Orientation-dependent Surface Energy Characterization of Magnetostrictive Alloys for Abnormal Grain Growth Modeling
Van Order, Michael Norman
Flatau, Alison B
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High sensitivity semiconductor, optical, and sensing devices require single crystal metals for their isotropy which allows for unique material properties. However they are expensive and difficult to produce. By using abnormal grain growth (AGG) techniques, our group can produce single-crystal-like materials that achieve ~90% performance of true single-crystals at ~5% of the cost. Fully understanding AGG mechanisms is crucial to the growth of high quality, cost effective alloy fabrication. Many advances have been made to understand key parameters of AGG, and we postulate that the final piece lies in understanding the surface energy of our alloys. While several surface energy measurement techniques have been developed for low-energy plastic surfaces, high-energy metal surfaces have largely been ignored due to the complexity of sample preparation and experimentation. This dissertation investigates three measurement techniques targeted for high-surface-energy iron-alloy crystal facets. The first of these techniques, the gallium drop contact angle method, examined a droplet of liquid metal gallium resting on our metal sample. By recording the shape of this droplet, a value for surface energy of targeted crystal orientations is acquired using our derived thermodynamically-based mathematical model. This study experimentally confirmed trends that are predicted in theoretical models, but identified that oxide formation on the sample surface interferes with acquisition of accurate quantitative results. This revelation led to a more robust study that expands on classic drop shape analysis techniques and eliminates complications associated with oxide layer formation. For this, multiple oxide removal procedures were performed and analyzed using X-ray photoelectron spectroscopy. The most promising procedures are polishing in an inert atmosphere and ion bombardment cleaning. Immersing the sample in an oil environment isolates this unstable iron-alloy surface from air and prevents oxidation. While in this environment, samples are probed with a deionized water droplet and a shape analysis is performed to calculate surface energy values using the Schultz method. This dissertation describes modifications to this method that utilize a technique previously only used on plastics to prevent water from spreading. I hypothesize that patterning sample surfaces with an ion mill will stabilize droplets during shape measurements, thus generating reliable surface energy calculations. Success of each technique could allow metallurgists to finally experimentally measure surface energy for any metal surface, thus providing confirmations of theory and sparking new ideas of how grain growth in metals can be controlled and even manipulated.