| This dissertation encompasses the fabrication of Al/B 4C composites, the characterization of the microstructures, the mechanical tests and the cost analysis. Mechanical milling at cryogenic temperature, known as cryomilling, was applied to synthesize the nanostructured Al/B 4C composite powders. The as-cryomilled composite powders, either completely cryomilled or blended with certain fractions of the coarse-grained Al, were consolidated with conventional powder metallurgy methods. The microstructures of the as-cryomilled powder and the consolidated bulk composites were investigated extensively by using various advanced characterization techniques. The mechanical properties were investigated uniaxially either in tension or in compression. Moreover, the potential of commercialization for this composite material was studied with technical cost modeling.; In the first part of this work, cryomilling was used to synthesize the Al/B4C nanocomposite powders with the particulate B4C reinforcement (submicron-to-several micron) uniformly distributed in the nanocrystalline Al matrix. Cryomilling provided an extremely low temperature for the processing of this composite, which prevented the formation of secondary phases that are detrimental to mechanical properties. Furthermore, the repeated cold welding and fracturing processes involved in cryomilling resulted in a strong metallurgical reinforcement/matrix interface. The cryomilling process was explored in detail, including the effect of process conditions on microstructure and the mechanisms for microstructural evolution during cryomilling.; In the second part of this work, the cryomilled composite powders were consolidated into bulk materials, and the mechanical properties, including quasi-static and the dynamic properties, were investigated in detail and the relationships between the microstructure and mechanical properties are discussed. The bulk nanocomposites exhibited extremely high strength but limited ductility. Therefore, coarse-grained Al was introduced into the nanocomposites to improve the ductility, which gave rise to the tri-modal composites, which consisted of B4C particles, nanocrystalline Al, and coarse-grained Al.; In the third part of this work, technical cost modeling was used to evaluate the potential for the commercialization of this composite, with the focus on the most novel step, cryomilling. Both processing and economic parameters were incorporated into the modeling. The cryomilling was demonstrated to be both scalable and economically feasible. The effects of the processing parameters on the processing cost for cryomilling were explored. |
