<p>  </p> <p>This work explored the dynamic behavior and failure mechanisms of an additively manufactured high-entropy alloy (HEA) when subjected to high-strain rate shock impacts. A laser-induced projectile impact testing (LIPIT) setup was used to study the dynamic behavior of the Cantor alloy CoCrFeMnNi samples manufactured using a directed-energy deposition technique. HEA flyers were accelerated by a pulse laser to velocities up to 1 km/s prior to impact with lithium fluoride glass windows. A photon Doppler velocimetry (PDV) system recorded the velocity of the flyer during the acceleration and subsequent impact. From this velocity profile, the Hugoniot coefficient and sound speed of the HEA samples were determined.</p> <p><br></p> <p>Upon determination of key shock parameters, spallation occurring due to shock was analyzed. Using the same LIPIT and PDV systems as the earlier testing, aluminum flyers of various thicknesses were accelerated into HEA samples. The back-surface velocity profiles of the HEA samples showed a characteristic “pullback” caused by the interaction of the tensile stress waves indicative of spall occurrence in the material. The magnitude of this pullback and the material properties determined in the first experiments allow for the measurement of spall strength at various strain-rates. This data is compared to previous data looking at similar HEAs manufactured using traditional methods. A comparison of this data showed that the spall strength of the HEA samples was equivalent to that of similar alloys but at significantly higher strain rates. As an increased strain-rate tends to result in increased spall strengths, further examination was needed to determine the reasons for this decreased spall strength in the AM samples.</p> <p><br></p> <p>Post-shock specimen recovery allowed for the failure mechanisms behind the spallation to be observed. Scanning electron microscope (SEM) images of the cross-section of the samples showed ductile fracture and void growth outside of the predicted spall region. Further imaging using energy dispersive spectroscopy (EDS) showed the presence of potentially chromium-oxide deposits in regions outside of the predicted spall plane. It is hypothesized that these regions created nucleation points about which spallation occurred. Thus, to achieve spall strength in AM HEAs equivalent to strengths in traditionally-casted alloys, the AM sample must be refined to reduce the occurrence of these deposits and voids.  </p>

<p dir="ltr">Nanocrystalline materials have unique properties such as high ultimate strength and superior hardness. However, they also exhibit some disadvantages, such as low thermal stability. An effective strategy to address this issue is alloying with other materials. Grain boundaries play a pivotal role in property prediction due to their orientation between grains and the complexity of their structure. The prediction of structure-property relationships for GBs with microstructural complexity represents a difficult challenge.</p><p dir="ltr">To understand the effects of dopants on the material properties of grain boundaries, we constructed some bicrystal models for Al and Mg-doped Al (Al-Mg) alloys. Findings from shearing simulations of these GBs indicate that the GB structure and dopant distribution can influence GB migration. Dopants inhibit GB migration at certain GBs, effectively reinforcing these GBs. Shear-coupled GB migration in pure Al, as well as dopant inhibition of GB Al-Mg alloys, both contribute to the mechanisms of GB migration.</p>

