Additive Friction Stir Deposition of Al-Ce Alloys for Improved Strength and Ductility

Additive friction stir deposition (AFSD) is a solid-state additive manufacturing (AM) technique that breaks down large constituent particles into more refined and uniformly disturbed microstructure. AFSD was used to print Al-Ce alloys. Current commercial Al-alloys upon elevated temperatures go through dissolution and coarsening of strengthening precipitates causing mechanical degradation of these alloys. Al-Ce alloys do not have this issue as cerium's low solubility restricts dissolution into the aluminum matrix at elevated temperatures, thus giving great thermal stability to the microstructure. Al-Ce alloys lack solid solubility that affects the solid solution strengthening and precipitation strengthening. Al-Ce alloys have limitation at room temperature as they can only reach a maximum of ~65 MPa yield strength. Elements like magnesium have been added to alloy to enable solid solution strengthening, and scandium to enable precipitation strengthening to improve strength before going through the AFSD process. By adding new elements to the Al-Ce alloys, an increase in the yield strength from ~60 MPa to ~200 MPa was achieved before AFSD. The casted alloys form coarse particles that reach 300 µm in size; resulting in stress concentration that causes material fracture before necking, giving >10% ductility. AFSD breaks down these coarse particles to increase strength and …
Date: December 2023
Creator: Davis, Devin Fredric
System: The UNT Digital Library

Thermodynamics, Kinetics and Mechanical Behavior of Model Metallic Glasses

The thermophysical properties and deformation behavior of a systematic series of model metallic glasses was investigated. For Zr-based metallic glasses with all metallic constituents, the activation energy of glass transition was determined to be in the range of 74-173 kJ/mol while the activation energy of crystallization was in the range of 155-170 kJ/mol. The reduced glass transition temperature was roughly the same for all the alloys (~ 0.6) while the supercooled liquid region was in the range of 100-150 K, indicating varying degree of thermal stability. In contrast, the metal-metalloid systems (such as Ni-Pd-P-B) showed relatively higher activation energy of crystallization from short range ordering in the form of triagonal prism clusters with strongly bonded metal-metalloid atomic pairs. Deformation mechanisms of all the alloys were investigated by uniaxial compression tests, strain rate sensitivity (SRS) measurements, and detailed characterization of the fracture surface morphology. For the metal-metal systems, plasticity was found to be directly correlated with shear transformation zone (STZ) size, with systems of larger STZ size showing better plasticity. In metal-metalloid amorphous alloys, plasticity was limited by the distribution of STZ units, with lower activation energy leading to more STZ units and better plasticity. The alloys with relatively higher plasticity …
Date: December 2023
Creator: Akhtar, Mst Alpona
System: The UNT Digital Library

Electrochemical Behavior of Catalytic Metallic Glasses

Metallic Glasses are multi-component alloys with disordered atomic structures and unique and attractive properties such as ultra-high strength, soft magnetism, and excellent corrosion/wear resistance. In addition, they may be thermoplastically processed in the supercooled liquid region to desired shapes across multiple length-scales. Recently developed metallic glasses based on noble metals (such as Pt and Pd) are highly active in catalytic reactions such as hydrogen oxidation, oxygen reduction, and degradation of organic chemicals for environmental remediation. However, there is a limited understanding of the underlying electrochemical mechanisms and surface characteristics of catalytically active metallic glasses. Here, we demonstrate the influence of alloy chemistry and the associated electronic structure on the activity of a systematic series of Pt42.5−xPdxCu27Ni9.5P21 bulk metallic glasses (BMGs) with x = 0 to 42.5 at%. The activity and electrochemically active surface area as a function of composition are in the form of volcano plots, with a peak around an equal proportion of Pt and Pd. These amorphous alloys showed more than two times the hydrogen oxidation reactivity compared to pure Pt. This high activity was attributed to their lower electron work function and higher binding energy of Pt core level that reduced charge-transfer resistance and improved electrocatalytic activity …
Date: July 2023
Creator: Mahajan, Chaitanya
System: The UNT Digital Library
Exploring the Synergistic Effects of MXene-based Nanocomposites for Superlubricity and Friction/Wear Reduction on Rough Steel Surfaces (open access)

