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Abstract
Nanocrystalline thin films and coatings are an exciting class of nano-materials with properties superior to their bulk counterparts. Strengths exceeding that of steel, excellent corrosion resistance, and enhanced fatigue endurance have been reported. The root of these impressive properties is generally attributed to the ubiquitous presence of grain boundaries, which allow the materials to deform by means different from traditional dislocation driven plasticity. Successful implementation of nano-structured materials in commercial applications hinges on a fundamental understanding of the new grain boundary mediated deformation mechanisms.
This thesis seeks to develop a more comprehensive understanding of the stress driven grain boundary migration mechanism. Particular focus is placed on how intrinsic system parameters affect its operation. Efforts were directed toward testing a variety of nanocrystalline metallic films to explore whether stress driven boundary motion is universal to all nanocrystalline metals. Microtensile thin film specimens were synthesized, tested in uniaxial tension, and the evolution of the microstructure was studied with electron microscopy.
Stress driven boundary motion resulting in grain coarsening was detected in Pt highlighting that the mechanism continues to be active in face-center-cubic materials. Hexagonal Mg was also tested but the mechanism was not readily apparent. The findings suggest that for each material, the stress driven boundary mechanism is sensitive to the microstructure as the grain size, impurity content, boundary mobility, etc. can provide an environment more favorable for other deformation mechanisms that overshadow any boundary motion events.
To study the effect of grain boundary character on the mechanism, experimental techniques that combine in situ mechanical testing and orientation imaging microscopy were developed. Nanocrystalline Al thin films were investigated and the large grains formed by stress driven boundary motion had no orientation preference. The mobile interfaces in Al characterized with orientation mapping point to non pure twist boundaries whose plane deviates from the (111) orientation. These observations are compared to theoretical models and molecular dynamics simulation studies examining mobility for a variety of grain boundary configurations.
