Additive manufacturing has developed as a strong tool in making parts with complex shapes and high qualities. The microstructure evolution of the AlSi10Mg alloy manufactured through Laser Engineered Net Shaping (LENS®) and Selective Laser Melting (SLM) AM techniques is studied through electron microscopy characterization. The solidified melt pools of both as-deposited samples exhibit a “fish scale” pattern that is inherent to the AM process. A very fine cellular microstructure is seen due to the high thermal gradients and cooling rates on the order of 103-106K/s. Eutectic silicon forms along the cell boundaries of the aluminum matrix. The rapid solidification and non-equilibrium conditions result in the formation of metastable phases. X-Ray Diffraction and Differential Scanning Calorimetry analysis were carried out to identify the phases in the as-deposited samples. In-situ and post-mortem TEM characterization were conducted to observe the formation of silicon and Mg2Si precipitates. The Mg2Si precipitates segregated along the cell boundaries in both of the as-deposited samples. The π-Al8Si6Mg3Fe phase was also observed to segregate along the cell boundary. High resolution transmission electron microscopy studies revealed a crystallographic orientation relationship between the [001] Al matrix and the [110] Si primary phase. A comparative study between the microstructure of the LENS® and SLM as-deposited samples revealed a finer microstructure in the as-deposited SLM sample. A conventional T6 heat treatment caused the cellular network to break down and the Si particles to coarsen. Microhardness tests were conducted on the as-deposited samples and heat-treated samples to correlate the mechanical properties with the microstructure. The hardness of the as-deposited samples was 105 HV which is higher than the hardness of a cast counterpart.

Interest in additive manufacturing (AM) has grown exponentially in recent decades and is now being used in many different industries, such as the aerospace, automotive, and biomedical device industries. Unfortunately, the high cost of feedstock powder material, the need for high energy lasers, and a low rate of production have limited the use of this powerful technique on a large scale. Among all the parameters that are crucial in the quality of the parts, the relationship between starting feedstock powder and the quality of the part is not well explored. The common feedstock used in AM, gas atomized powder, requires a high amount of energy and inert gas to be produced. Therefore, gas atomized powder production is costly and not environmentally desirable. AM is believed to produce lower waste compared to the conventional manufacturing process due to minimal required post-processing; however, unless more sustainable starting materials and continuous powder reuse are implemented, the process is very wasteful. The primary focus of this dissertation is on understanding the role of the starting powder on the AM process sustainability in addition to the properties of additively manufactured parts. Powder size and morphology strongly influence powder flow and powder packing density, both of which are critical to successful AM processing. Therefore, the relationship between powder morphological features and flowability was explored and concluded that flowability of powders are heavily influenced by the particles size and shape. In order to reduce the amount of waste produced in laser directed energy deposition (L-DED) process, gas atomized powder was reused, and the reused powders and manufactured parts were characterized. The results indicate that although particles undergo severe changes as they are being reused in AM, the mechanical properties of the manufactured parts show minimal changes.

The production of powder from waste material for AM was explored. High energy milling and cryomilling were employed to recycle waste materials to be used as a starting material in AM. The results show that the size and morphology of the produced powder are significantly influenced by the production method, which was modified by tailoring the processing parameters. In addition, the feasibility of depositing aluminum matrix composites has been investigated as a way to improve the mechanical properties of parts manufactured with milled powder. The composite single tracks deposited in L-DED showed comparable morphologies to the single tracks deposited with gas atomized powder. Understanding the effects of using milled powder prepared as composite or recycled powder on the mechanical and microstructural properties of AM parts will be investigated in future work.

The 21st century presents humanity with unique challenges, addressing which requires constant improvements in manufacturing technologies and engineering materials. In this dissertation, two areas of modern material research are explored: additive manufacturing and high entropy materials.

First, the problem of microstructural inhomogeneities within additively manufactured metal parts is addressed. Specifically, the effect of process parameters and feature thickness on the microstructures of 316L stainless steel components fabricated by laser powder bed fusion (LPBF) is examined. A standard benchmark geometry developed by the National Institute of Standards and Technology, which contained walls of 0.5, 2.5 and 5.0 mm in thickness, is used. Optical microscopy, finite element analysis, scanning electron microscopy and electron backscatter diffraction reveal dramatic microstructural differences in features of different thickness within the same component. The feature thickness influences the cooling rate, which in turn impacts the melt pool size, solidification microstructure, grain morphology and density of geometrically necessary dislocations. The relationship between feature size and grain morphology is dependent on the energy input used during LPBF. Such behavior suggested that local manipulation of LPBF process parameters can be employed to achieve microstructural homogeneity within as-printed stainless steel components.