<p>  </p> <p>Concentrated solar power (CSP) is a technology that utilizes focused sunlight to heat a high-temperature medium (such as a molten salt). Heat from this medium can be transferred to a working fluid (such as supercritical CO2) that is then used to drive a turbine to generate electricity. Alternatively, the hot medium/fluid can be pumped into tanks for thermal energy storage (TES), for heat extraction later to generate dispatchable electricity and/or for electricity production at night or on cloudy days. By increasing the fluid temperature to <u>></u>750oC and utilizing TES, CSP can become more cost competitive with fossil-based electricity production. Current CSP systems utilize molten nitrate salts for heat transfer and TES that are known to thermally degrade at temperatures >600oC. To achieve temperatures <u>></u>750oC, molten chloride salts, such as ternary MgCl2-KCl-NaCl compositions, are being considered as heat transfer and thermal energy fluids for next generation CSP plants due to their higher temperature stability, low cost, and availability. </p> <p>In this work, it was demonstrated that MgCl2-containing molten salts are prone to oxidation in ambient air at 750oC, which can enhance corrosion of the containment materials and alter the thermophysical properties of the fluid. An alternative, low-cost, earth-abundant, MgCl2-free, oxidation-resistant molten salt, a eutectic CaCl2-NaCl composition, was developed, along with a corrosion mitigation strategy, to enable the slow growth of protective oxide layers on metals that are resistant to dissolution by such MgCl2-free molten chloride salts. </p> <p>This strategy was expanded to other low-cost, oxidation resistant compositions, such as eutectic BaCl2-CaCl2-KCl-NaCl with tailored chemical and thermophysical properties for CSP and TES. The melting temperature, heat capacity, oxidation resistance, and crystallization behavior were measured for eutectic a BaCl2-CaCl2-KCl-NaCl(17.5-47.8-3.3-31.4 mol%) (BCKN) salt and a MgCl2-KCl-NaCl (40-40-20 mol%) salt. BCKN salt was shown to have a similar melting temperature while having a higher heat capacity and far better oxidation resistance. </p> <p>The corrosion of the nickel-based superalloy Haynes 214 was studied in molten MgCl2-KCl-NaCl (40-40-20 mol%) salt at 750oC under inert atmosphere conditions using a custom-built rotating-disc corrosion testing apparatus that maintained laminar fluid flow on the sample. Non-protective external Cr-, Al-, and Mg- oxide layers were formed on Haynes 214 that were prone to spallation. Internal oxidation of Al was also observed along with Cr depletion zones within Haynes 214.  Corrosion kinetics were evaluated to quantify the role of fluid flow for application of this alloy for use in containment and transportation of this molten chloride salt. </p>

<p>With the emergence of additive manufacturing (AM), there is a need to understand the role of microstructures resulting from AM on the mechanical performance of the material. Ti-6Al-4V alloys are widely used within the aerospace industry as well as other industries to achieve high strength, low weight premium performance parts. There is a desire to utilize AM to produce Ti-6Al-4V, although these materials need to be qualified prior to their use in safety critical applications. Within the qualification of AM Ti-6Al-4V in aeronautics, fatigue loading is a crucial aspect to. It has been seen that within AM Ti-6Al-4V, prior β boundaries can be locations of microscopic localization of plastic strain which often lead to fatigue crack initiation. This thesis aims to further understand and predict the role of AM Ti-6Al-4V microstructures in dictating fatigue behavior. Specifically, the goal was to gauge the contributions of two microstructural features resulting from AM, prior β boundaries and α lathe-shaped grains, to the localization behavior. With the need to understand and predict the emergent behavior of the material system, crystal plasticity finite element (CPFE) methods were used in this thesis as the main method. </p> <p><br></p> <p>Within the context of CPFE, there is an existing gap in the current literature of realistic synthetic microstructures of Ti-6Al-4V that capture both the prior β boundaries and α lathes. With the ability to generate realistic FE models, the effects of the microstructural features can be better studied and characterized. The first portion of this thesis focuses on the generation of such synthetic microstructures which are simulated within the CPFE framework. An emphasis is placed on modeling the prior β boundaries and α grains. As these generated models are statistically equivalent to actual microstructures, material characterization via EBSD was performed on specimen that were used in the experimental fatigue testing. With the framework’s ability to generate synthetic microstructures that consider one prior β grain or multiple β grains (and thus prior β boundaries), simulations were conducted on both conditions of microstructures. </p> <p><br></p> <p>In the second portion of this thesis, simulations are conducted on two conditions of synthetic microstructures: models which contain 𝛼 lathes associated with one prior 𝛽 grain and models which contain multiple prior 𝛽 boundaries and the respective 𝛼 lathes. The goals of the simulations included: (1) lifing the different synthetic microstructures using a fatigue lifing model by way of the accumulated plastic strain energy density (APSED), (2) analyzing the microscopic localization of APSED at the prior β boundaries, and (3) analyzing the effects of the α lathes on the microscopic localization. This investigation aimed to further shed light on the effects of the additive manufacturing process and the implications of the resulting microstructure on the fatigue properties of AM Ti-6Al-4V. Furthermore, physics-based prognosis strategies similar to what is employed here will enable the rapid qualification of materials/structures and the ability to tailor component design on fatigue performance. </p>