Exploring the Synergistic Effects of MXene-based Nanocomposites for Superlubricity and Friction/Wear Reduction on Rough Steel Surfaces

The aim of this thesis is to advance the field of solid lubrication science by developing coatings that provide reliable performance in ambient conditions, work on rough surfaces, and are amenable to industrial size and design complexities. Two different coating systems, Ti3C2Tx-MoS2 and Ti3C2Tx-Graphene Oxide blends, were studied in this work. The Ti3C2Tx-MoS2 nanocomposites were spray-coated onto rough 52100-grade steel surfaces, and their tribological performance was evaluated in a ball-on-disk configuration in a unidirectional sliding mode. The test results indicate that Ti3C2Tx-MoS2 coatings achieved superlubricity, which has not been previously reported for either pristine material under macroscale sliding conditions. The observed synergistic mechanism enabled the superlative performance, which was explained by the in-situ formation of a robust tribolayer responsible for sustained lubricity even at high contact pressures (>1.1 GPa) and sliding speeds (0.1 m/s). Processing, structure, and property correlation studies were conducted to understand the underlying phenomena. Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy were used to reveal the formation of the tribolayer. The Ti3C2Tx-Graphene Oxide blends were also spray-coated onto rough-bearing steel surfaces, and their tribological assessment was carried out in ambient environmental conditions and high contact pressures in a ball-on-disc experimental setup. The coatings led to …
Date: July 2023
Creator: Macknojia, Ali Zayaan
System: The UNT Digital Library

Next Generation Friction Stir Welding Tools for High Temperature Materials

The historical success of friction stir welding (FSW) on materials such as aluminum and magnesium alloys is associated with the absence of melting and solidification during the solid-state process. However, commercial adoption of FSW on steels and other non-ferrous high-strength, high-temperature materials such as nickel-base and titanium-base alloys is limited due to the high costs associated with the process. In this dissertation, the feasibility of using an FSW approach to fabricate certain structural components made of nitrogen containing austenitic stainless steels that go into the vacuum vessel and magnetic systems of tokamak devices was demonstrated. The FSW weldments possessed superior application-specific mechanical and functional properties when compared to fusion weldments reported in the technical literature. However, as stated earlier, the industrial adoption of FSW on high temperature materials such as the ferrous alloys used in the present study is greatly limited due to the high costs associated with the process. The cost is mainly dictated by the high temperature FSW tools used to accomplish the weldments. Commercially available high temperature FSW tools are exorbitantly priced and often have short lifetimes. To overcome the high-cost barrier, we have explored the use of integrated computational materials engineering (ICME) combined with experimental prototyping …
Date: July 2023
Creator: Gaddam, Supreeth
System: The UNT Digital Library

Small Scale Fracture Mechanisms in Alloys with Varying Microstructural Complexity

Small-scale fracture behavior of four model alloy systems were investigated in the order of increasing microstructural complexity, namely: (i) a Ni-based Bulk Metallic Glass (Ni-BMG) with an isotropic amorphous microstructure; (ii) a single-phase high entropy alloy, HfTaTiVZr, with body centered cubic (BCC) microstructure; (iii) a dual-phase high entropy alloy, AlCoCrFeNi2.1, with eutectic FCC (L12) -BCC (B2) microstructure; and (iv) a Medium-Mn steel with hierarchical microstructure. The micro-mechanical response of these model alloys was investigated using nano-indentation, micro-pillar compression, and micro-cantilever bending. The relaxed Ni-BMG showed 6% higher hardness, 22% higher yield strength, and 26% higher bending strength compared to its as-cast counterpart. Both the as-cast and corresponding relaxed BMGs showed stable notch opening and blunting during micro-cantilever bending tests rather than unstable crack propagation. However, pronounced notch weakening was observed for both the structural states, with the bending strength lower by ~ 25% for the notched samples compared to the un-notched samples. Deformation behavior of HfTaTiVZr was evaluated by micropillar compression and micro-cantilever bending as a function of two different grain orientations, namely [101] and [111]. The [111] oriented micropillars demonstrated higher strength and strain hardening rate compared to [101] oriented micropillars. The [111] oriented micropillars showed transformation induced plasticity …
Date: July 2023
Creator: Jha, Shristy
System: The UNT Digital Library
Tribocatalytically-Active Coatings for Enhanced Tribological Performance and Carbon-Based Tribofilm Formation (open access)