Second, a novel approach toward manipulation of residual stresses in additively manufactured parts is explored. Alloy design involving engineering of solid-state transformations is proposed as a way to change the residual stress state of the as-printed components. To this end, parts fabricated from two metals (pure Fe and Fe-50Cu) are evaluated. Residual stress measurements demonstrate that surface residual stresses can be successfully manipulated by adjusting the alloy composition. It is hypothesized that solid-state transformations experience by the Fe and Fe-rich phases are responsible for the observed differences in residual stresses. This study is the first to suggest using residual stress as a design criterion in alloy engineering for metal additive manufacturing.

Finally, a new high entropy silicide material for applications in electronics is proposed. The CALculation of PHAse Diagrams (CALPHAD) approach is used to identify two candidate single-phase high entropy silicides (HES): the ternary (CrMoTa)Si2 and the quinary (CrMoTaVNb)Si2. Both candidate compositions are experimentally synthesized via electron beam evaporation followed by heat treatment in vacuum, which facilitates the solid-state reaction. Both the ternary (CrMoTa)Si2 and the quinary (CrMoTaVNb)Si2 HES form a single phase with a C40 hexagonal crystal structure, validating our CALPHAD phase formation predictions. This work reports the first experimental realization of a thin film high entropy silicide material.

Being the lightest structural metal, Mg is a promising candidate in numerous energy-saving and environmental-friendly applications. The research interest in Mg and its alloys continues to rise in past decades, however, their commercial applications were held back by some longstanding intrinsic difficulties. Belonging to the hexagonal close packed family of metals, Mg has anisotropic slip and twinning systems, leading to strong textures invariably associated with wrought Mg and small strains-to-failure compared to those of current structural metals. Further enhancement of the performance in Mg requires mechanistic understanding of the deformation and innovative design strategies.

Microstructure is known to have tremendous influence on the mechanical and functional properties of materials. To shed light on the microstructural design for low density, high strength and formable Mg alloys, the objective of this dissertation is to advance the fundamental understanding of how microstructure affects their mechanical properties. Particular focus is on examining the influence of grain morphology (equiaxed or laminated), grain size (micro- or nano-crystalline), twin mesh features (with or without a high density of intersecting twins) and dilute solute addition of Y in polycrystalline Mg. For this purpose, we designed the experimental strategies leveraging powder metallurgy or surface mechanical treatment approaches to achieve samples with various internal microstructures. Bulk form and in-situ mechanical testing together with multiscale microstructure characterization were carried out on the Mg samples being synthesized.

In this dissertation, we have demonstrated strong grain morphology and grain size effects on the active deformation modes in polycrystalline Mg, resulting in a reversed yield strength anisotropy and a large compressive strain >120% at room temperature, in the microlaminated and nanocrystalline Mg, respectively, that have not been observed in conventional Mg. The toughening effect of twin meshes was explored by in-situ TEM nanocompression, and was successfully achieved in a gradient twin meshed Mg polycrystal at the macroscale. Lastly, the alloying effect of Y was analyzed experimentally in regard to the slip and twinning behavior. The difference in activation barriers among the slip modes was reduced by Y addition, and the mobility of twin boundaries may be affected by the segregation of Y atoms on such boundaries.

The promotion of renewable energy in electric energy grids is motivated by the adverse effects from the use and production of conventional energy sources. To accommodate large amounts of renewable penetration, the deployment of energy storage systems is required to manage variable renewable resources such as wind and solar. In recent years, utility-based energy systems have been undergoing a significant transformation to increasingly integrate energy storage systems. However, these devices are diverse in terms of not only their use-phase performance, but also the design parameters and processing materials used for their production and waste generated throughout their life cycle. Thus, sustainability assessment toward those various energy storage systems is urgently needed. In contrast, advancing customized sustainability assessment to cover the rarely addressed sustainability issues on emerging energy storage systems is also important.

In this dissertation, we have conducted comprehensive sustainability assessment on several energy storage systems from the perspectives of environmental and human health impact, chemical hazard, economic feasibility, and decision making. More specifically, a comprehensive investigation of the potential environmental and human health impact, chemical hazards, and economic feasibility associated with the production of three flow batteries: vanadium redox, zinc bromide, and all-iron flow batteries; is performed through life cycle impact assessment, chemical hazard assessment and techno-economic analysis. Further, the toxicity and hazard of processing chemicals used for the manufacturing of low band gap polymers in organic photovoltaics were assessed through a chemical hazard assessment approach. In addition, a multi-criteria decision analysis is applied to select different chemical toxicity data sources in order to promote a more comprehensive chemical hazard assessment.