<p>    </p> <p>Electrical steels are one type of soft magnetic material. They are based on Fe-Si alloys and are widely used for magnetic cores in transformers and electric motors. It is well known that Fe- 6.5Si wt% is the most efficient composition; however, at such a high silicon concentration (6.5wt.% = 12.1 at.% Si in Fe), the poor workability of the alloy makes it unacceptable for industrial production via conventional sheet steel rolling processes. This problem was approached in two different ways. First, a machining-based approach that suppresses the mechanisms that lead to cracking during conventional rolling was implemented for processing of thin metal strips. Two related machining-based sheet production technologies called free machining (FM), and hybrid cutting extrusion (HCE) were used to produce strips of high resistivity electrical steel. The maximum strip width achieved was 50 mm, and it was produced with a combination of FM and light rolling with a surface roughness comparable to cold-rolled sheet surfaces. Second, a new experimental alloy Fe-4Si-4Cr wt% was developed with improved magnetic properties compared to ~ Fe-3.2Si wt% and outstanding workability. Results report that the new experimental alloy has an electrical resistivity of 85 ± 3 𝜇Ω ∙ 𝑐𝑚 which is higher than Fe-6.5%Si. Also, the results on the Fe-4Si-4Cr workability show that this new alloy can withstand 75% cold-rolled reduction. The magnetic properties characterization was done via standard stacked toroid testing, and results show that Fe-4Si-4Cr experimental alloy exhibits excellent magnetic performance with a reduction in core losses of 33% at 400 Hz compared to commercial alloys with ~ Fe-3.2Si wt%. Recrystallization kinetics and texture evolution in the experimental alloy were evaluated for traditionally rolled and machining-based samples. Results were used to construct annealing maps. These maps represent the stages of the annealing process for a range of temperature versus time conditions, i.e., the annealing maps are a graphical summary showing the different stages of the annealing process for the Fe-4Si-4Cr experimental alloy in the two conditions. Despite the significant differences in the deformation texture of the two conditions, the recrystallization kinetics were similar. Finally, the two conditions retained the as-deformed texture in the intermediate annealing but to a lesser degree after completing a full anneal. In the case of the rolled sample, it is possible to trace the original texture fibers (γ-fiber, the partial α-fiber, and the θ -fiber) in the fully annealed data, but the texture intensity is just 2.5 mrd. On the other hand, the texture of the fully annealed HCE sample changes as compared to the as-deformed condition, located close to (110)[112] with a surprisingly strong peak of ~ 25 mrd. </p>

<p dir="ltr">The study aims to prove the need for higher quality production of weight plates which fall under US6746380B2 (expired May 10, 2021), assigned to USA Sports Inc. Literature justifies quality control standards. Selected literature validates posed hypotheses, and a product study is completed using weight plate specimens selected for physical quality testing to prove the common (repeated offense) existence of weight plates which fall out of the designated weight tolerance. Findings of physical product testing are collected and set against each other to determine differences in levels of quality control. Through extensive product testing (quantified within the study’s research methodology), novel quality control ideals are identified for product improvement in the study’s recommendations. Next steps are suggested to improve the understanding and utilization of quality control to work toward creating a sustainable and consistently high-quality product. Findings from the study are available to be used amongst companies in the fitness industry who produce weight plates.</p>