Tribocatalytically-Active Coatings for Enhanced Tribological Performance and Carbon-Based Tribofilm Formation

In this study, we investigate the fundamental mechanisms defining the approach for addressing tribological challenges in mechanical systems via the use of the tribocatalytically active coating. The coating is designed using an electrodeposition process and consists of a hard amorphous cobalt-phosphorous matrix with the incorporation of tribocatalytically-active nickel and copper. Our focus is on understanding the effect of the tribocatalytic elements, Cu vs Ni, on the coating's performance in high-contact stress conditions, generating local heating, shear, and compression. By optimizing the relative composition and mechanical characteristics of the coating, we aim to enhance its tribological performance in the presence of a hydrocarbon environment. Through extensive characterization of the wear tracks using SEM/EDS and Raman analyses, we identify the formation of a protective carbon-based tribofilm on the coating's surface during sliding as the key factor behind its excellent performance. Our findings not only contribute to the understanding of material transformations in the contact but also offer a robust and versatile approach to addressing tribological challenges in mechanical systems. The development of this innovative coating opens up new possibilities for promoting the formation of protective tribofilms and improving the performance of mechanical components operating in low-viscosity fuels and synthetic oils.
Date: July 2023
Creator: Al Sulaimi, Rawan
System: The UNT Digital Library

In-situ Electrochemical Surface Engineering in Additively Manufactured CoCrMo for Enhanced Biocompatibility

Laser-based additive manufacturing is inherently associated with extreme, unprecedented, and rapid thermokinetics which impact the microstructural evolution in a built component. Such a unique, near to non-equilibrium microstructure/phase evolution in laser additively manufactured metallic components impact their properties in engineering application. In light of this, the present work investigates the unique microstructural traits as a result of process induced spatial and temporal variation in thermokinetic parameters in laser directed energy deposited CoCrMo biomedical alloy. The influence of such a unique microstructural evolution in laser directed energy deposited CoCrMo on electrochemical response in physiological media was elucidated and compared with a conventionally manufactured, commercially available CoCrMo component. Furthermore, while investigation of the electrochemical response, such a microstructural evolution in laser directed energy deposited CoCrMo led to in-situ surface modification of the built components in physiological media via selective, non-uniform electrochemical etching. Such in-situ surface modification resulted in enhanced biocompatibility in terms of mammalian cell growth, cell-substrate adhesion, blood compatibility, and antibacterial properties indicating improved osteointegration, compared to a conventionally manufactured, commercially available CoCrMo component.
Date: May 2023
Creator: Mazumder, Sangram
System: The UNT Digital Library
The Influence of Particle Morphology and Heat Treatment on the Microstructural Evolution of Silver Inks for Additively Manufactured RF Applications: A Comparison between Nanoflake and Reactive Inks (open access)

The Influence of Particle Morphology and Heat Treatment on the Microstructural Evolution of Silver Inks for Additively Manufactured RF Applications: A Comparison between Nanoflake and Reactive Inks

