Thesis Proposal- Advisor: Chelsey Hargather

First-Principles Study into Vacancy Formation Energy and the Migration Barrier of High-Entropy Alloys

Jones Hall 227

Abstract:
Diffusion is the main process of mass transfer within a material. Diffusion directly effects phase transformations, mechanical properties, and failure mechanisms of the material. Many materials scientists and others in the engineering community have been studying the diffusion of metals and their alloys. Common methods for experimentally calculating diffusion coefficients in an HEA is costly and time consuming. This sparked the need for a better way to compute diffusion coefficients within a metal alloy, leading to the application of computational techniques which use first-principles calculations based on
density functional theory. The calculations of diffusion related properties in metals systems with an impurity are well established, but research is lacking for the calculation of diffusion coefficients in more complicated systems. The next step in diffusion modeling using first-principles techniques is to use the established techniques for new ternary medium-entropy alloys (MEAs) or quinary high-entropy alloys (HEAs). The goal of this thesis is to calculate properties that contribute to diffusion coefficients and activation energy in fcc MEAs/HEAs. The diffusion properties include chemical potential, vacancy formation energy (VFE) and the diffusion migration barrier. The diffusion properties are investigated for each element: Co, Cr, and Ni, in the ternary CoCrNi MEA. calculating diffusion-related properties in a MEA or HEA is difficult due to the random solid-solution nature of the MEAs and HEAs. In random alloys, the local environment impacts the properties of a diffusing atom. To find the diffusion properties of a MEA or HEA, vacancy formation energy of each type of atom in the system and the migration energy of each type of atom in the system must be calculated.
VFE refers to the amount of energy required to remove an atom from its location within the lattice structure of a material. The migration barrier is the amount of energy that an individual atom needs to squeeze through the lattice structure into a neighboring vacancy. In the present work, a method for finding the VFE and the migration barrier in CoCrNi is presented with considerations to effects from the varying local environment. The diffusion properties of the CoCrNi system are calculated using magnetic and nonmagnetic calculation settings. Additionally, calculations are performed with the both generalized gradient approximation exchange-correlation functional as implemented by Perdew, Burke, and Ernzerhof (PBE) and the adapted variation applied for solids (PBEsol). Four calculation settings are used for the diffusion calculations, and the efficacy of each in terms of accuracy of the calculated property and time for obtaining result is discussed. The settings are situated so that each system is calculated twice per exchange-corrleation functional approximation, one uses magnetic inputs and the other excludes magnetism. In the present work, a method for finding the VFE in a high-entropy alloy is presented which considers effects from the varying local environment. To justify the methods of this work, the calculations are performed on a commonly studied ternary system, CoCrNi. The exploration into this system will includes discussion on why the magnetic systems are more practical than the nonmagnetic when using the PBE approximation yet the opposite is true using the PBEsol approximation. The energy values calculated in this work are computed for each type of atom, then compared their corresponding pure system energies and other computational calculations for ternary systems and quinary systems. The pure systems use a 36 atom or 72 atom supercell, the ternary system use a 72 atom supercell, while the quinary systems use a 100 atom supercell for their respective calculations. The VFE and chemical potential show that the PBE
approximation used in this work compares closely with the reported values in literature. We see throughout this work that the magnetic systems using the PBE approximation are shown to produce values that are most in line with the literature and the theory. The non-magnetic systems using PBE approximation show to produce values that are smaller than the pure values reported by literature. The
calculations involve the PBEsol approximations concluded to similar value between magnetic and nonmagnetic
systems. This calculated energy for VFE and chemical potential were shown to be larger than all other calculated values by at least 0.5 eV/atom, meaning that the non-magnetic calculations are more time efficient with minimal loss in accuracy than its magnetic counterparts.

Thesis Proposal- Advisor: Bhaskar Majumdar

ALLOY DESIGN AND PROCESS DEVELOPMENT FOR

ADDITIVE MANUFACTURING OF NICKEL-BASE SUPER ALLOYS

Jones Hall 159

Abstract

Additive manufacturing (AM), also known as 3D printing, is a rapidly growing manufacturing technology. It allows the production of complex geometries and near-net shapes, which can be difficult or impossible to achieve using traditional manufacturing methods. In the case of metals, the process involves building up a three-dimensional object layer-by-layer using a computer-controlled laser or electron beam to melt and fuse metal powder or wire. Key challenges of this process are the extremely rapid solidification of small melt pools in high temperature-gradient fields, residual stresses imposed by the surroundings, and others. Consequently, there are many conventional alloys that are difficult to fabricate using AM because they create various defects such as cracks.