<p> This dissertation describes the recent advancements in micromechanical testing that inform how deformation mechanisms in austenitic stainless steels (SS) are affected by the presence of irradiation-induced defects. Austenitic SS is one of the most widely utilized structural alloys in nuclear energy systems, but the role of irradiation on its underlying mechanisms of mechanical deformation remains poorly understood. Now, recent advancement of microscale mechanical testing in a scanning electron microscope (SEM), coupled with site-specific transmission electron microscopy (TEM), enables us to precisely determine deformation mechanisms as a function of plastic strain and grain orientation.</p> <p> </p> <p>We focus on AISI 304L SSs irradiated in EBR-II to ~1-28 displacements per atom (dpa) at ~415 °C and contains ~0.2-8 atomic parts per million (appm) He amounting to ~0.2-2.8% swelling. A portion of the specimen is laser welded in a hot cell; the laser weld heat affected zone (HAZ) is studied and considered to have undergone post-irradiation annealing (PIA). An archival, virgin specimen is also studied as a control. We conduct nanoindentation, then prepare TEM lamellae from the indent plastic zone. In the 3 appm He condition, TEM investigation reveals nucleation of deformation-induced <i>α</i>’ martensite in the irradiated specimen, and metastable <i>ε</i> martensite in the PIA specimen. Meanwhile, the unirradiated control specimen exhibits evidence only of dislocation slip and twinning; this is unsurprising given that alternative deformation mechanisms such as twinning and martensitic transformation are typically observed only near cryogenic temperatures in austenitic SS. Surface area of irradiation-produced cavities contribute enough free energy to accommodate the martensitic transformation. The lower population of cavities in the PIA material enables metastable <i>ε</i> martensite formation, while the higher cavity number density in the irradiated material causes direct <i>α</i>’ martensite formation. In the 0.2 appm He condition, SEM-based micropillar compression tests confirm nanoindentation results. A deformation transition map with corresponding criteria has been proposed for tailoring the plasticity of irradiated steels. Irradiation damage could enable fundamental, mechanistic studies of deformation mechanisms that are typically only accessible at extremely low temperatures. </p>

<div>Defects are unavoidable, life-limiting and dominant sites of damage and subsequent failure in a material. Ni-based superalloys are commonly used in high temperature applications and inevitably found to have defects in the form of inclusions, voids and microscopic cracks which are below the resolution of standard inspection techniques. A mechanistic understanding of the role of defects in such industrially relevant bulk polycrystalline material is essential for philosophies of design and durability to follow and ensure structural integrity of components in the inevitable presence of such defects. The current understanding of defect-mediated damage, in bulk Ni superalloys, is limited by experimental techniques that can capture the local micromechanical state of the material surrounding the defect. In this work, we combine mechanical testing with in-situ, non-destructive 3-D X-ray characterization techniques to obtain rich multi-modal datasets at the microscale to interrogate complex defect-microstructure interactions and elucidate the mechanisms of failure around defects. The attenuated X-ray beam, after passage through the material, is utilized through computed micro-tomography to characterize the defects owing to its sensitivity to density differences in the material. The diffracted X-ray beam, after illuminating the material, is employed through high energy diffraction microscopy in various modes to interrogate the evolving micromechanical state around the discovered defects.</div><div>Three case studies are performed with specimens made of a Ni-based superalloy specially designed and fabricated to have internal defects in the form of: (i) an inclusion, (ii) a microscopic crack, and (iii) voids. In each case, the grain scale information is investigated to reveal heterogeneity in the local micromechanical state of the material as a precursor for the onset of failure. Models and simulations based on finite element or crystal plasticity are utilized, wherever necessary, to assess the factors essential to the underlying mechanism of failure. In the first case study, the detrimental effects of an inclusion in initiating a crack upon cyclic loading is interrogated and the state of bonding, residual stresses, and geometrical stress concentrations around the inclusion are demonstrated to be of utmost importance. In the second case study, the propagation of a short fatigue crack through the microstructure is examined to reveal the crystallographic nature of crack growth through the (i) alignment of the crack plane with the most active slip system, (ii) the correlation between the crack growth rate and the maximum resolved shear stresses, and (iii) the dependence of the crack growth direction on microplasticity within grains ahead of the crack front. In the third case study, the role of voids in ductile failure under tensile loading is explored to illuminate the activation and operation of distinct mechanisms of inter-void shear and necking under the control of the local state of stress triaxiality and the local plasticity within the grains at critical sites of fracture.</div><div>In summary, a grain scale description of the micromechanical state has been unambiguously determined through experiments to examine the heterogeneity around defects in the material. It has enabled us to identify and isolate the nature of factors essential to the activation of specific mechanisms at the onset failure. The grain scale thus provides an ideal physical basis to understand the fundamentals of defect mediated damage and failure instilling trust in the predictive capabilities of models that incorporate the response of the grain structure. The generated datasets can be used to instantiate and calibrate such models at the grain level for higher fidelity. </div>