In recent years, advancements in additive manufacturing (AM) technologies have paved the way for 3D-printed flexible hybrid electronics (FHE) and created opportunities for extending these gains to RF applications. However, printed metal interconnects and devices are typically characterized by high porosity and chemical impurities that significantly limit their electrical conductivity and RF performance compared to bulk equivalents. Using direct ink writing (DIW), two silver inks, a nanoflake suspension and a nanoparticle-reactive ink, were investigated to understand the relationship between free interfacial energy, sintering behavior, DC conductivity, and RF loss. The printed silver samples were characterized using scanning electron microscopy, x-ray diffraction, and x-ray photoelectron spectroscopy to monitor microstructural evolution, grain size and orientation, and chemical purity as a function of heat treatment temperature. Three heat treatments were applied to each ink: the manufacturer's recommendation, 225°C for 30 minutes, and 350°C for 30 minutes. Four-wire structures and coplanar waveguides were printed to compare the DC and RF performance up to 18 GHz, respectively. The results show that ink formulations that facilitate larger grains, high density, and good chemical purity have superior RF performance. A low resistivity of 1.4 times bulk Ag, average of 0.8% greater RF loss factor than evaporated Ag, …
Date: May 2023
Creator: Summers, Jason Masao
System: The UNT Digital Library
Laser Powder Bed Fusion of H13 Tool Steel: Experiments, Process Optimization and Microstructural Characterization (open access)

Laser Powder Bed Fusion of H13 Tool Steel: Experiments, Process Optimization and Microstructural Characterization

This work focused on laser powder bed fusion (LPBF) of H13 tool steel to examine microstructure and melt pool morphology. Experiments were conducted with varying laser power (P) in the range of 90-180 W and scan speed (v) in the range of 500-1000 mm/s. layer thickness (l) and hatch spacing (h) were kept constant. Volumetric energy density (γ) was calculated using the above process parameters. In order to find a relation between the recorded density and top surface roughness with changing process parameters, set of equations were derived using the non-dimensional analysis. For any chosen values of laser power, scan speed, hatch spacing and layer thickness, these equations help to predict top surface roughness and density of LPBF processed H13 tool steel. To confirm the universal relation for these equations, data of In718 and SS316L processed in LPBF was input which gave a R-square of >94% for top surface roughness and >99% for density. A closed box approach, response surface model, was also used to predict the density and surface roughness which allows only in the parametric range. Material microstructures were examined to identify the melting modes such as keyhole, transition and conduction modes. X-ray diffraction data revealed that there …
Date: May 2023
Creator: Channa Reddy, Sumanth Kumar Reddy
System: The UNT Digital Library

Structures of Multicomponent Silicate and Borosilicate Glasses from Molecular Dynamics Simulations: Effects of Iron Redox Ratio and Cation Field Strength

Multicomponent silicate and borosilicate glasses find wide technological applications ranging from optical fibers, biomedicine to nuclear waste disposal. As a common component of earth's mantle and nuclear waste, iron is a frequent encounter in silicate and borosilicate melts and glasses. The redox ratio in glass matrix defined by the ratio of ferrous and ferric ions is dependent on factors such as temperature, pressure, and oxygen fugacity. Understanding their roles on the short- and medium-range structure of these glasses is important in establishing the structure-property relationships which are important for glass composition design but usually difficult to obtain from experimental characterization techniques alone. Classical molecular dynamics simulations were chosen in this dissertation to study iron containing glasses due to challenges in experimental techniques such as NMR spectroscopy originated from the paramagnetic nature of iron. Magnesium is also a common element in the oxide glass compositions and its effect on the structure of boroaluminosilicate glasses were also investigated. Magnesium ion (Mg2+) has relatively higher cation field strength than other modifier cations and its structural role in oxide glasses is still under debate. Therefore, investigating the effects of cation field strength of modifier cations in light of MgO in boroaluminosilicate glasses is also …
Date: May 2023
Creator: Tuheen, Manzila Islam
System: The UNT Digital Library
Developing Modern Atom Probe Tomography for Nonmetals (open access)

Developing Modern Atom Probe Tomography for Nonmetals

Atom probe tomography as a 3D atomic-scale characterization tool has seen considerable development in the past decade, both in systems improvement and theoretical understanding. However, the time and expertise required from the outset of experimentation to analyzed results is highly asymmetric between metals and nonmetals. For complex oxides, this difficulty can be exemplified by GdBa2Cu3O7-x based high-temperature superconducting coated conductors. The objective of this dissertation is to further establish the experimental and theoretical knowledge required to effectively, and compositionally, study nanoscale defects in nonmetals using atom probe tomography; specifically, those influencing the electromagnetic properties of RBa2Cu3O7-x high temperature superconducting coated conductors. The results from this dissertation can be applied to other complex oxides, nitrides, carbides, or other nonmetallic systems, through the creation and use of an extensive open-source Python package, APAV.
Date: December 2022
Creator: Smith, Jesse Daniel
System: The UNT Digital Library

