Yinkai, Lei
(2016)
Understanding Materials Behavior from Atomistic Simulations: Case study of Al-containing High Entropy Alloys and Thermally Grown Aluminum Oxide.
Doctoral Dissertation, University of Pittsburgh.
(Unpublished)
Abstract
Atomistic simulation refers to a set of simulation methods that model the materials on the atomistic scale. These simulation methods are faster and cheaper alternative approaches to investigate thermodynamics and kinetics of materials compared to experiments. In this dissertation, atomistic simulation methods have been used to study the thermodynamic and kinetic properties of two material systems, i.e. the entropy of Al-containing high entropy alloys (HEAs) and the vacancy migration energy of thermally grown aluminum oxide.
In the first case study of the dissertation, a computational scheme for evaluating the entropy of HEAs has been developed. Entropy is a key factor for the phase stability of HEAs. However, it has not been well understood yet. In this study, atomistic simulation methods have been used to quantify the configurational and vibrational entropy of HEAs for the first time. Modified embedded atom method was used to describe the interatomic interactions in HEAs. Monte Carlo simulation and thermodynamic integration method were used to calculate the thermodynamic properties such as entropy and free energy. This scheme has been tested on AlxCoCrFeNi HEAs. The results show that a reasonable evaluation of the entropy of AlxCoCrFeNi HEAs can be obtained by the developed scheme. The FCC
to BCC phase transition in this alloy system has also been captured by the calculated free energy. Importantly, it is found that atomic vibrations have an important effect on the quantitative prediction of the compositional boundary of the FCC-BCC duplex region in the AlxCoCrFeNi HEA system. The calculated entropy has been validated by comparing the atomic ordering in the simulated HEAs to the HEAs in experiments. The good agreement between the simulations and experiments indicates that the developed computational scheme captured the non-ideality in HEAs which is the key to understand the entropy of HEAs.
In the second case study of this dissertation, the charge effect on the vacancy diffusion in α-Al2O3 has been investigated. It has been known that the charge state has an effect on the formation energy of vacancies. However, the relation between the charge state and the migration energy of vacancies is unknown yet. In this study, density functional theory calculations have been used to investigate the charge effect on the vacancy migration energy. It is found that the vacancy migration energy depends strongly on the charge state of the vacancy. This dependency is explained by the shift of the defect levels associated with the vacancy and the electron occupancy on the defect levels. These findings for the first time built a link between the electronic structure and the migration of vacancy in metal oxides. This information indicates a novel approach to tune the diffusion kinetics by modifying the electronic structure of metal oxides.
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| Item Type: |
University of Pittsburgh ETD
|
| Status: |
Unpublished |
| Creators/Authors: |
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| ETD Committee: |
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| Date: |
26 January 2016 |
| Date Type: |
Publication |
| Defense Date: |
1 October 2015 |
| Approval Date: |
26 January 2016 |
| Submission Date: |
19 October 2015 |
| Access Restriction: |
3 year -- Restrict access to University of Pittsburgh for a period of 3 years. |
| Number of Pages: |
104 |
| Institution: |
University of Pittsburgh |
| Schools and Programs: |
Swanson School of Engineering > Materials Science and Engineering |
| Degree: |
PhD - Doctor of Philosophy |
| Thesis Type: |
Doctoral Dissertation |
| Refereed: |
Yes |
| Uncontrolled Keywords: |
atomistic simulations, Monte Carlo, density functional theory, modified em- bedded atom method, thermodynamic integration, high entropy alloy, configurational entropy, vibrational entropy, α-Al2O3, vacancy migration energy, charge effect, defet level. |
| Date Deposited: |
26 Jan 2016 16:11 |
| Last Modified: |
26 Jan 2019 06:15 |
| URI: |
http://d-scholarship.pitt.edu/id/eprint/26131 |
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