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Description
Electric-current assisted manufacturing techniques have been at the forefront of manipulating metal microstructures. These newly processed microstructures often have great fatigue resistant properties. Nevertheless, many of these microstructures do not allow for a reversal of dislocation motion. Dislocations accumulate at grain boundaries and, ultimately, cause fatigue failure. A newly proposed bimodal microstructure, that can make dislocation motion reversible, has been developed. This bimodal microstructure contains micrometer sized grains separated by a network of ultrafine nanocrystalline structure. This structure requires the processing of the grain boundaries with a technique called electric-current assisted grain boundary local engineering (EAGLE). EAGLE utilizes the physics of Joule heating to melt the grain boundaries of metals without affecting the grain interiors. The grain boundaries will, then, be rapidly cooled to form a highly refined grain nanocrystalline structure. The purpose of this thesis is to examine the optimized operating parameters of EAGLE. In particular, the optimal dc voltage and the duration of current pulse were to be determined. Although several metals have been studied, Ti metal was especially scrutinized. Three microcrystalline models were created and simulated with FEA. These models include the bi-grain, hexagon-grain, and realistic-grain model. A pulse of current with predetermined voltages were applied to the models. The bi-grain model serves to prove the feasibility of localized grain boundary heating. The hexagon-grain model serves to observe the Joule heating of varyingly oriented grain boundaries. Finally, the realistic-grain model serves to accurately simulate the heating of the grain boundary in a true microcrystalline structure. The pulse duration and voltage of current that provides the most ideal temperature distribution was to be determined. The concluded optimized parameters were the result of the realistic- grain model simulation.