Development and Thermo-Mechanical Testing of Low Hysteresis Shape Memory Alloy for Satellite Actuators

Shape memory alloys (SMAs) have gained much attention as a powerful source of actuation due to their improved performance, reduced size, and reduced complexity between components as well as having a high work output density. Their primary mechanism of actuation relies on a non-diffusional cyclic phase transformation from martensite to austenite, where the amount of thermal energy needed per cycle is directly associated with the hysteresis width between the austenite final and martensite final temperatures. Consequently, a narrower gap between those two temperature ranges requires a much lower energy demand to produce the actuation needed. Previous studies have indicated that the hysteresis width is linked to a strong coherence between the austenite/martensite interface. It has been shown that elemental additions to NiTi-based SMAs can further improve this coherency. Another huge challenge facing this unique technology is linked with its thermo-mechanical stability. Binary NiTi SMAs often exhibit significant transformation temperature shifts after each thermo-mechanical cycle, which can contribute to a shorter lifespan. The primary goal of this project is to identify and develop thermo-mechanically stable, low hysteresis shape memory alloys (LHSMAs) for actuator applications. To accomplish this goal, elemental additions of Cu, Co, Hf, and Pd were incorporated into NiTi-based SMAs …
Date: December 2022
Creator: Montagnoli, Andre Luiz
System: The UNT Digital Library

Influence of Externally Applied Magnetic Field on the Mechanical Behavior of Paramagnetic Materials

Current ways to alter the microstructure of materials are usually through heat treatments, alloying, and other physical metallurgical methods. Recent efforts in the 21st century are focused on altering the microstructure of a material without physical contact which can be achieved through exposure to a magnetic field (MF). The motivation of this research is to study the quantum effects by subjecting solid-state metals to exposure of MFs. Many of the popular metals currently used in industry are paramagnetic. The ability to alter the microstructure and thus properties of these paramagnetic materials through a magnetic field would open new avenues to the field of research, including, potentially, a pollution-free, non-contact route. The effects of a magnetic field on the mechanical properties of paramagnetic materials were observed through compression testing of the pure paramagnetic material samples induced in a magnetic field. XRD and PPMS were used to relate flow stress to dislocation density and magnetic property of the samples when exposed to the presence of a static magnetic field. The effects of a magnetic field relative to alloyed paramagnetic materials were observed through the same procedure as pure paramagnetic elements. For this purpose, -Ti alloys were chosen as literature suggests a phase …
Date: December 2022
Creator: Reeder, Jessica Phoebe
System: The UNT Digital Library

Investigation of the Processing-Induced Transition from Shape Memory to Strain Glass of Ni-Ti and Fe-Mn-Al-Cr-Ni Alloys

In this study, we observed the effects of the processing-induced method on two different shape memory alloys (SMAs). First, we compare the transformation behavior of a martensitic NiTi SMA during thermal cycling using wide angle synchrotron radiation X-ray diffraction (WAXS). Based on the thermal cycling results, three observations about processing-induced SGAs as compared to SMAs can be seen: (1) retention of distorted austenite at high and low temperatures, (2) broadening of diffraction peaks in WAXS and disappearance of the thermal peaks in DSC measurements both due to induced strain, and (3) gradual increase in the amount of the martensitic phase. Second, we applied a processing-induced method to a FeMnAlCrNi alloy to examine the possibility of forming a strain glass alloy in an Fe-based system through sufficient dislocation formation via plastic deformation. This alloy was subjected to various percentages of cold work and characterized using scanning electron microscopy, differential scanning calorimetry, Vickers hardness, WAXS data. The results indicate with 50% thickness reduction, stress-free thermal cycling no longer exhibits a measurable phase transformation, suggesting the successful formation of strain glass alloy through sufficient dislocation. The results of this research contribute significantly to the advancement of strain glass alloys (SGAs), especially with respect …
Date: December 2022
Creator: Ashmore, Bailey Nicole
System: The UNT Digital Library

