Fundamental Study of Nanoparticles Enabled High Performance Copper
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Fundamental Study of Nanoparticles Enabled High Performance Copper


High performance (hardness, strength, electrical/thermal conductivity, ductility, Young’s modulus, and corrosion resistance, etc.) copper (Cu) based materials are in strong demand in industry for numerous applications. However, it has been a long-standing challenge to achieve high performance Cu by scalable and cost-effective fabrication due to the limits of traditional metallurgy. Pure Cu is very soft and improving the mechanical properties of Cu comes at great expense of electrical and thermal conductivity. The properties of traditional Cu alloys have reached certain limits. Nanotechnology enabled metallurgy provides a new pathway to achieve significant performance enhancement of Cu. The overarching goal of this dissertation is to advance the fundamental understanding of nanoparticle effects on micro/nano-structures and properties of Cu/Cu alloys, thereby overcoming the existing limits of Cu/Cu alloys. The specific research objectives are to develop effective processing methods to synthesize and disperse suitable nanoparticles in Cu/Cu alloys and to break the limits of grain refinement, phase modification, and property enhancement in Cu/Cu alloys by nanoparticles.Two ex-situ processing methods were utilized to fabricate Cu matrix nanocomposites with a wide volume fraction range of well-dispersed nanoparticles. To enable the size control of the nanoparticles and expand the choices of nanoparticles for Cu/Cu alloys, novel in-situ synthesis of nanoparticles and Cu matrix nanocomposites were developed. In-situ TiB2 nanoparticles with an average size of 65 nm in the Cu matrix were synthesized using fluoride salts as precursors and Al as the reduction agent via casting. It was also discovered that Al in molten Cu can stabilize TiB2 nanoparticles. This finding overcame the challenge of incorporating TiB2 nanoparticles into molten Cu. In addition, Cu with in-situ W nanoparticles (average size as small as 132.7 nm) was cast using tungsten oxide microparticles as the precursor. Cu/WC nanocomposites can exhibit simultaneously enhanced microhardness, strength, and Young’s modulus without significant degradation of the electrical/thermal conductivity. The as-solidified Cu/40 vol.% WC showed a yield strength over 1000 MPa, a Young’s modulus over 250 GPa, and still maintained reasonable electrical and thermal conductivity. In addition, a CuAlMg/TiB2 nanocomposite was designed and fabricated by casting. Results showed that TiB2 nanoparticles effectively increased the hardness while causing less deterioration of the electrical conductivity compared to alloying of Al and Mg. Nanoparticle-enabled grain modification was investigated. Bimodal grain structure has been demonstrated effective to overcome the strength-ductility tradeoff in nanostructured materials. We report a new and scalable method to fabricate bimodal grained metals, i.e., casting followed by regular hot rolling, when the metals are loaded with nanoparticles. After hot rolling, as WC nanoparticles restricted grain growth, the nanoparticle-rich zones retained ultrafine grains, while the grains in the nanoparticle-sparse zones grew to microscale. This new pathway has great potential to advance the scalable manufacturing of bimodal grained metals for various applications. In addition, it was discovered that microparticles (CrB and CrB2) with surface nanofeatures can enable ultrafine/nanograined Cu via slow cooling. CrB/CrB2 microparticles, formed by coalescence of nanoparticles in Cu matrix, displayed surface nanofeatures, which induced substantial grain refinement and stabilization down to the ultrafine/nanoscale. The UFG Cu/CrB and Cu/CrB2 samples exhibit exceptional thermal stability, comparable to UFG Cu induced by nanoparticles, without coarsening after annealing at 600 ̊C for 1 h. The nanoparticle enabled phase modification in Cu alloys was studied targeting the intermetallic phase, the solid solution, and the precipitates, which potentially offers a profound impact on alloy design. High-Al bronze (Cu-14Al) has high hardness, wear resistance, but extremely low ductility, resulted from the brittle intermetallic phases especially at the grain boundaries. Nanoparticles tuned the morphology and distribution of the intermetallic phases, effectively increasing ductility. In addition, Cu-4Al is a corrosion-resistant alloy, comprised of Al solid solution in Cu. Dispersed TiB2 nanoparticles not only significantly increased its hardness and modulus, but also mitigated the intergranular corrosion, which is anticipated to arise from the reduced grain boundary energy of serrated grain boundaries in the nanocomposite sample and the partially blocked dealumination by nanoparticles at the grain boundaries. For many alloys, bulky and brittle intermetallic phases are detrimental to their properties, and the refinement of those phases is considered a long-time challenge. Here, we show that nanoparticles can modify and refine such intermetallics to the ultrafine/nanoscale during solidification. Cu-ZrCuSi pseudo-binary alloy was nano-treated by WC nanoparticles. Significant morphology transformation and refinement of ZrCuSi down to the ultrafine/nanoscale was achieved upon the addition of 4 vol.% WC nanoparticles, which arose from the nanoparticle-enabled phase control during solidification. The CuZrSi alloy with 4 vol.% WC nanoparticles exhibit simultaneously enhanced hardness, strength, and plasticity over pure CuZrSi. Cu-Cr alloys are a class of high-strength high-conductivity Cu alloys. However, the strength of Cu-Cr alloys by precipitation-hardening has reached a certain limit. Cu-Cr alloy containing W nanoparticles was fabricated. W nanoparticles accelerated the precipitation, leading to a significant reduction in the peak-aging time. Moreover, the nano-treated sample exhibit increased peak microhardness and improved thermal stability. Thus, nano-treating the Cu-Cr alloys is promising to break the limits of current Cu-Cr alloys. In summary, this dissertation has demonstrated the effectiveness of dispersing nanoparticles into Cu/Cu alloys to achieve high performance Cu. First, nanoparticles with suitable volume fractions can effectively enhance the mechanical properties without significantly deteriorating the electrical/thermal properties of pure Cu. Second, nanoparticles can modify the grain structures of Cu during solidification, enabling bimodal grained and ultrafine grained Cu. Third, nanoparticles can enable Cu alloys with much improved properties by refining intermetallics to ultrafine/nanoscale (CuZrSi alloy), enhancing aging behavior (Cu-Cr alloy), mitigating intergranular corrosion (Cu-4Al), and modifying the morphology and distribution of the brittle phases to improve ductility (Cu-14Al). This approach breaks the limits of traditional liquid metallurgy and lays a foundation for the rational design of high performance metals with dispersed nanoparticles for widespread applications.

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