This research focuses on high temperature Ni-base alloys that are well-known in the aerospace gas turbine industry. Among these alloys, IN718 is an excellent weldable alloy and very suitable for AM. However, the alloy loses strength beyond 650-700C. For higher temperature capability, the traditional alloys suffer from severe microcracking during AM and some solutions have been formulated using lower volume fraction g/g' alloys. This research considers high volume fraction (> 60 vol.%) g/g' strengthened CM247LC as a particular example, and addresses approaches that may mitigate the microcracking problem. The methodology involves- (i) CALculation of PHAse Diagram (CALPHAD) and both equilibrium and Scheil non-equilibrium analyses to understand the basic solidification issues, (ii) utilizes hot-tear models (Kou model) in the welding literature to predict solidification cracking in an alloy, and most importantly, (iii) investigate the microstructures leading to microcracking. Based on such mechanism understanding, minor alloy changes are formulated for microcrack mitigation. Partial validation of the alloy design approach is done using laser line scans of arc-melted buttons and assessing the microstructure and density of cracks. This approach is used and initially validated, in place of procuring powders of different compositions, in order to reduce the time and enormous cost of custom-made powders of multiple compositions. The results show that this approach is indeed successful in arriving at an alloy composition around the base CM247LC alloy, where cracking is completely eliminated for the first time in this high volume-fraction L1₂ phase strengthened alloy. Modeling and microstructural observations provide initial understanding, but further research is needed to establish the mechanisms. Future work will focus on procuring alloy powders of this composition to validate actual applications in additive manufacturing and address mechanical properties and microstructural stability at high temperatures.

Keywords: Additive Manufacturing; Ni-base superalloys; CM247LC; microcracking; process parameter.

PhD Defense- Advisor: David Burleigh

Development of Neutron Diffraction Texture Analysis Techniques for the Study of TATB Based Explosives

Jones Hall 155

Abstract

Explosives based on 1,3,5-triamino-2,4,6-trinitrobezene, (TATB), are known for exhibiting good explosive performance and low sensitivity to thermomechanical insult. These materials exhibit anisotropic, irreversible expansion during thermal cycling known as “ratchet growth”. This phenomenon is not well understood, and several mechanisms have been proposed in the literature. These theories attribute ratchet growth to the anisotropic microstructural properties of TATB and are difficult to validate experimentally. In this dissertation, the means to investigate the crystallographic properties of TATB, including crystallographic texture, with diffraction techniques are developed. A methodology for integration of experimental and software coordinate systems for the HIPPO neutron diffractometer was developed using a single crystal quartz sample. Automation of diffraction analysis necessary for performing custom diffraction analysis is also developed and demonstrated using centrifugally cast stainless steel samples. Finally, neutron diffraction on TATB based explosives is conducted. Simultaneous measurement of lattice and bulk strains within explosives samples are performed and the implications of texture and strain measurement on the ratchet growth mechanism are discussed.

Thesis Proposasl- Advisor: Nikolai Kalugin

Delivering Circularly Polarized Mid Infrared Light to Liquid Helium Temperatures and High Magnetic Fields for Generation of Floquet-Bloch States in 2D Materials

Jones Hall 155

Abstract

Floquet-Bloch experiments have been performed using pulsed laser systems, but we have managed to get Floquet-Bloch measurements from a continuous wave system. In this paper we will discuss the system used, how we improved the system between experiments, and what material restrictions we faced when designing our system. Using a metallized fiber, we were able to couple mid-infrared light and deliver it down to our sample to get a power of ~100 mW at the sample. After making improvements, we were able to get this power up to 2 mW / . Our experiments focused around transport measurements, but the continuous wave system still could be used to probe the band structure in order to provide a better picture of the Floquet-Bloch band structure transformation.