Investigation of Room Temperature Sputtering and Laser Annealing of Chalcogen Rich TMDs for Opto-Electronics

Chalcogen-rich transition-metal dichalcogenide (TMD) magnetron sputtering targets were custom manufactured via ball milling and sintering in the interest of depositing p-type chalcogen-rich films. Room temperature radio frequency (RF) magnetron sputtering produced ultra-thin amorphous precursor of WSx and MoSx (where x is between 2-3) on several different substrates. The influence of working pressure on the MoS3 content of the amorphous films was explored with X-ray photoelectron spectroscopy (XPS), while the physical and chemical effects of sputtering were investigated for the WSx target itself. The amorphous precursor films with higher chalcogenide content were chosen for laser annealing, and their subsequent laser annealing induced phase transformations were investigated for the synthesis of polycrystalline 2H-phase semiconducting thin films. The role of laser fluence and the number of laser pulses during annealing on phase transformation and film mobility was determined from Raman spectroscopy and Hall effect measurement, respectively. Hall effect measurements were used to identify carrier type and track mobility between amorphous precursors and crystalline films. The p-type 2H-TMD films demonstrates the ability to produce a scalable processing criterion for quality ultra-thin TMD films on various substrates and in a method which is also compatible for flexible, stretchable, transparent, and bendable substrates.
Date: August 2022
Creator: Gellerup, Branden Spencer
System: The UNT Digital Library
Processing-Structure Relationships of Reactive Spark Plasma Sintered Diamond Composites (open access)

Processing-Structure Relationships of Reactive Spark Plasma Sintered Diamond Composites

Traditional lightweight armor ceramics such as boron carbide (B4C) and silicon carbide (SiC) are used alone or together in varying amounts to create monolithic protective plates. These materials exhibit relatively small differences in hardness, flexure strength, and fracture toughness. Many of the routes taken during the synthesis of the powder and sintering of the plates using traditional ceramic processing techniques have long processing times, tend to leave asperities within the microstructure, and have unwanted secondary phases that lower the performance of these materials. In lieu of the incremental changes in the above properties, it is thought that adding diamond particulates to the ceramic matrix will dramatically improve the mechanical properties and overall performance. With the reduced cost of synthetic diamond and the commercial development of more rapid spark plasma sintering (SPS), this work develops a novel reactive SPS process to fabricate near fully dense SiC-TiC-diamond composites at various processing temperatures with minimal graphitization and full adhesion to the ceramic matrix. It was found that samples with up to ~97% theoretical density can be fabricated with no quantifiable graphite content within the characterization ability using advanced X-ray diffraction and microscopy techniques.
Date: August 2022
Creator: Garcia, Christian
System: The UNT Digital Library

Scuffing and Wear Prevention in Low Viscosity Hydrocarbon Fuels

To design high pressure fuel system components that resist wear and scuffing failure when operated in low viscosity fuels, a comprehensive study on the tribological performance of various existing coating materials is necessary. This thesis aims to provide the relative performance of a variety of coating materials across different fuel environments by testing them in conditions that model those experienced in fuel pumps. The relative performance of these coatings are then indexed across a variety of material properties, including hardness, elastic modulus, wettability, and the interaction between the surface and the various types of fuel molecules.
Date: August 2022
Creator: Dockins, Maddox Wade
System: The UNT Digital Library
Characterization and Chemical Analysis of Fundamental Components for Lead Acid Batteries (open access)

Characterization and Chemical Analysis of Fundamental Components for Lead Acid Batteries