Overall, this dissertation aims to provide insight into developing a strategy for comprehensive evaluation of the production and deployment of sustainable energy systems, with a focus on the effects of the product design and materials selection choices on the environmental footprint, hazard potential and economic feasibility of these novel energy devices. This strategy is designed to guide the selection of energy technologies in order to help utilities and regional governments to support their renewable energy penetration goals while minimizing environmental externalities.

Directed Energy Deposition (DED), one of the additive manufacturing techniques, has seen rapid growth over the last decade in multiple industries, including aerospace, medical devices, and energy systems. Although showing great potential and having proven itself a promising manufacturing alternative, the complex physics and highly non-equilibrium processes involved in DED exhibit unique challenges in optimizing the design parameters and controlling the overall quality of the final parts. Often times the entire process is treated as a “black box”, in which the effort has been focused on correlating the final built properties with input parameters, without fully understand the governing physics. This dissertation aims to provide a deeper understanding of the fundamental physics at each stage during the DED process, with the aid of in situ process monitoring tools and numerical simulations. In the first study, particle flow behavior of a gas atomized metallic powder (Inconel 718) and a ceramic powder (TiC) during the coaxial nozzle injection was studied through both experimental and numerical analysis. A 3-dimensional COMSOL numerical model was formulated to simulate the particle trajectories under the influence of gas flow, particle-gas interactions, and particle-wall collisions. High-speed photography was used to examine and compare the powder flow behavior of the Inconel 718 and TiC particles. The results reveal distinct differences in particle velocity and spatial distribution between the Inconel 718 and TiC particles, due to the dissimilar particle sizes, morphologies, material densities, and particle collision behavior. Next, thermal behavior of coated particles during the interaction with the molten pool was studied by constructing heat and phase transfer models using COMSOL software. Transient temperature and phase fields were calculated for coated and uncoated stainless steel (SS316L) and Zn-Al particles under different particle size, coating thickness, molten pool temperature and coating material conditions. Particle residence time values, defined as how long the particle can exist before undergoing a certain phase change, were determined from the calculations. The results show large variations in residence time due to the external parameters, especially the thermal diffusivity of the coated materials. Recommendations regarding coating material selection for retaining the Zn-Al particle were made based on the calculation results. In the third study, macroscopic thermal histories for a cuboidal build were analyzed in a 3-dimensional heat transfer model. Hatch rotation angle of 0 and 90 degrees were compared. A more randomized thermal history was observed for the 90-degree hatch rotation angle, which may inhibit epitaxial grain growth due to the dissimilar heat flux from pre-deposited layers. Finally, the tensile deformation behavior and phase formation in two nickel-based superalloys, Inconel 718 and Haynes 282, plus TiC-reinforced metal matrix composites of these alloys, were characterized experimentally for DED as-deposited samples. Scheil solidification calculations were also performed to understand the role of Nb (in the Inconel 718) and excess Ti and C (from the partially dissolved TiC reinforcement particles) on the phase formation sequence during solidification. The results reveal that the excess Ti+C leads to Nb-rich carbide formation in the Inconel MMCs, which depletes the Nb in the matrix and thus limits the formation of Laves and ” phases. This leads to the Inconel MMC matrix having a similar composition as in the Nb-free Haynes MMC matrix, and yields similar tensile behavior in the two sets of bulk, as-deposited MMC samples. These studies, combined, provide insight into the importance of materials properties and phase transformations in comprehensively understanding the DED process to facilitate expanded application in the industry.

