Degree Type

Dissertation

Date of Award

2018

Degree Name

Doctor of Philosophy

Department

Mechanical Engineering

Major

Mechanical Engineering

First Advisor

Ganesh Balasubramanian

Second Advisor

Duane D. Johnson

Abstract

Computational investigations of structural, chemical, and deformation behavior in high entropy alloys (HEAs), which possess notable mechanical strength, have been limited due to the absence of applicable force fields. First by employing Lennard-Jones (LJ) type potential, we explore the atomic origins of the structural phase transformations (PTs) in AlxCrCoFeNi multi-principal element alloys (MPEAs) using classical molecular dynamics (MD) simulations. We report that the amorphous phase exists above the melting temperatures of the participating elements when the concentration of Al 20 %, while for Al > 20 %, a gradual molten-amorphous phase transition is noted. To extend investigations for mechanical properties, we propose a set of intermolecular potential parameters for a quinary Al-Cr-Co-Fe-Ni

alloy, using the available ternary Embedded Atom Method and Lennard-Jones potential in classical molecular-dynamics simulations. The simulation results are validated by a comparison to first-principles Korringa-Kohn-Rostoker (KKR) - Coherent Potential Approximation

(CPA) [KKR-CPA] calculations for the HEA structural properties (lattice constants and bulk moduli), relative stability, pair probabilities and high-temperature short-range ordering. The simulation (MD)-derived properties are in quantitative agreement with KKR-CPA calculations

(first-principles) and experiments. We study AlxCrCoFeNi for Al ranging from 0 $leq$ x $leq$ 2 mole fraction, and and that the HEA shows large chemical clustering over a wide temperature range for x < 0.5. At various temperatures high-strain compression promotes atomistic rearrangements in Al0:1CrCoFeNi, resulting in a clustering-to-ordering transition that is absent for tensile loading. Large fluctuations under stress, at higher temperatures, are attributed to the thermo-plastic instability in Al0:1CrCoFeNi. With the proposed EAM-LJ potential parameters, we then analyze the dislocation dynamics in the FCC Al0:1CrCoFeNi HEA. During plastic deformation, we find that dislocation nucleation and mobility plays a pivotal role in initially triggering twin boundaries followed by the generation of intrinsic and extrinsic stacking

faults in the alloy. At room temperature, we find dislocation annihilation contributes to the shear resistance of the alloy eecting a serration laden plastic ow of stress as uniaxial strain is increased. Designing advanced materials for high-temperature applications is a challenging

problem. Traditionally superalloys, especially Ni-based, have been the to-go materials whenever strength, and oxidation/corrosion resistance is required in applications ranging across gas turbines to engines. Refractory elements have a high melting point and are ideal candidates to design refractory based MPEAs. We utilized classical Hume-Rothery rules, like Valence Electron Concentration (VEC), size-effect , alongside density functional theory predicted global stability parameter like formation enthalpy and thermodynamic linear response predicted local stability criteria like short-range order (SRO) to explore the 5D design space in a holistic manner for a quinary refractory MPEA. Our investigation revealed the specific design regimes ideal for enhanced electronic stability alongside desired mechanical strength for a novel refractory alloy series based on Mo-W-Ta-Ti-Zr. Findings suggest that the elastic strength of predicted alloy, composition having greater content of Mo and W (at. %), surpasses current commercial high-temperature alloy (Ti-Zr-Mo). Transport properties of refractory MPEAs are also investigated as a potential new class of high-performance thermoelectric material. Investigations in XTa-MoW MPEAs (X=Ti, V, Nb, Zr) revealed dispersion effects and critical doping concentration that helps in tuning the figure-of-merit (ZT) by a factor of 6 at 1250 K. Further investigations in refractory MPEAs revealed twinning-induced pseudoelasticity in (MoW)0:85(TaTi)7:5Zr7:5. Atomistic insights through structural analysis and role of temperature was carried out in this work. Materials for actuators and bio-medical stents are some of the possible application areas for the predicted pseudoelastic MPEA. We design a robust computational framework that couples the metaheuristic cuckoo search technique with classical molecular dynamics simulations to explore the structure-composition phase space of multicomponent alloys. Thereby the predictive scheme explores a vast materials landscape and accelerates the elemental selection for discovery of novel multicomponent alloys. Structural design of MPEAs is generally based on brute-force Monte-Carlo (MC) techniques which are often computationally demanding and less reliable. We proposed a novel exploratory technique to numerically design initial lattice structures for MPEAs for quantum or atomistic calculations that maintains desired cubic symmetry, at required compositions at a fraction of the computational requirements of current algorithms/frameworks. Structural design of BCC alloy systems from binary to quinay were successfully performed and verified with two different density functional approaches.

Copyright Owner

Aayush Sharma

Language

en

File Format

application/pdf

File Size

178 pages

Available for download on Saturday, November 21, 2020

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