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ItemCharacterizing Plastic Deformation Mechanisms in Metal Thin Films using In Situ Transmission Electron Microscopy Nanomechanics(Georgia Institute of Technology, 2023-04-19) Stangebye, SandraThe demand for smaller, smarter and faster devices has motivated continued research into understanding the mechanical behavior of small-scale materials used to create micron-sized features for devices such as flexible or stretchable electronics or micro electromechanical systems (MEMS). Nanocrystalline (NC) and ultrafine-grained (UFG) metal thin films show increased strength when compared to their coarse-grained equivalents, and as a result, have been proposed as viable solutions to high-strength MEMS materials. The increased yield strength is generally attributed to the high volume of grain boundaries (GB) which impede conventional dislocation glide. Unfortunately, the increase in strength is accompanied by a decrease in ductility. NC and UFG metals also exhibit an increase in strain-rate sensitivity and decrease in measured activation volume compared to their coarse-grained equivalents, both of which imply different atomistic mechanisms control the deformation. There remains a lack of quantitative characterization of these deformation mechanisms which hinders material design towards exception mechanical properties. In this work, the plastic deformation mechanisms that govern the mechanical properties of NC and UFG metal thin films are investigated through in situ transmission electron microscopy (TEM) nanomechanical experiments. This technique allows for the simultaneous observation of the active deformation mechanisms and quantification of the mechanical properties during monotonic and stress-relaxation experiments. Experiments were performed on NC Al and UFG Au specimens with different microstructures (grain sizes, thickness, texture), including irradiated UFG Au. A variety of deformation mechanisms have been identified, including dislocation nucleation and absorption at GBs, inter- and intragranular dislocation glide, and GB migration. It was found that the radiation damage in the irradiated UFG Au served as effective pinning points for transgranular dislocation glide, however, stress-assisted GBM was still active and effectively removed radiation damage as the defects were absorbed by the GB during migration. This resulted in defect-free (‘cleaned’) regions that can support unrestricted dislocation glide, suggesting that stress-assisted GBM is a healing mechanisms for radiation damage in UFG metals. The measured activation volume was found to increase with increasing grain size, decreasing stress level, and the addition of radiation damage. These values were compared with existing models to suggest that there is likely a competition between active displacive- and diffusive-type deformation mechanisms and that the contribution of the two depends on the microstructure. Furthermore, stress-assisted GB migration was studied in detail to investigate how the local microstructure influences boundary migration. This is completed by combining orientation mapping with in situ TEM straining to document the stress-induced migration behavior across boundaries of different structure.
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ItemAdvanced Microstructural Characterization of Ferroelectric and Antiferroelectric Fluorite-Structure Binary Oxide Thin Films for Memory Applications(Georgia Institute of Technology, 2022-08-22) Lombardo, Sarah F.The need for novel, super-high K dielectric gate oxides has substantially increased in recent years. With the equivalent oxide thickness in advanced nodes reaching a limit, materials innovation can enable increased dielectric constants in gate oxide stacks beyond the current limit, providing significant enhancement in logic technologies. Capacitance enhancement and super-high K dielectric gate stacks require the use of FE materials to be stabilized in an otherwise unstable state, resulting in an effective static negative capacitance (NC). However, the structural ferroelectric pathways in fluorite-structure binary oxides (i.e., HfO2 and ZrO2) – offering full scalability and CMOS compatibility – is not well-known. Additionally, since the discovery of antiferroelectricity in ZrO2, it has been well-recognized that the electrical characteristics associated with the field-induced phase transition in these materials can solve some of the most pressing challenges in modern microelectronics (energy efficiency, sub-Boltzmann logic technologies, memory and neuromorphic applications, etc.). While this sets the stage for post-scaling electronics, the physical origins of ferroelectricity and antiferroelectricity in ZrO2-based thin films has yet to be unanimously confirmed nor the phase transition experimentally visualized. Significant gaps remain in our fundamental understanding of the structure-property relationships in polycrystalline HfO2/ZrO2-based FE/AFE thin films. Since polarization correlates with crystal structure, the application of an electric field alters the microscopic features, e.g. grain orientation, phase, size, and sub-grain characteristics (interphase boundaries and domain walls) of these materials. This complex field-induced evolution of microstructure enables electrical characteristics such as multi-level cell capabilities for embedded non-volatile memory, analog synapses, and abrupt transitions for artificial neurons. On the other hand, such evolution of microstructure poses significant challenges to performance including cycle-to-cycle and device-to-device variation, reliability, and endurance. Due to the end of dimensional scaling of transistors, materials innovation is more crucial now than ever before to the advancement of microelectronics and modern computing. The goal of the work presented in this thesis is to advance our fundamental understanding of the structure-process-performance relationships in ferroelectric and antiferroelectric ZrO2-based thin films by establishing a foundation for high-throughput, automated microstructural analysis via a synergistic advanced microscopy characterization approach, thereby providing significant insight into processes necessary to optimize these material properties and enhance device performance while reducing power consumption in post-scaling electronics. Here, a variety of advanced microscopy characterization techniques are employed, and sample preparation procedures developed for the purpose of characterizing and quantifying the microstructural properties associated with material performance as a function of processing for polycrystalline zirconia-based thin films. These techniques include in situ TEM biasing, high resolution transmission electron microscopy (HRTEM), STEM, DF-TEM, and NBED. The first step in identifying structure-performance relationships presented here is the direct imaging of the polarization switching at the atomic and mesoscopic scales with applied bias, which suggests the presence of a field-induced phase transition with an applied field. Secondly, both local and statistical microstructural analysis of atomic structure, grain size distribution, orientation, and epitaxy are achieved and compared for both ferroelectric and antiferroelectric zirconia-based thin film capacitors, revealing structural pathways for ferroelectric phase stabilization as a function of doping and substrate. Thirdly, in addition to cross-section analysis, plan-view microstructural analysis is achieved, allowing for direct statistical quantification of real-space polycrystalline microstructure in ferroelectric zirconia-based thin films. The results from this work showcase the statistical, high-throughput characterization capabilities afforded by advanced electron microscopy for the purposes of furthering our understanding of the microstructure-property-process relationships in ferroelectric and antiferroelectric polycrystalline thin film materials.
