九色导航

Thesis Defense- 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 engineering metals of recent scientific interest. HEAs usually consist of five or more alloying components in roughly equiatomic proportions that can form single-phase or dual-phase solid solutions 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 material properties. The properties can include better ductility and strength relationship, lighter and stronger, higher oxidation resistance, superior resistance to corrosion, and better fracture resistance when compared to traditional alloys. Because of these qualities, HEAs are candidate materials to replace traditional alloys as structural engineering materials.

However, before HEAs can replace traditional alloys, their mechanical properties including thermodynamic and elastic properties must be understood. In industrial applications, the understanding of thermodynamic and elastic properties of refractory alloys are crucial because their utility depends on these properties. The application of refractory alloys depends on their resistance to different processes such as high temperature and high oxidation. This means that in their application, they must have low thermal expansion and high strength at elevated temperatures. Thermal expansion and strength at elevated temperatures are part of their thermodynamic and elastic properties in general. So to understand how these alloys will react under these types of conditions, these properties must be well understood. This thesis explores thermodynamic and elastic properties of BCC 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, elastic constants, bulk modulus,  shear modulus, Young鈥檚 modulus, and Poisson鈥檚 ratio are calculated as a function of temperature and discussed in this thesis. 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 three quarternary and three quinary systems, NbTaTiZr, NbTaTiV, AlMoNbV, HfNbTaTiZr, MoNbTaVW, 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 calculated 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 to validate the methodology of the thermodynamic modeling. Elastic properties such as elastic constants, bulk, shear and Young鈥檚 moduli for pure metals are also in close agreement with literature values. After method validation on pure metals this work then applies the modeling process to more complicated systems such as RHEAs and is further validated on AlMoNbV. For the first time, the finite temperature thermodynamic properties of all 24 atomic configuration permutations of a quaternary RHEA are calculated. At most, 1.7% difference is found between the resulting properties as a function of atomic configuration, indicating that the atomic configuration of the SQS has little effect on the calculated thermodynamic properties. The behavior of thermodynamic properties among the RHEAs studied is discussed based on valence electron concentration and atomic size. Among the quaternary RHEAs studied, namely AlMoNbV, NbTaTiZr, and NbTaTiV, it is found that the presence of Zr contributes to higher entropy. Additionally, at lower temperatures, Zr contributes to higher heat capacity and thermal expansion compared to the alloys without Zr, possibly due to its valence electron concentration. At higher temperatures, Al contributes to higher heat capacity and thermal expansion, possibly due its ductility. Among the quinary systems, the presence of Mo, W, and/or V causes the RHEA to have a lower thermal expansion than the other systems studied. When comparing the systems with the NbTaTi core, the addition of Al increases thermal expansion, while the removal of Zr lowers the thermal expansion. Enthalpy, entropy and thermal expansion of all six RHEA systems were calculated and results discussed.

The elastic properties of all 24 atomic configurations of AlMoNbV were calculated and it was found that elastic constants had a standard deviation of up to 6.4%, with elastic constant C44 being the highest, while C11 and C12 varied by 1.85% and 2.28% respectively. When comparing 1-, 40, 50, and 64-atom cell sizes, elastic constants varied by at most 1.2% different, showing that cell size, shape, and orthogonality does not vary the results of the elastic properties significantly. MoNbTaVW, out of all six RHEAs studied had the highest bulk, shear and Young鈥檚 moduli, and lowest thermal expansion, giving promise to its use and utility in refractory settings. This may be due to its high valence electron concentration. Other results and trends are discussed thoroughly in this thesis. This thesis offers novel insights for cutting down computational time and resources while calculating properties of systems that have not previously been published before.

Keywords: High-Entropy Alloys, Refractory, BCC Body-Centered Cubic, Density Functional Theory, Elastic Properties, Thermodynamic Properties

PhD Proposal Defense- Advisor: David Burleigh

First-Principles Calculations of

Intrinsic and Extrinsic Point Defects in AlGaN

Jones A 101

Abstract

Aluminum Gallium Nitrogen (AlGaN) is a wide band gap semiconductor alloy which is useful in producing AlGaN/GaN high electron mobility transistors (HEMTs). In semiconductors, point defects produce levels which can either occur in the band gap or near the band levels and affect the performance of HEMTs. Understanding these defects is therefore necessary for understanding the properties of AlGaN/GaN HEMTs under application. Here, a comprehensive density functional theory (DFT) analysis of a wide range of intrinsic and extrinsic defects was performed in zinc blende (cubic) AlN and AlGaN using the local moment counter charge method. An attempt is made at creating an effective defect band gap using the defect band gaps for AlN and AlGaN as was done previously for GaN.

Keywords: AlGaN, cubic, defect levels, charged defects, LMCC, Jahn-Teller distortions, transition metals

Thesis Defense- 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 156

Abstract

Previously, it had been very challenging to send mid-infrared light into liquid helium temperatures and magnetic fields higher than 8T, and we have designed a system to deliver more power with more accuracy than previous solutions and used it to detect Floquet-Bloch features in a continuous wave system. To detect Floquet-Bloch features in a graphene sample, a system needed to be designed that is capable of delivering high power mid infrared light in both linear and circular polarization to our sample in a 18T, 1K environment at the bottom of a cryostat. Using a series of proprietary fibers from Guiding Photonics, we were able to deliver the mid infrared light down into the cryostat, and used a specially designed alignment system which would hold its position from the bench down to the cryostat in order to ensure that the laser light would be delivered to the sample. We were able to demonstrate band modification in graphene鈥檚 band structure, opening the gate for further experiments into Floquet-Bloch band modification. Additionally, this system would be able to be used for numerous other experiments which were previously unattainable due to an inability to deliver mid infrared light down to a sample in a cryostat environment.

Keywords: Floquet-Bloch; Magnetic Field; Graphene; Circular and Linear Polarization; Mid Infrared   

 

Thesis Proposal- Advisor T. David Burleigh 

CORROSION RESISTANCE OF 316L STAINLESS STEEL IN HCl

Jones Annex 104

Abstract

Containers made from annealed 316L austenitic stainless steel materials are highly susceptible to corrosion attack when exposed to hazardous radioactive laboratory wastes. Several byproducts such as hydrochloric acid are usually formed as a result of degradation by radiolysis of the PVC wastes stored in the containers. The question is whether subsequent release of these byproducts that condensed around the internal wall surface will significantly lead to corrosion attack of the 316L stainless steel containers by reducing the wall thickness and with time reduces the expected service life.

The main objective of this work is to determine the design lifetime of containers made of 316 low carbon austenitic stainless steel materials when exposed to vapor of HCl or immersed in varying concentrations of hydrochloric acid and FeCl₃ solution using different test methods such as weight loss, potentiodynamic polarization and electrochemical impedance spectroscopy techniques.

Keywords: Corrosion, Stainless steel, 316L, Radiolysis, HCl 鈥