PhD Defense- Advisor: John McCoy

Environmentally Enhanced Aging of Epoxy Bisphenol A Diglycidyl Ether (DGEBA) cured with Diethanolamine (DEA): Hydrothermal and Uniaxially Stressed

Weir 132

Zoom Meeting: https://zoom.us/j/98394476524

Abstract

A variety of time-dependent tests were performed on the epoxy DGEBA/DEA in order to explore environmentally enhanced aging. These tests were of two broad types: compressive creep/recovery and hydrothermal aging. The creep/recovery tests were performed over a range of temperatures and load/unload protocols. At the end of each test, the samples were typically loaded through yield. The hydrothermal aging investigated the effect of water absorption on DSC thermograms where both absorbed mass and the enthalpy of desorption were measured and correlated. Creep/recovery testing procedures ranged from simple creep tests to complicated load/unload cyclic tests. The complexity of the tests increased as the project advanced in order to probe more nuanced system response. Tests involved: 1) creep for an extended time followed by a rapid unload to zero stress and a reload through yield; 2) creep followed by an extended recovery; 3) multiple creep/recovery cycles with various creep and recovery times; 4) an initial creep with a large "preconditioning" load followed by an extended recovery and then multiple small stress creep/recovery probe cycles.
These test procedures were motivated by a number of theoretical predictions. First was the Eyring model linking strain rate to the applied stress. This theory turns out to be widely applicable to many of the test types performed in this study permitting prediction of the Eyring activation volume. The activation volume is found to decrease during compressive creep and to be approximately 0.5 nm3 after a few hours at temperatures ~10°C below Tg. Second was Boltzmann superposition which predicts the system response to complex load/unload cycles based on the response to a single extended creep test. This is found to give correct qualitative predictions, but is only quantitative for extremely small stresses where experimental noise makes comparison difficult. Third were simple spring-dashpot models (i.e., Maxwell and Voigt-Kelvin models) which justified the Nutting model of recovery and permitted systematic analysis of storage and loss in cyclic tests. Fourth was the Doolittle equation linking characteristic rates or timescales with free volume.
A major portion of the creep/recovery tests followed the "preconditioning" protocol mentioned above where an initial extended creep period (potentially under high stress) is followed by a long recovery and then a series of small load/unload cycles. Principle quantities used in analysis were 1) the energy storage and loss during load/creep/unload, and 2) the strain during the recovery. The storage/loss analysis was conveniently analyzed with a Maxwell Model, and the recovery, with a Voigt-Kelvin model with a shear thinning dashpot fluid. Incorporating the effect of the initial loading stress using the Eyring model into the storage/loss analysis predicts an activation volume in good agreement with values found through other types of tests.
Hydrothermal aging was explored with the specific intent of understanding the signature of sorbed water in DSC thermograms. Samples were soaked for varying times with mass measured periodically. Thermograms showed clear evidence of the presence of water with the heat of desorption correlating with mass absorbed. Surprisingly, samples after multiyear aging under laboratory conditions (room temperature and humidity) showed pronounced evidence of sorbed moisture.

Keywords: Epoxy; Physical Aging; Hydrothermal Aging; Creep; Recovery

 

 

Thesis Proposal- Advisor: Bhaskar Majumdar

ALLOY DESIGN AND PROCESS DEVELOPMENT FOR ADDITIVE MANUFACTURING OF Ni-BASE SUPER ALLOYS

Jones Hall 106

Abstract:

Additive manufacturing (AM), also known as 3D printing, is a rapidly growing manufacturing technology. It allows the production of complex geometries and near-net shapes, which can be difficult or impossible to achieve using traditional manufacturing methods. In the case of metals, the process involves building up a three-dimensional object layer-by-layer using a computer-controlled laser or electron beam to melt and fuse metal powder or wire. Key problems of this process are extremely rapid solidification of small melt pools in high temperature-gradient fields, residual stresses imposed by the surroundings, and others. Consequently, there are many conventional alloys that are difficult to fabricate using AM because they create various defects such as cracks.