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ItemFailure Mechanisms in Additively Manufactured Ferrous Metals Under Dynamic Loading Conditions(Georgia Institute of Technology, 2022-08-22) Koube, Katie D.Additive Manufacturing (AM) via 3D printing offers the ability to tailor materials with microstructures along various length scales so that properties may be optimized for specific use cases. Though methods for producing metallic parts which are fully dense and homogenous on the macroscale have been largely resolved, AM metals fabricated through laser powder bed fabrication (LPBF) possess highly heterogeneous hierarchical microstructures which affect their mechanical properties and failure response. These microstructures result from a complex thermal processing history and can be influenced by anything from laser settings to platform heat or gas flow rate and gas type. While the quasistatic mechanical behavior of AM stainless steels has been extensively studied, the effects of these microstructural heterogeneities have not been well characterized in a dynamic loading environment, and thus, the failure mechanisms of AM materials under these conditions are poorly understood. The first part of this dissertation investigates the role of local microstructure in Stainless Steel 316L (SS316L), through both the intentional control of fabrication process and as a byproduct of processing, in determining the spall behavior of AM materials and seeks to understand both spatially and temporally the heterogeneous failure modes which may be present. The second part explores the role of processing on the microstructure of direct ink write (DIW) fabricated metals (alloys) from their oxide components and seeks to understand the significance of rheological and thermodynamic factors which drive the process of successful printing, reduction, and sintering in alloyed and single element metals. The spall properties for LPBF SS316L were measured in both the in-plane (IP) and through-thickness (TT) build directions for a fully dense as-built part. Additionally, the effects of mesoscale porosity on spall were measured in the IP direction. When random and intentional porosity was added throughout the LPBF SS316L material, the spall failure modes displayed local heterogeneities where observed damage depended on the amount of porosity as well as the distance from the pores. Nano-CT scans of select impacted samples reveal local strain accommodation through pore damage and solidification of SS316L powder that dampens or even locally eliminates the spall response. The overall results show that porosity plays a critical role in slowing the shock wave propagation, effectively shifting the spall plane towards the rear free surface, and in some cases eliminating it entirely. Ferrous materials including elemental iron, SS316L, and the Cantor alloy were fabricated from their oxide constituents and 3D printed using DIW. The volume of particles in solution was optimized through the addition of a dispersant and the use of bimodal particle distributions. Reduction pathways which take advantage of the highly negative Gibbs Free Energy of mixing allow for reduction of Mn, and Cr oxides in both the Cantor alloy and SS316 to create alloys from stable oxides which would normally not be suitable for reduction. Alloys manufactured using DIW have an isotropic grain orientation and were fabricated with greater than 90% density. Demonstrating the capabilities of DIW as a solid-state processing test bed as well as a potential low-cost metal AM technique in addition to improving certain solid-state processing short falls, including minimizing the development of a core-shell microstructure.
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ItemAdditive Manufacturing of Carbon Steels Through Direct Ink Write Printing of Oxide Precursors(Georgia Institute of Technology, 2022-05-05) Stiers, Collin D.A method for additive manufacturing of various carbon steels from low cost and stable oxide powders is presented. This method uses direct ink write (DIW) processes to extrude inks composed of oxide powders, plastic binders, and solvents. Oxide powders are synthesized into viscous inks through mechanical mixing with a plastic binder and solvents. Inks are then extruded under ambient conditions into three dimensional (3D) architectures. The 3D printed green bodies solidify on contact with air after which they are subjected to a reducing process at elevated temperatures in hydrogen-rich environments to burn off the polymer binder and reduce the oxide powders, yielding metal alloys with controlled compositions. While this approach has been demonstrated in previous publications for various alloys, adding carbon, an important element in most industrial steels, has been a persistent challenge. This paper demonstrates an approach to introduce carbon during the reduction process, resulting in through-thickness carburization of the final parts. Post-printing, the parts can be heat treated to achieve desirable characteristics through well-established methods.
