1. GARS Directorate Seminar

    "The Influence of Grain Misorientation on Grain Growth in Nanocrystalline Metals"

    Presented by Justin G. Brons, University of Alabama

    Tuesday, April 2, 2013, 10:30 am
    Building 130, Modular Conference Room

    Hosted by: Dr. Simerjeet Gill and Dr. Lynne Ecker

    As the grain size approaches the nanometer regime, the structure can exhibit peculiar grain growth behavior. For example, nanocrystalline metals have been reported to undergo both widespread coarsening and dramatic abnormal grain growth (AGG) due to annealing, tensile deformation, fatigue, and indentation at moderate-to-low temperatures (<0.5Tm). This research investigated the underpinnings of grain growth in two cases: (1) grain growth via annealing and (2) grain growth via indentation at 77 K, to determine what types of boundaries contribute to grain growth. It is not possible to measure the frequency with which boundaries are eliminated, but it is possible to determine which grain boundaries are shrinking and which are growing, based on relative boundary fractions. A step-wise annealing schedule was employed on Cu and Ni thin films to track individual grains and grain boundaries to quantify their character using precession-enhanced electron diffraction (PED). This allowed the tracking of the grain size and the evolution of specific boundary types, as well as the overall texture of the film. Cu exhibited normal grain growth, while Ni underwent AGG. The onset of AGG was accompanied by an increase in the "3 and "9 boundaries. The high mobility of "3 allowed the boundaries to migrate out of the system by moving to the surface. Once these boundaries were no longer present, AGG ceased. Additionally, a series of nanostructured Cu films, with and without a high density of twin boundaries, were subjected to indentation at 77K. Subsequent PED determined the crystallographic texture and grain-to-grain misorientation to understand which grain boundary types were present after mechanically-induced grain growth. In the twinned films, the undeformed region retained a high density of twin boundaries, whereas the pile-up region showed grain coarsening and an order of magnitude decrease in twin boundaries. The film without twins exp