Although markets for alternative batteries, such as Li-ion, are growing, Pb-alloy batteries still dominate the market due to their low cost and good functionality. Even though these Pb-alloy batteries have been around since their discovery in 1859, little research involving advanced characterization techniques, such as synchrotron radiation X-ray diffraction (SR-XRD) and transmission electron diffraction (TEM) have been performed on Pb-alloys and sulfation, a failure mode in lead acid batteries, with regards to thermally- and electrochemically-induced changes at the atomic and microstructural scale. Therefore, there is a need to close this scientific gap between research and the application of Pb-alloy battery material. The main objectives of this research are to examine the process of sulfation and its growth mechanisms as well as to study the effects of minor alloying additions in Pb-alloy material. In the first case, nucleation and growth mechanisms of PbSO4 nano- and micro-particles in various solutions are examined using TEM to potentially reduce or control the buildup of PbSO4 on battery electrodes over time. The time dependency of particle morphology was observed using various reaction conditions. This insight can provide avenues to reduce unwanted buildup of PbSO4 on battery electrodes over time which can extend battery life and …
Date: May 2022
Creator: Wall, Michael T
System: The UNT Digital Library

Considerations in Designing Alloys for Laser-Powder Bed Fusion Additive Manufacturing

This work identifies alloy terminal freezing range, columnar growth, grain coarsening, liquid availability towards the terminal stage of solidification, and segregation towards boundaries as primary factors affecting the hot-cracking susceptibility of fusion-based additive manufacturing (F-BAM) processed alloys. Additionally, an integrated computational materials engineering (ICME)-based approach has been formulated to design novel Al alloys, and high entropy alloys for F-BAM processing. The ICME-based approach has led to heterogeneous nucleation-induced grain refinement, terminal eutectic solidification-enabled liquid availability, and segregation-induced coalescence of solidification boundaries during laser-powder bed fusion (L-PBF) processing. In addition to exhibiting a wide crack-free L-PBF processing window, the designed alloys exhibited microstructural heterogeneity and hierarchy (MHH), and thus could leverage the unique process dynamics of L-PBF to produce a fine-tunable MHH and mechanical behavior. Furthermore, alloy chemistry-based fine tuning of the stacking fault energy has led to transformative damage tolerant alloys. Such alloys can shield defects stemming from the stochastic powder bed in L-PBF, and consequently can prevent catastrophic failure despite the solidification defects. A modified materials systems approach that explicitly includes alloy chemistry as a means to modify the printability, properties and performance with F-BAM is also presented. Overall, this work is expected to facilitate application specific manufacture with …
Date: May 2022
Creator: Thapliyal, Saket
System: The UNT Digital Library

Corrosion Behavior of High Entropy Alloys in Molten Chloride and Molten Fluoride Salts

High entropy alloys (HEAs) or complex concentrated alloys (CCAs) represent a new paradigm in structural alloy design. Molten salt corrosion behavior was studied for single-phase HEAs such as TaTiVWZr and HfTaTiVZr, and multi-phase HEAs such as AlCoCrFeNi2.1. De-alloying with porosity formation along the exposed surface and fluxing of unstable oxides were found to be primary corrosion mechanisms. Potentiodynamic polarization study was combined with systematic mass–loss study for TaTiVWZr, HfTaTiVZr, and AlCoCrFeNi2.1 as a function of temperature. Electrochemical impedance spectroscopy (EIS) was used for monitoring the corrosion of TaTiVWZr and HfTaTiVZr in molten fluoride salt at 650 oC. TaTiVWZr and AlCoCrFeNi2.1 showed low corrosion rate in the range of 5.5-7.5 mm/year and low mass-loss in the range of 35-40 mg/cm2 in molten chloride salt at 650 oC. Both TaTiVWZr and HfTaTiVZr showed similar mass loss in the range of 31-33 mg/cm2, which was slightly higher than IN 718 (~ 28 mg/cm2) in molten fluoride salt at 650 oC. Ta-W rich dendrite region in TaTiVWZr showed higher corrosion resistance against dissolution of alloying elements in the molten salt environment. AlCoCrFeNi2.1 showed higher resistance to galvanic corrosion compared to Duplex steel 2205 in molten chloride salt environment. These results suggest the potential use …
Date: May 2022
Creator: Patel, Kunjalkumar Babubhai
System: The UNT Digital Library

Design and Performance of Metal Matrix Composite Composed of Porous Boron Carbide Created by Magnetic Field-Assisted Freeze Casting Infiltrated with Aluminum (A356)