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ItemCorrelating microstructure with the corrosion properties of Aluminum and Stainless Steel alloy(Georgia Institute of Technology, 2022-04-25) Desai Choundraj, JahnaviThe primary research objective was to develop a relationship between the microstructure and the material’s susceptibility to degradation in corrosive environments. Susceptibility to, and mechanisms of, corrosion were investigated using multiscale electron microscopy techniques. The specific material systems studied were Aluminum alloy 5456 and additively manufactured (AM) stainless steel 316L. Corrosion experiments combined with EBSD analysis were used to determine the microstructure influence on β phase precipitation and intergranular corrosion in Al5456 alloy. This combined approach facilitates the rapid characterization of a large number of grain boundaries (~28,000 in this study), providing a statistical framework for understanding the results produced in the earlier studies, which focused on relatively very small number of grain boundaries. The influence of extrinsic characteristics of grain boundary such as local dislocation density were investigated, and qualitative/quantitative observations were reported. Clear trends with GND were observed, with the fraction of uncorroded grain boundaries decreasing with the increase in GND density. For AM steel, the influence of native oxide film on the passive film characteristics and localized corrosion of SLM (selective laser melting) 316L stainless steel were studied. The analysis showed that corrosive attack varied between initial attack of Cr and Mo-enriched dislocation cell boundaries to cell interior depending on the presence of a native oxide film on the initial sample surface. A corrosion mechanism has been proposed to explain this variation in corrosion attack behavior. The Pitting behavior of these samples was evaluated, and the general corrosion performance was compared with the wrought counterparts.
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ItemEffects of Microstructure on Crack Initiation in AA6451 and Crack Propagation in AA3xxx(Georgia Institute of Technology, 2020-03-31) Yoo, Yung SukAluminum alloys have been enjoying the spotlight in recent years as the next generation alloy for a wide variety of applications. Their potentially waste-free recyclability, excellent corrosion resistance, and desirable balance in physical properties—low density and high strength-to-weight ratio—makes them an ideal candidate material for efficient and environmentally-friendly products. Mechanical properties of aluminum alloys can be engineered to suit the requirements for different functions by controlling the microstructural features. Naturally, the variety of alloying elements, microstructural features, and thermomechanical processes produce complex microstructures that deform heterogeneously under different mechanical loading conditions. To get a better understanding of the failure mechanism of aluminum alloys, this dissertation will explore the effects of dispersoids, a type of second phase particle, on the crack initiation and propagation behaviors. A multiscale electron microscopy-approach was employed to characterize different aspects of the microstructure and their localized deformation behavior. This work is divided into two parts. The first part will delve into the crack initiation mechanism of AA6451 during three-point bending and the influence of microstructural features on each step of the process. It will also discuss the effects of variation in alloying elements and tempering conditions on the microstructure evolution and localized deformation behavior of AA6451. The second part involves studying the crack propagation behavior of deep drawn and necked AA3xxx. The dispersoid effects on crack growth direction will be discussed in depth. These findings will ultimately help scientists gain a better mechanistic understanding of defect interactions during extreme stress.
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ItemInvestigating the influence of microstructure on corrosion susceptibility: A multiscale electron microscopy approach(Georgia Institute of Technology, 2020-03-25) Key, Jordan W.In recent years, researchers have been leveraging developments in novel experimental methods and automated processing and analysis to establish processing-structure-property links in a more robust and statistical manner. One prime area that would benefit from such an approach is corrosion studies. Corrosion is an important societal issue with a broad and varied impact, and the corrosion-related maintenance and repair imposes a large expense on the global economy. This makes it important to better understand and predict corrosion behavior in order to design superior corrosion resistant materials. This work investigates the role of local microstructure in determining the corrosion behavior of materials, at the mesoscale and the nanoscale, through the combination of automated image processing and rapid, detailed characterization. This work is divided into two parts. At the mesoscale, detailed microstructural characterization through SEM and EBSD analysis is combined with automated image processing to develop first order correlations between pit initiation and grain orientation, intermetallic particle proximity, grain boundary proximity, and local dislocation density in 5083 aluminum. At the nanoscale, in situ TEM oxidation investigations of Fe thin films are combined with automated image processing to track dynamic processes in real time. Information on oxidation front propagation behavior and kinetics, as well as crystallographic evolution, is extracted. These findings improve the understanding of the influence of microstructure on corrosion and lay the groundwork for further developments of these methodologies.