite of an initial ECAE (e= 2.0), the lamella refinement in the material processed by flux-melt + ECAE + swaged + drawn (total e = 4.6) is 73+/-12 nm and comparable to the 84+/-12 nm obtained in the flux-melt + swaged + drawn (total e = 2.6) material. The incorporation of directional solidification (DS) + swaged + drawn further reduces the lamella thickness to 59+/-13 nm (total e = 2.6). This suggests that the initial ECAE has a minimal effect on the reduction in the lamella thickness. Typical fcc <111> + <100> duplex texture is produced in the axis-symmetric swaging + drawing deformation in both the Cu and Ag phases. The off-eutectic Ag and Cu phases wire texture intensities are 27% and 76% higher than the eutectic. The texture in both rolled Cu-Ag composites are Brass {110}<-112>+ a weak S1{124}<21-1> and S2{123}<41-2> components. At e ~ 4.5 the Ag and Cu phases of the off-eutectic composite has 13% and 48% stronger Brass intensity than the eutectic, respectively. In spite of the very high Cu content in the off-eutectic Cu-Ag composite, the texture of the rolled composite is Brass, even in the Cu phase. This suggests that the deformation in the composite is dominated by the mechanism of the deformation in the Ag. At large strains, the lamellae alignment in the rolling (L-S) direction is displaced due to the formation of shear bands. Both texture and shear bands results in anisotropy of the mechanical strength and electrical conductivity. Molecular Dynamics (MD) is used to provide a better understanding of the deformation mechanisms during rolling and wire drawing of the Cu-Ag nanocomposites. This is simulated using uniaxial tension, compression, and nano-indentation of the nanocomposites' bilayers. Initial relaxation of the Cu-Ag bilayers reveals the formation of a semi-coherent Cu/Ag interface which later serves as both the source for nucleation of partial dislocations and a barrier to the moving dislocations. The MD results suggest that the deformation in the Cu-Ag bilayer system occurs in four stages: (1) elastic deformation up to extremely high stresses, (2) nucleation of the leading partial dislocations from the interface on multiple slip systems, (3) propagation of the partial dislocations which leads to the formation of stacking faults, (4) the formation of multiple dislocation loops and stacking fault systems which propagate and expand into the material at much lower stress values. Eventually, the influence of large dislocation densities and interface effects dominates the materials and the stress in the system begins to increase slightly, a trait of possible strain-hardening. The leading partial dislocations nucleate first in the Ag layer for the proeutectic material on the {111} plane in the <112> directions for both tension and compression simulations. This may explain why the dominant texture that is observed in the off-eutectic material is Brass, even in the proeutectic Cu-rich regions. The simulated [111] nano-hardness result is 378 HV and 384 HV which is higher than the measured nano-hardness of 267 and 288 for eutectic and off-eutectic drawn wires, respectively. The [111] simulated stress-strain curves show much higher value of yield stress (~8 GPa) than the experimental result (~0.9 GPa). / A Dissertation submitted to the Department of Mechanical Engineering in partial fulfillment of the requirements for the
degree of Doctor of Philosophy. / Fall Semester, 2010. / October 18, 2010. / Cu-Ag, Nanostructured, Eutectic, Off-eutectic, Molecular Dynamics, Lamella / Includes bibliographical references. / Peter N. Kalu, Professor Co-Directing Dissertation; Ke Han, Professor Co-Directing Dissertation; Okenwa Okoli, University Representative; William S. Oates, Committee Member; Farrukh S. Alvi, Committee Member; Anthony D. Rollett, Committee Member.
| Identifer | oai:union.ndltd.org:fsu.edu/oai:fsu.digital.flvc.org:fsu_253887 |
| Contributors | Davy, Charney A. (Charney Anchilyn), 1977- (authoraut), Kalu, Peter N. (professor co-directing dissertation), Han, Ke (professor co-directing dissertation), Okoli, Okenwa (university representative), Oates, William S. (committee member), Alvi, Farrukh S. (committee member), Rollett, Anthony D. (committee member), Department of Mechanical Engineering (degree granting department), Florida State University (degree granting institution) |
| Publisher | Florida State University, Florida State University |
| Source Sets | Florida State University |
| Language | English, English |
| Detected Language | English |
| Type | Text, text |
| Format | 1 online resource, computer, application/pdf |
| Rights | This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s). The copyright in theses and dissertations completed at Florida State University is held by the students who author them. |
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