Fang, Zhengwu
(2024)
Atomic-scale In Situ TEM Investigation of Grain Boundary Deformation Behavior in FCC Gold.
Doctoral Dissertation, University of Pittsburgh.
(Unpublished)
Abstract
The deformation mechanism of metallic polycrystalline materials is known to transform from dislocation-mediated plasticity into grain boundary (GB)-mediated plasticity when the crystal size decreases to sub-10 nm. Numerous studies have been performed in the past decades to reveal the mechanisms of GB deformation. However, the dynamic process of GB deformation at the atomic scale remains largely unclear due to the lack of real-time experimental observation. In this dissertation, in-situ high-resolution transmission electron microscopy (HRTEM) combined with molecular dynamics (MD) simulations have been employed to investigate the typical GB deformation behavior in face-centered cubic (FCC) gold (Au) nanocrystals at the atomic scale.
Dynamic GB structural transformation arising from reversible facet transformation and GB dissociation was observed during the migration of faceted GBs. It is found that the types of steps/disconnections mediating the migration of GB facets determine which pathway the GB structural transformation follows. Moreover, the loading dependence of GB facet transformation and the roles of GB junctions in accommodating GB migration and structural transformation are clarified.
Regarding the case of mixed tilt-twist GB, two distinct migration patterns showing the opposite signs of shear-coupling factor were observed, and further revealed to be mediated by the motion of GB disconnections with different crystallographic parameters and exhibit different lattice correspondence relations. MD simulation results confirm that the two distinct migration patterns could be activated under different stress/strain states. Furthermore, excess GB sliding and GB plane reorientation were found to accommodate the GB migration in both experiments and simulations, as to establish a point-to-point lattice correspondence during GB migration.
Additionally, the deformation-induced formation and annihilation of a typical HAGB in an Au nanocrystal upon reciprocating bending deformation was investigated. HAGB formation underwent the process of accumulation, alignment, further accumulation, and eventually exhaustion of geometrically necessary dislocations. In comparison, HAGB annihilation was accomplished by the synergic operation of GB structure reconstruction, emission of partial and full dislocations, and twinning.
This dissertation advances the fundamental understanding of atomic-scale GB deformation behavior in FCC materials and provides important guidelines for the future development of ductile nanocrystalline materials and reliable nanocrystal components in nano electromechanical systems devices.
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Details
| Item Type: |
University of Pittsburgh ETD
|
| Status: |
Unpublished |
| Creators/Authors: |
|
| ETD Committee: |
|
| Date: |
11 January 2024 |
| Date Type: |
Publication |
| Defense Date: |
30 August 2023 |
| Approval Date: |
11 January 2024 |
| Submission Date: |
7 September 2023 |
| Access Restriction: |
No restriction; Release the ETD for access worldwide immediately. |
| Number of Pages: |
145 |
| Institution: |
University of Pittsburgh |
| Schools and Programs: |
Swanson School of Engineering > Mechanical Engineering |
| Degree: |
PhD - Doctor of Philosophy |
| Thesis Type: |
Doctoral Dissertation |
| Refereed: |
Yes |
| Uncontrolled Keywords: |
Grain boundary migration; grain refinement; in-situ TEM; Grain boundary structural transformation |
| Date Deposited: |
11 Jan 2024 19:28 |
| Last Modified: |
11 Jan 2024 19:28 |
| URI: |
http://d-scholarship.pitt.edu/id/eprint/45390 |
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