UC Irvine

One of the important characteristics of crystalline microstructures is how the crystal orientation changes in space, often abruptly to form grain boundaries. Quantifying crystalline microstructure can be accomplished in many ways, ranging from measurements of grain size, to long range network analyses. For example, studies of how the density of grain boundaries affects strength have led to the discovery of both the Hall-Petch relation and its breakdown in nanocrystalline metals. In another case, measurements of grain boundary character have been central to the success of grain boundary engineering in improving corrosion resistance. Where the first field focuses on the quantity of grain boundaries, the second emphasizes their qualities. This sort of analytical division can be very productive because it defines focused research problems, but it also prescribes limits around the possible findings. This thesis bridges some of these inevitable gaps by applying more expansive local orientation correlation metrics to situations where they were not, or could not, be used in the past.

Starting with the analytically simplest case, we measured the types of grain boundaries found in nanocrystalline metals prepared by different processing routines. This is of special interest because the extreme density of grain boundaries in nanocrystalline metals exaggerates the importance of their character. Despite its importance, practical limits on microscopy previously prevented most prior research from analyzing boundary character. The development of the grain boundary character distributions have been examined to provide insight into the mechanisms responsible for their formation. Next, we use longer range metrics to unravel how the complicated topology of grain boundary engineered microstructures is formed. Studying the grain boundary network topology of these materials helps to rationalize their processing and clarify several prior studies that relied on two-point metrics. Similar grain boundary engineered materials will then be used to explore how boundary type affects grain size strengthening. The result is a new measure of how much twin boundaries contribute to yield strength, which is important to understanding the strength of advanced materials with high twin fractions. The next chapter describes a thermomechanical method for grain boundary engineering nanocrystalline metals, where conventional techniques cannot be applied. Grain boundary network measurements of these materials are then applied to understand the mechanisms at work, revealing new information about the response of nanocrystalline metals to cyclic deformation. This has value for developing new processing methods and understanding changes that may occur during service. In each section, new insights are gained by applying different local orientation correlations than have been typical in prior inquiries.