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Kinetics of Detwinning and Grain Bou...

Sun, Hao .

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Kinetics of Detwinning and Grain Boundary Structural Transformation in Chemical Vapor Deposited Nickel.

Record Type:	Electronic resources : Monograph/item
Title/Author:	Kinetics of Detwinning and Grain Boundary Structural Transformation in Chemical Vapor Deposited Nickel./
Author:	Sun, Hao .
Published:	Ann Arbor : ProQuest Dissertations & Theses, : 2020,
Description:	198 p.
Notes:	Source: Dissertations Abstracts International, Volume: 81-12, Section: B.
Contained By:	Dissertations Abstracts International81-12B.
Subject:	Computational physics. -
Online resource:	http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=27737405
ISBN:	9798645485948

Kinetics of Detwinning and Grain Boundary Structural Transformation in Chemical Vapor Deposited Nickel.
Sun, Hao .

Kinetics of Detwinning and Grain Boundary Structural Transformation in Chemical Vapor Deposited Nickel. - Ann Arbor : ProQuest Dissertations & Theses, 2020 - 198 p.

Source: Dissertations Abstracts International, Volume: 81-12, Section: B.

Thesis (Ph.D.)--University of Toronto (Canada), 2020.

This item must not be sold to any third party vendors.

Nickel carbonyl chemical vapor deposition (CVD) is a high-efficiency process used to produce nickel shell molds, which integrates high yield strength, reasonable ductility, and strong corrosion resistance. These advantageous properties arise from its bimodal grain structure, which consists of a nanocrystalline matrix embedded with large columnar grains filled with nanotwins. However, most nanotwins and nano-grains are removed after annealing at 600 ℃; meanwhile, plentiful dislocations are generated. While the driving force for detwinning is well-understood, the mechanisms for detwinning and dislocations generation without any external load remain elusive. Based on computational simulation results, this thesis investigated the kinetics of detwinning in CVD nickel. First, the necessity of dislocations for detwinning is explained based on theoretical analysis: detwinning requires a long-range driving force for all the atoms in the twin lamella, a force that cannot be provided merely by coherent twin boundaries (CTBs). However, if there were plentiful dislocations inside a twin lamella, each atom would "sense" a driving force to eliminate these dislocations, making it easier to form a stable embryonic detwinning nucleus. Next, molecular dynamics simulations were conducted, and the results showed that the dislocations found inside CVD nickel after annealing are intrinsic grain boundary dislocations (IGBDs) on CTBs with misorientation angles deviated from the theoretically predicted value. The separation between the IGBDs and CTBs is driven by the internal stress intensified by grain growth in the nanocrystalline regime of CVD nickel. The stress intensification arising from grain growth is attributed to the grain boundary excess free volume (BFV). The extrapolation of the simulation results into the range of experimental strain rates suggests that the IGBDs can separate from CTBs due to creep at 800 oC. These dislocations then trigger the formation of incoherent twin boundaries (ITBs) from the intersection of CTBs and grain boundaries. The migration of the ITBs removes the nanotwins connecting to them. Overall, unlike grain growth, stress is necessary for detwinning. Our understanding of CSL boundaries, the temperature-dependent BFV, and the kinetic of detwinning is instrumental in controlling the microstructure of nanotwins and nanocrystals at elevated temperatures.

ISBN: 9798645485948Subjects--Topical Terms:

3343998
Computational physics.
Subjects--Index Terms:

Detwinning

Kinetics of Detwinning and Grain Boundary Structural Transformation in Chemical Vapor Deposited Nickel.
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020 $a 9798645485948
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100 1 $a Sun, Hao . $3 3547459
245 1 0 $a Kinetics of Detwinning and Grain Boundary Structural Transformation in Chemical Vapor Deposited Nickel.
260 1 $a Ann Arbor : $b ProQuest Dissertations & Theses, $c 2020
300 $a 198 p.
500 $a Source: Dissertations Abstracts International, Volume: 81-12, Section: B.
500 $a Advisor: Singh, Chandra Veer.
502 $a Thesis (Ph.D.)--University of Toronto (Canada), 2020.
506 $a This item must not be sold to any third party vendors.
520 $a Nickel carbonyl chemical vapor deposition (CVD) is a high-efficiency process used to produce nickel shell molds, which integrates high yield strength, reasonable ductility, and strong corrosion resistance. These advantageous properties arise from its bimodal grain structure, which consists of a nanocrystalline matrix embedded with large columnar grains filled with nanotwins. However, most nanotwins and nano-grains are removed after annealing at 600 ℃; meanwhile, plentiful dislocations are generated. While the driving force for detwinning is well-understood, the mechanisms for detwinning and dislocations generation without any external load remain elusive. Based on computational simulation results, this thesis investigated the kinetics of detwinning in CVD nickel. First, the necessity of dislocations for detwinning is explained based on theoretical analysis: detwinning requires a long-range driving force for all the atoms in the twin lamella, a force that cannot be provided merely by coherent twin boundaries (CTBs). However, if there were plentiful dislocations inside a twin lamella, each atom would "sense" a driving force to eliminate these dislocations, making it easier to form a stable embryonic detwinning nucleus. Next, molecular dynamics simulations were conducted, and the results showed that the dislocations found inside CVD nickel after annealing are intrinsic grain boundary dislocations (IGBDs) on CTBs with misorientation angles deviated from the theoretically predicted value. The separation between the IGBDs and CTBs is driven by the internal stress intensified by grain growth in the nanocrystalline regime of CVD nickel. The stress intensification arising from grain growth is attributed to the grain boundary excess free volume (BFV). The extrapolation of the simulation results into the range of experimental strain rates suggests that the IGBDs can separate from CTBs due to creep at 800 oC. These dislocations then trigger the formation of incoherent twin boundaries (ITBs) from the intersection of CTBs and grain boundaries. The migration of the ITBs removes the nanotwins connecting to them. Overall, unlike grain growth, stress is necessary for detwinning. Our understanding of CSL boundaries, the temperature-dependent BFV, and the kinetic of detwinning is instrumental in controlling the microstructure of nanotwins and nanocrystals at elevated temperatures.
590 $a School code: 0779.
650 4 $a Computational physics. $3 3343998
650 4 $a Mechanics. $3 525881
650 4 $a Materials science. $3 543314
653 $a Detwinning
653 $a Grain growth
653 $a Intrinsic grain boundary dislocations
653 $a Molecular dynamics simulations
653 $a Nanotwins
653 $a Nickel carbonyl chemical vapor deposition
690 $a 0794
690 $a 0216
690 $a 0346
710 2 $a University of Toronto (Canada). $b Mechanical and Industrial Engineering. $3 2100959
773 0 $t Dissertations Abstracts International $g 81-12B.
790 $a 0779
791 $a Ph.D.
792 $a 2020
793 $a English
856 4 0 $u http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=27737405