This research focuses on high temperature Ni-base alloys that are well known in the aerospace gas turbine industry. Among these alloys, IN718 is an excellent weldable alloy and very suitable for AM. However, the alloy loses strength beyond 650-700^⸰C. For higher temperature capability, the traditional alloys suffer from severe microcracking during AM and some solutions have been formulated using lower volume fraction /' alloys. This research considers high volume fraction (> 60 vol.%) /' strengthened CM247LC as a particular example, and addresses approaches that may mitigate the microcracking problem. The methodology involves: (i) Calculation of Phase Diagram (CALPHAD) and both equilibrium and Scheil non-equilibrium analyses to understand the basic solidification issues, (ii) utilizes hot-tear models (Kou model) in the welding literature to predict solidification cracking in an alloy, and most importantly, (iii) investigate the microstructures leading to microcracking. Based on such mechanism understanding, minor alloy changes are formulated for microcrack mitigation. Partial validation of the alloy design approach is done using laser line scans of arc-melted buttons, and assessing the microstructure and density of cracks. This approach is used and initially validated, in place of procuring powders of different composition, in order to reduce the time and enormous cost of custom-made powders of multiple compositions. Preliminary work has been successful in formulating one to two compositions that may mitigate microcracks. Future work will focus of actual AM of procured powders of the selected composition, and establishing a processing window within which microcrack-free material may be manufactured.

 

Keywords: Additive Manufacturing, Ni-base superalloys, CM247LC, microcracking, process parameter.

Thesis Proposal- Advisor: John McCoy

Limitations of Maleimide Resins to Produce Diels-Alder Networks

Jones Hall 225

Zoom

(https://zoom.us/j/96048238603,

ID: 960 4823 8603)

Abstract:

The primary goal of the present work was to study the nature and effects of maleimide homopolymerization on thermosets crosslinked using Diels-Alder chemistry, a popularly studied dissociative chemistry among covalent adaptable networks (CANs). Dominate at relatively mild temperatures (20-70°C), the forward Diels-Alder (fDA) reaction proceeds as a [4+2] cycloaddition between a diene (furans) and dienophile (maleimides) to produce cycloadducts that undergo retro-DA (rDA) reaction at elevated temperatures to regenerate the precursors. Maleimide homopolymerization becomes observable at temperatures in excess of 110°C, which presents a challenge since the rDA reaction typically occurs around the same temperature range where it is feasible to (re-)process these DA materials. Several furan-maleimide thermosets were synthesized and studied to better understand the effect of and enumerate strategies to reduce the extent of network stiffening associated with the maleimide homopolymerization. The self-polymerization produces succinimide moieties that significantly change the thermomechanical behavior and preclude further flowability. This has implications for similar systems using maleimides because these systems are often studied for their novel intrinsic self-healing, recyclability, and 3D-printable qualities. Various strategies can be undertaken to reduce the effect of maleimide homopolymerization including the use of reduced maleimide content, using a free-radical inhibitor such as hydroquinone, and using a longer, more flexible prepolymer backbone. The preliminary work completed thus far was done using rheological and differential scanning calorimetry (DSC) methods. Additional work included determination of the kinetic parameters of the fDA and rDA reactions using gel criteria.

 

Thesis Proposal- Advisor: Chelsey Hargather

Ab Initio Calculations of Elastic and Thermodynamic Properties of High Entropy Alloys and Their Alloying Components

Jones Hall 227

Abstract:

High-entropy alloys (HEAs) are a class of alloyed metals of recent scientific interest. HEAs consist of five or more alloying components in roughly equiatomic proportions that can form a single phase such as face-centered cubic (FCC), body-centered cubic (BCC), or hexagonal close packed (HCP). They take on the name high entropy because they exhibit unusually high entropy of mixing. The reason for the interest in HEAs in the scientific community is that these alloys can display promising combinations of materials properties. The properties can include better ductility and strength relationship , lighter but stronger qualities , higher temperature resistance , improved oxidation resistance , more resistance to corrosion , and superior fracture resistance , among others. Because of these qualities, HEAS are candidate materials to replace traditional alloys in the metallurgical industry. However, before HEAs can replace traditional alloys, their mechanical properties including thermodynamic and elastic properties must be understood. This thesis proposal explores elastic and thermodynamic properties of body-centered cubic refractory high-entropy alloys through density functional theory computation and modeling. By using Vienna Ab initio Simulation Package (VASP), thermodynamic and elastic properties such as Debye temperature, Helmholtz Free energy, entropy, enthalpy, heat capacity, thermal expansion, bulk modulus, shear modulus, Young’s modulus, and Poisson’s ratio are calculated as a function of temperature and discussed in this thesis proposal. First, the computational method is compared to well-known experimental values of pure systems Ag, Ni, Al,Ta, Ti, and V for procedural validation and then used on BCC refractory HEAs including NbTaTiZr, HfNbTaTiZr, MoNbTaVW, NbTaTiV, and AlNbTaTiV with 40 and 60 atom cells for the quarternary systems, and 50 and 75 atom cells for the quinary systems.
This work has modeled the entropy and enthalpy of the pure metals, Al and Nb to be in close agreement with NIST JANAF reported experimental values. This was done for validating the methodology of the thermodynamic modeling. Elastic properties such as elastic constants, bulk, shear and Young’s moduli for pure metals are also in close agreement with literature values. For example, the bulk modulus of Nb was found to be 170 GPa, in good agreement with a computational value of 172.3 GPa. The elastic constant C11 was calculated to be 119 compared to reported literature value of 116. After method validation this work applies the modeling process to more complicated systems such as RHEAs. Specifically, enthalpy and entropy properties of HEA systems AlNbTaTiV and MoNbTaVW were calculated and results discussed.

Thesis Proposal- Advisor: Paul Fuierer

DRY AEROSOL DEPOSITION OF MICROWAVE DIELECTRICS AND METALLIZATIONS

Jones Hall 106

Abstract:

Microwaves plays a huge role in modern communication. Microwave dielectric ceramics (MWDCs) used for modern microwave communication components such as resonators, waveguides, and antennas, require tunable dielectric constant (K) and high quality factor (Q).

Additive manufacturing (AM) is attractive to build passive components, filters, antennas, resonators, and custom circuits, but current technology and materials (with a range of dielectric constant, K, values) are limited. Dry aerosol deposition (DAD) can be considered as a fully AM approach. It is a novel kinetic spray process that can make fully dense, ultrafine grain ceramic coatings at room temperature. DAD offers densification of ceramics films on a variety of low-melting substrates at room temperature. The DAD process can produce robust thick films and low-profile 3D dielectric ceramic structures, potentially at a low cost. This project aims to prove DAD as a viable AM process for producing embedded low-profile 3D dielectric structures and to offer a new technology option for AM of high-K materials.

Preliminary work has been done with a MWDC called BaNd2Ti4O12 (BNT), a complex perovskite with a high-K, high-Q, and low temperature coefficient of resonant frequency (τf). Optimized process parameters were determined for a commercial, samarium-doped BNSmT powder through multiple experiments. DAD films were inspected for thickness, roughness, overall quality, and adhesion. BNSmT powder was sprayed on bulk copper and FR4 substrates. Satisfactory process parameters for DAD copper feedstock powder were also determined. BNSmT films were electroded with DAD copper for dielectric measurements. It has been demonstrated that co-manufacturing of dense MWDC along with copper metallization can be done in the same DAD apparatus.

Thesis Proposal- Advisor: Chelsey Hargather

First-Principles Study into High-Entropy Alloys and Forming Vacancies

Jones Hall 227

Abstract:

Diffusion is the main process of mass transfer within a material. Diffusion directly effects phase transformations, mechanical properties, and failure mechanisms of the material. Many materials scientists and others in the engineering community have been studying the diffusion within metals and their alloys. Common methods for experimentally calculating diffusion coefficients in an HEA is costly and time consuming. This sparked the need for a better way to compute diffusion coefficients within a metal alloy, leading to the application of computational techniques which use first-principles techniques. The calculations of diffusion related properties in metals systems with an impurity are well established, but research is lacking for the calculation of diffusion coefficients in more complicated systems. The next step in diffusion modeling using first-principles techniques is to use the established techniques on new ternary medium-entropy alloys (MEAs) or quinary high-entropy alloys (HEAs). MEAs are similar to HEAs with configurational entropy reduced to between R and 1.5*R where R is the gas constant. These MEAs are comprised of ternary systems and are used as initial information about the atoms to describe how they behave with increased configurational entropy. The goal of this thesis is to calculate values that contribute to diffusion coefficients in fcc MEAs and HEAs. Calculating diffusion-related properties in a MEA or HEA is difficult due to its random solid-solution properties. In random alloys, the local environment impacts the properties of a diffusing atom. To find the diffusion properties of a MEA or HEA, input values needed are, vacancy formation energy of each type of atom in the system and the migration energy of each type of atom in the system, among other variables. Vacancy formation energy (VFE) refers to the amount of energy required to remove an atom from its location within the lattice structure of a material. The VFE differs for each type of atom in a system depending on the concentration of each element and its local environment in the alloy. Calculating VFE is the focus of this thesis. In the present work, a method for finding the VFE in a high-entropy alloy is presented which takes into account effects from the varying local environment. To justify the methods of this work, the calculations are performed on a commonly studied ternary system, CoCrNi. After justifying the methods using a ternary system, the same methods are applied to a quinary system, CoCrFeMnNi. The exploration into these systems will include an analysis into the effects of magnetism within the ground state energy of these system by calculating the vacancy formation energy with and without magnetic inputs. These calculations were performed using the projector augmented wave (PAW) pseudo-potentials and the generalized gradient approximation exchange-correlation functional that is applied for solids as implemented by Perdew, Burke, and Ernzerhof. The energy values calculated in this work are computed for each type of atom, then compared their corresponding pure system energies and other computational calculations for ternary systems and quinary systems. The pure systems use a 36 atom supercell, the ternary system use a 72 atom supercell, while the quinary systems use a 125 atom supercell for their respective calculations. Future work beyond this thesis proposal will apply the VFE method to other, less-studied quinary systems such as CoCrCuFeNi. Calculating vacancy migration energy may also be explored and the importance for reporting differences between magnetic and non-magnetic systems will be determined.

PhD Defense- Advisor: Matthew Herman

Advanced Formulation of Highly Loaded Composites via Biomimetic Interfacial Reinforcement

Speare 15

Abstract:

Technological advancement is often inspired by nature, promoting scientists and engineers to continually attempt to develop new material systems based on materials found in nature. To strongly bind themselves to a variety of marine surfaces, mussels produce a strong adhesive protein that is high in dopamine chemical unit concentration. Dopamine is rich in catechol groups at the interface, which act as adhesion promoters. Synthetic dopamine, capable of undergoing self-polymerization under ambient conditions and becoming polydopamine (PDA), has been demonstrated to form controllable nanometer thickness films which are capable of promoting the adhesion between the filler and binder system in a highly loaded composite. The improvement of mechanical properties by promoting interfacial adhesion, as well as a uniform surface film to promote our formulation efforts, are of particular interest to our team. This work includes the investigation of PDA as it pertains to our experiments on tailoring crystal-binder adhesion properties in both plastic-bonded explosives and high-fidelity surrogates. Neutron reflectometry data will be presented to demonstrate the controllable nature of PDA film growth and the film’s structure. The effects of particle size and binder selection were studied in order to baseline our experiments to determine how much impact these variables can have and how they impact the final composite performance. 

Paper Critique- Advisor: Michaelann Tatis

Biomechanical characterization of ex vivo human brain using ultrasound
shear wave spectroscopy

JONES HALL 225

Abstract

The characterization of brain tissue is crucial to better understand neurological disorders. Mechanical characterization is an emerging tool in that field. The purpose of this work was to validate a transient ultrasound technique aimed at measuring dispersion of mechanical parameters of the brain tissue. The first part of this work was dedicated to the validation of that technique by comparing it with two proven rheology methods: a rotating plate rheometer, and a viscoelastic spectroscopy apparatus. Experiments were done on tissue mimicking gels. Results were compared on storage and loss modulus in the 20–100 Hz band. Our method was validated for the measurement of storage modulus dispersion, with some reserves on the measurement of loss modulus. The second part of this work was the measurement of the mechanical characteristics of ex vivo human white matter. We were able to measure the dispersion of the storage and loss modulus in the 20–100 Hz band, fitting the data with a custom power law model.

Article Reference:
E. Nicolas, S. Call ́e, S. Nicolle, D. Mitton, and J. P. Remenieras, “Biomechanical
characterization of ex vivo human brain using ultrasound shear wave spectroscopy,” Ultrasonics, vol. 84, pp. 119–125, 2018