<p>Powder bed fusion (PBF) process is a metal additive manufacturing process, which can build parts with any complexity from a wide range of metallic materials. Research in the PBF process predominantly focuses on the impact of a few parameters on the ultimate properties of the printed part. The lack of a systematic approach to optimizing the process parameters for a better performance of given material results in a sub-optimal process limiting the potentialof the application. This process needs a comprehensive study of all the influential parameters and their impact on the mechanical and microstructural properties of a fabricated part. Furthermore, there is a need to develop a quantitative system for mapping the material properties and process parameters with the ultimate quality of the fabricated part to achieve improvement in the manufacturing cycle as well as the quality of the final part produced by the PBF process. To address the aforementioned challenges, this research proposes a framework to optimize the process for 316L stainless steel material. This framework characterizes the influence of process parameters on the microstructure and mechanical properties of the fabricated part using a series of experiments. These experiments study the significance of process parameters and their variance as well as study the microstructure and mechanical properties of fabricated parts by conducting tensile, impact, hardness, surface roughness, and densification tests, and ultimately obtain the optimum range of parameters. This would result in a more complete understanding of the correlation between process parameters and part quality. Furthermore, the data acquired from the experimentsare employed to develop an intelligent parameter suggestion multi-layer feedforward (FF) backpropagation (BP) artificial neural network (ANN). This network estimates the fabrication time and suggests the parameter setting accordingly to the user/manufacturers desired characteristics of the end-product. Further, research is in progress to evaluate the framework for assemblies and complex part designs and incorporate the results in the network for achieving process repeatability and consistency.</p><br>

<p>Modern structural materials utilize tailored microstructures to retain peak performance within the most volatile operating conditions. Features such as grain size, grain boundary (GB) character and morphology and secondary phases are just a few of the tunable parameters. By tailoring these types of microstructural features, the deformation behavior of the material is also altered. The localization of plastic strain directly correlated to material failure. Thus, a systematic approach was utilized to understand the effect of microstructural features on the localization of plastic deformation utilizing digital image correlation (DIC). First, at the macroscopic scale, strain accumulation is known to form parallel to the plane of maximum shear stress. The local deviations in the deformation pathways at the meso-scale are investigated relative to the plane of maximum shear stress. The deviations in the deformation pathways are observed to be a function of the accumulated local plastic strain magnitude and the grain size. Next, strains characterized via DIC were used to calculate a value of incremental slip on the active slip systems and identify cases of slip transmission. The incremental slip was calculated based on a Taylor-Bishop-Hill algorithm, which determined a qualitative assessment of deformation on a given slip system, by satisfying compatibility and identifying the stress state by the principle of virtual work. Inter-connected slip bands, between neighboring grains, were shown to accumulate more incremental slip (and associated strain) relative to slip bands confined to a single grain, where slip transmission did not occur. These results rationalize the role of grain clusters which lead to intense strain accumulation and thus serve as potential sites for fatigue crack initiation. Lastly, at GB interfaces, the effect of GB morphology (planar or serrated) on the cavitation behavior was studied during elevated temperature dwell-fatigue at 700 °C. The resulting γ′ precipitate structures were characterized near GBs and within grains. Along serrated GBs coarsened and elongated <a>γ′ </a>precipitates formed and consequently created adjacent regions that were denuded of γ′ precipitates. Dwell-fatigue experiments were performed at low and high stress amplitudes which varied the amount of imparted strain on the specimens.<a> Additionally, the regions denuded of the γ′ precipitates were observed to localize strain and to be initial sites of cavitation.</a> <a>These results present a quantitative strain analysis between two GB morphologies, which provided the micromechanical rationale for the increased proclivity for serrated GBs to form cavities.</a></p>