Dispersion, Microstructure, and Mechanical Behavior of Metal-Carbon Nanotube Composites

Bulk metallic glass (BMG) and ultra-fine grain (UFG) metal powders have emerged as a new class of potential structural materials. However, fabrication of fully amorphous alloys and UFG metals with relatively large sample size still poses a challenge, the mechanical behavior need to be further improved to achieve better combination of strength and ductility. Recently, carbon nanotubes (CNTs) have been reported to enhance the mechanical behavior of conventional polycrystalline metals. Thus a fundamental understanding of the relationship between processing, microstructure, and mechanical behavior as it pertains to the materials of interest is necessary. This dissertation selected Cu-based amorphous powders and UFG Ni to study dispersion, processing, microstructure, and mechanical behavior of metal-CNT composites. CNTs were dispersed using a non-covalent zwitterionic (ZW) surfactant, cohesion behavior of CNTs onto metallic powder is described using hydration of metal particles in the ZW-CNT suspension, and metal-CNT composite powder consolidation was performed by spark plasma sintering (SPS), in an effort to understand the effects of CNTs in the metal matrix and further our fundamental understanding of the mechanisms that govern the contribution of CNT to the metal matrix.;Carbon nanotubes (CNTs) were dispersed in gas atomized Cu47.5Zr 47.5Al5 (CZA) and Cu50Zr50 (CZ) amorphous powders, in an effort to elucidate the mechanisms of cohesion of CNTs onto metallic powders. CNTs were homogenously dispersed in water using a zwitterionic (ZW) surfactant. Then CZA and CZ powders were submersed in the ZW-CNT suspensions with varying amounts of dwell time in an ultrasonic bath. The ZW-CNT-metal powder suspensions were dried, and CNT-metal composite powders were obtained after decomposition of the surfactant by calcination. Zeta potential measurements on ZW-CNT-metal powder suspensions and scanning electron microscopy investigation into the CNT-metal composite powders both indicated an ideal dwell time, for a specific alloy composition, of metallic powders in ZW-CNT suspension to achieve optimal cohesion of CNTs onto metallic powder surfaces. The results are rationalized on the basis of hydrolysis of metal ions into suspension creating a net positive charge on the metallic powder surfaces, and the interaction between the charged powder surfaces and the charged hydrophilic head groups of ZW, which has the other end attached to CNTs.;Ultra-fine grain (UFG) Ni and multi-walled carbon nanotubes (CNTs) powders were prepared using non-covalent functionalization to promote cohesion between the metal powders and CNTs. After consolidation using spark plasma sintering, the resulting composites had densities above >97% with well-dispersed CNT reinforcement. A pronounced decrease in average grain size was observed for Ni-CNT samples vs. pure Ni SPS consolidated at the same temperatures. Tensile testing resulted in comparable fracture strengths with significantly decreased fracture strain. Using a shear lag model, with reinforcement lengths consistent with grain edge lengths produced good agreement with experimental results. Twinning behavior in Ni consolidated specimens was carefully investigated in an effort to discuss the mechanisms governing the evolution of annealing twins with the presence of CNTs in the matrix.