High entropy oxide (HEO) materials consist of five or more oxide components in equimolar amounts, which form a single-phase state after processing. The first HEO developed was a transition metal HEO (CoCuMgNiZn)O (referred to here as TM-HEO). These materials exhibit interesting functional properties and hold excellent promise in battery applications. However, the influence of processing on the mechanical and electrical behavior of TM-HEOs is not well studied, the understanding of which is critical for implementation into practical applications. Here we first investigate the role of microstructure on mechanical properties in TM-HEO. Vickers indentation and nanoindentation methods were used to explore the influence of grain size on hardness, elastic modulus, and fracture toughness of single-phase TM-HEO. Hardness values exhibit a Hall-Petch relationship at larger grain sizes and an inverse Hall-Petch relationship at nanocrystalline grain sizes. Variations in elastic modulus and fracture toughness are attributed to several failure mechanisms discussed. This grain-size dependent mechanical behavior emphasizes the necessity of microstructure control when designing TM-HEO materials for applications such as batteries.Concerns with the safety and sourcing of lithium-ion batteries have prompted significant research into sodium-based systems. TM-HEOs are well suited for sodium battery applications due to their ability to accommodate a substantial quantity of mobile charge carriers, while also demonstrating promising cycling stability, conductivity, and battery capacity retention. The second investigation focuses on the underexplored influence of sodium doping, processing, and microstructure on charge transport in bulk sintered (CoCuMgNiZn)1-xNaxO (Na-HEO). We find that the conductivity increases with increasing dopant amount, up to 1.4x10-5 S∙cm-1 at x=0.33. Much of this increase is attributed to the high grain boundary conductivity, which originates from a NaxCoO2 layered structure that forms in the grain boundaries. The relative contributions of the grain boundaries and the grain bulk to the charge transport are discussed, along with how processing conditions and composition can be used to engineer conductive grain boundaries in Na-HEO. The third study introduces additional electrical property tuning by attempting to control secondary phase formation, particularly NaxCoO2, through heat treatment. This phenomenon of secondary phase formation is well studied in the undoped TM-HEO but has not been investigated in conjunction with Na-HEO. We find that Na-HEO shows evidence of the layered grain boundary phase after heat treatment, but also exhibits similar TM-HEO copper rich tenorite. Notably, secondary phase volume fraction reaches up to 13%, with evidence of large variations in microstructure and cation electronic state based on the heat treatment temperature. Multi-phase samples also demonstrate the highest reported conductivity of 2.03 x 10-4 S∙cm-1 for any room temperature Na-HEO study. Together, these findings underscore the potential for property control in TM-HEO systems, both mechanical and functional. Further analysis of multi-phase Na-HEO grain boundaries and composition is recommended to confirm grain boundary charge transport contributions.

This thesis includes two case studies on sustainable manufacturing: (1) Production of Metals for Additive Manufacturing; and (2) Brass Jewelry Manufacturing. Additive manufacturing (AM) produces components with complex geometries more efficiently and with higher material yields as compared to traditional material forming technologies. However, AM processes generally require fine powder with a narrow size distribution, which is expensive to produce, is energy intensive and its production results in a negative environmental impact. To address this challenge, we report on a study of the cost feasibility and environmental impact focusing on global warming potential and cumulative energy demand associated with the production of different powder feedstocks that are suitable for Laser Engineered Net-Shaping (LENS®), a directed energydeposition AM technique. LENS® is a widely used AM technique that requires metal powders as the starting feedstock. In our study, the associated costs, and environmental impacts from two commonly used AM materials, 316L stainless steel and AlSi10Mg, are analyzed. Moreover, we determined the economic costs of the various processing steps and the various factors affecting production, using techno-economic analysis (TEA). To evaluate the environmental impact associated with the different approaches used for powder feedstock production, we used life cycle impact assessment (LCIA). The results of the TEA and LCIA analyses demonstrate that amongst the various feedstock studied – gas atomized powder, recycled machine chip powder, and recycled gas atomized powder - recycled gas atomized powder provided the greatest economic benefit and lowest environmental impact. Our results provide a framework that can be used to assess how recycling can be implemented to reduce the costs and environmental impact of feedstock materials, and thereby enable the sustainable development of AM technologies. Brass alloys are frequently used in fashion jewelry due to their physical properties, such as color, shininess, toughness, corrosion-resistance, and ability to be crafted at low temperatures. However, brass jewelry commonly includes toxic metals, such as lead and cadmium. In the brass jewelry project, I explored the sustainable development of brass jewelry materials, specifically focusing on the associated casting, processing, and supply chain issues. Another goal is to identify the causes of negative impacts on human health or environmental quality, while exploring the potential to apply the principles of the circular economy. Evaluating the historical costs of materials to determine the cost of creating an alloy is crucial to the keeping the cost of brass jewelry down; however, when designing the alloy this perspective must be weighed against the sustainability of the materials involved, including the inputs and outputs of producing the material and potential toxicity of the material. If a material is cheap, but toxic it cannot be used. Likewise, if a material is environmentally sustainable, but expensive, it is not likely to be used. In the first case study regarding the production of metal feedstock powders for additive manufacturing, it is observed that machine chips powder and reused gas atomized powder both reduce the economic costs and environmental impacts of the powders used to construct AM products. This study shows the importance of being informed of how feedstock powders are produced and their affects. In the second case study regarding brass jewelry, brass alloy design is instructed by the sustainability of the materials involved. While not all metals are suitable for replacing current compositions in brass alloys, sustainable material selection is informed by selecting for the material properties, future and current availability, and environmental impacts of elements for brass jewelry compositions.

