This work demonstrates how local, randomized tailoring of bond stiffness can affect the activation energy of diffusion in model alloys using density functional theory-based computations. This work is organized into two parts. The first part deals with the vacancy diffusion mechanism, and it compares the in–plane (IP) vs out-of-plane (OOP) diffusion paths in prototypical binary Mg-X (Ca, Y, and Gd) and ternary Mg-X (Ca, Y, and Gd)-Zn alloys. We examine how vacancy formation, migration, and solute vacancy binding energies in binary Mg-X alloys influence diffusion activation and correlated them with conventional diffusion model based solely on the solute sizes. Next, we explore how Zn addition to binary Mg-X (Ca, Y, and Gd) alloys influences the OOP activation energy barrier is discussed in terms of detailed energetic computations and bond characterization in the present work. Our results indicate that Zn addition further enhances the OOP activation energy barrier compared to corresponding activation energies in Mg binaries. This work concludes that engineering stiffer directional bonds via micro-alloying additions in Mg is a promising route to dramatically improve their high temperature creep response.
The second part of my work investigates the effects of Si, P, and S solutes on H interstitial diffusion mechanism in Ni. It examines how H interacts with vacancy, impurity atom, and vacancy-impurity atom defect pair by performing binding energy calculations. Results indicate that vacancy-impurity atom defect pair strongly traps the H atom compared to isolated defects. Finally, the effect of impurities on activation energy barrier of H diffusing in Ni is discussed by correlating migration energetics with bonding characteristics by performing charge density and electron density calculations. Our study validates experimental hypothesis of Berkowitz and Kane which postulates that P enhances the H diffusion in Ni. The present work also shows that H diffusion speeds up in Ni in the presence of Si and S solutes. In conclusion, we show that micro-alloying additions induce local lattice level pockets with covalent character, which substantially enhances the local bond stiffness. This will increase activation energy for vacancy diffusion mechanism while it reduces activation energy for interstitial H diffusion.
| Identifer | oai:union.ndltd.org:unt.edu/info:ark/67531/metadc1873854 |
| Date | 12 1900 |
| Creators | Paranjape, Priyanvada Madhukar |
| Contributors | Srivilliputhur, Srinivasan, Choudhuri, Deep, Dahotre, Narendra, Banerjee, Rajarshi, Scharf, Thomas |
| Publisher | University of North Texas |
| Source Sets | University of North Texas |
| Language | English |
| Detected Language | English |
| Type | Thesis or Dissertation |
| Format | xi, 118 pages, Text |
| Rights | Public, Paranjape, Priyanvada Madhukar, Copyright, Copyright is held by the author, unless otherwise noted. All rights Reserved. |
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