Magnetic field-assisted freeze-casting was used to create porous B4C ceramic preforms. An optimum slurry consisted of a mixture of B4C powders with 6 wt.% Er2O3 powder in an H2O-PVA solution and was cooled at a rate of 1 °C/min from room temperature to -30 °C resulting in porous green state ceramic preform with vertical channels. The Er2O3 powder was added to improve the magnetic response of the slurry. The preform was then sublimated to remove H2O and then sintered. The sintered ceramic preform was then infiltrated in the most vertically aligned channel direction with molten Al (A356) metal through a vacuum-assisted pump to create the metal matrix composite (MMC). Finite element analysis simulations were used to analyze and predict the anisotropic effect of B4C channel alignment on mechanical properties. The mechanical properties of the composite were then experimentally found via compression testing, which was compared with rule-of-mixtures and finite element modeling simulations, to analyze the effect of anisotropy due to magnetic field-assisted freeze-casting. This study reinforces the viability of cost-effective magnetic field-assisted freeze-casting as a method to create highly directional ceramic preforms, which can be subsequently metal infiltrated to produce MMCs with highly anisotropic toughness.
Date: May 2022
Creator: Gamboa, Gerardo
System: The UNT Digital Library
Effect of Modifier Cation Substitution on Structure and Properties of Bioactive Glasses from Molecular Dynamics Simulations (open access)

Effect of Modifier Cation Substitution on Structure and Properties of Bioactive Glasses from Molecular Dynamics Simulations

Bioactive glass is a type of third generation bioactive material that can bond to both soft and hard tissue with applications ranging from bone defect repair, coatings for metallic implants, to scaffolds for tissue engineering. Design of bioactive glasses for these applications rely on a detailed understanding of the structures of these glasses which are complicated and multicomponent. In this thesis, I have applied molecular dynamics (MD) simulations with interatomic potentials developed in our group to understand the effect of modifier cation substitution on the structures and properties of two series of bioactive glasses. Particularly, MD simulations are used to understand K2O to Na2O and MgO to CaO substitution on the short and medium range structures (such as cation coordination number, pair distribution function, Qn distribution, and ring size distribution) and properties (such as bulk and Young's moduli and CTE) of 55S4.1 bioactive glasses. As Na2O is incrementally substituted with K2O in 55S4.1, a decrease of the glass transition temperature (Tg) and an increase of CTE was observed, as well as a decreasing trend in the moduli. For the MgO to CaO substitution series, Mg2+ is mainly four-fold coordinated that suggests that it can play a role as a network …
Date: May 2022
Creator: Vu, Myra
System: The UNT Digital Library
Fabrication of the Novel Asymmetric Polymeric Materials via Bottom-Up Approach (open access)

Fabrication of the Novel Asymmetric Polymeric Materials via Bottom-Up Approach

Asymmetric polymeric materials can be formed by either top-down or bottom-up methods. Bottom-up methods involve assembling the atoms and molecules to form small nanostructures by carefully controlled synthesis, which results in a reduction of some of the top-down limitations. In this dissertation, thermal, tribological and antireflective properties of polymeric materials have been enhanced by introducing structural asymmetry. The overall performance of commercial polymeric coatings, e.g. epoxy and polyvinyl chloride, has been improved by conducting the blending methods, specifically, chemical modification (α,ω-dihydroxydimethyl(methyl-vinyl)oligoorganosiloxane), cross-linking (triallyl isocyanurate), and antioxidant (tris(nonylphenyl) phosphite) incorporation. The nonequilibrium polymeric structures (moth-eye and square array) have been developed for the ultrahigh molecular weight block copolymers via the short-term solvent vapor annealing self-assembly. The large domain size of the moth eye structure allows for improvement of the light transmittance particularly in the visible and near infrared ranges, while the square arrangement of the block copolymer opens the possibility of magnetic data storage application by the large magnetic nanoparticles' embedment or masking of the superconductors.
Date: May 2022
Creator: Hnatchuk, Nataliia
System: The UNT Digital Library