Laser Engineered Net Shaping, LENS®, is a multi-variable process of Additive Manufacturing that employs a high-power laser to deposit powder feedstock onto a substrate to build parts layer-by-layer. Advantages of this process are tailored microstructures with optimized structural properties for a variety of industry applications. The LENS® system parameters allow for variable user control but also introduce complexity into the process where every change in deposition procedure results in different effects on the build. To understand the connection between process parameters and build quality, each variable should be fully understood. The focus of this study was to isolate single variables and exhaustively analyze the effects on the deposition process. The first project investigates reusing powder feedstock for multiple cycles and the effects on powder properties and build quality. The second project examines the effect of varying the hatch rotation angle parameter by characterizing the resulting microstructural and mechanical properties. Microstructural and compositional characterization was done through several microscopy techniques to analyze the 316L SS powders, used throughout this thesis, and the manufactured parts. Bulk material properties were analyzed with compression, microhardness, and density testing. It was found that reusing powder for ten cycles causes minimal changes to the powder properties. The properties of builds tested from select cycles were unchanged. Varying the hatch rotation angle between layers of a deposition did affect microstructural development which influenced mechanical properties. Finally, the complexity of the LENS® system was discovered through extensive operation, and the care required to manufacture consistent parts is exemplified.

Entropy-stabilized oxides (ESO) consist of five or more oxide components that form a random solid solution structure after processing. ESO materials have garnered significant attention due to their interesting functional properties and the promise of undiscovered properties that may lie within the expanded compositional space that these materials possess. Multiple entropy stabilized oxides have been discovered and are being investigated around the world. In this study, (CoCuMgNiZn)O is investigated. This ESO composition, which was discovered in 2015, is known to exhibit a reversible phase transformation between the single-phase entropy stabilized rocksalt phase to a multi-phase state consisting of the primary rocksalt phase, a copper-rich tenorite secondary phase, and a cobalt-rich spinel secondary phase. The phase transformation between the single-phase state and the multi-phase state, and the specific phase concentrations, has been previously demonstrated to be a function of heat treatment temperature and grain size. In this study, we consider an additional variable – starting composition – in an effort to further understand the phase transformation behavior, as well as to explore the electrical behavior of this material. Thus, the goal of this research was to investigate the influence of the secondary phases on the electrical behavior of ESO materials, by manipulating the copper content in the materials, and consequently the phase composition of ESO materials, to better understand the structure-property relationships and the mechanisms that control electrical behavior in these versatile materials. To achieve this goal, the concentration of copper oxide (CuO) was varied during synthesis, and the post sintering heat treatment temperatures and times were also varied, in an effort to control the formation of secondary phases in the ESO material. Fully dense, bulk ESO samples were synthesized using solid-state powder processing and elevated temperature sintering, prior to secondary heat treatment. The electrical conductivity in the bulk samples was evaluated as a function of heat treatment temperature (450-900°C), time (0-48 hours), and frequency (500 mHz to 1 MHz), and the phase state and microstructure were characterized using various microscopy, diffraction, and spectroscopy techniques. By comparing equimolar (20% CuO), Cu-deficient (10% CuO), and Cu-enriched (30% CuO) compositions, the conductivity mechanisms are elucidated.The results of this experimental investigation indicate that the concentration and distribution of the secondary phases can be controlled by varying starting composition, heat treatment temperature and heat treatment time. The Cu-rich tenorite secondary phase ranged in concentration from 0-21 vol% and was distributed both at the grain boundaries and within the grains. The Co-rich spinel secondary phase ranged in concentration from 0-10 vol% and was distributed within the grains. The total conductivity ranged from approximately 10-12 S/cm to 10-5 S/cm, and the dielectric constant ranged from approximately 10 to 104. The highest values for both properties were observed in the 30% CuO sample heat treated at 750°C for 12 hr, and the lowest values were observed in the single-phase samples. The results of the frequency response and activation energy analysis, in combination with the microstructural characterization, indicate that the electrical conductivity enhancement is due to the presence of the Cu-rich tenorite secondary phase, which creates a conductive percolation path, or grain boundary network, along which charge transport is enhanced. Additionally, the Cu-rich tenorite secondary phase creates a capacitive effect at the interface regions between the primary rocksalt phase and the Cu-rich tenorite secondary phase, thereby increasing the dielectric constant. Overall, the results of this study highlight the ability to tailor the microstructure and phase state of ESO materials, and thereby manipulate their electrical behavior.