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Computational Studies of the Mechani...

Fan, Meng.

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Computational Studies of the Mechanical Behavior of Metallic Glasses.

紀錄類型:	書目-電子資源 : Monograph/item
正題名/作者:	Computational Studies of the Mechanical Behavior of Metallic Glasses./
作者:	Fan, Meng.
出版者:	Ann Arbor : ProQuest Dissertations & Theses, : 2019,
面頁冊數:	122 p.
附註:	Source: Dissertations Abstracts International, Volume: 81-10, Section: B.
Contained By:	Dissertations Abstracts International81-10B.
標題:	Mechanical engineering. -
電子資源:	http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=22587523
ISBN:	9798607313661

Computational Studies of the Mechanical Behavior of Metallic Glasses.
Fan, Meng.

Computational Studies of the Mechanical Behavior of Metallic Glasses. - Ann Arbor : ProQuest Dissertations & Theses, 2019 - 122 p.

Source: Dissertations Abstracts International, Volume: 81-10, Section: B.

Thesis (Ph.D.)--Yale University, 2019.

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

Metallic glasses (MG) are alloys with disordered atomic-scale structure. They possess promising mechanical properties, such as superior strength and hardness and a high elastic strain limit. Further, there is no theoretical framework to predict the complex spatiotemporal response of MG to applied deformations, and thus it is currently difficult to design and optimize their mechanical properties. In this thesis research, I use molecular dynamics (MD) simulations to study the mechanical properties of MG, such as the ductility, reversibility, elasticity, and yielding behavior on the atomic scale. In particular, this work identifies several key connections between atomic rearrangements and the macro-scale mechanical responses of MG. First, I characterize the nonlinear mechanical response of binary Lennard-Jones glasses subjected to athermal quasistatic (AQS) pure shear. I decompose the nonlinear stress versus strain into softening- and rearrangement-induced losses and study them as a function of strain, cooling rate and system size. I also characterize the structure of the potential energy landscape (PEL) in the shear strain direction, and find a dramatic change in the geometry of the landscape near the yielding transition.Mechanical yielding is poorly understood in MG. In the second project, I focus on shear strain near yielding. I identify several quantities that show significant changes as the strain is increased above yielding, including the system-size scaling of rearrangement size and frequency, and the decay exponent of energy drop distribution. Particularly, I find that the atomic rearrangement statistics can serve as order parameter for the yielding transition.Third, I identify the connection between rearrangement statistics and glass properties such as ductility and reversibility. I find that more rapidly cooled glasses show stronger rearrangements under applied AQS deformation, which further reduce stress accumulation and prevent catastrophic brittle failure. As a result, more rapidly cooled glasses are more ductile and less reversible. The intensity of the atomic rearrangements can thus be used to predict the brittleness of MG, even in the putative linear regime of stress versus strain. Metallic glasses are a promising materials class for micro- and nano-resonators since they are amorphous and can be fabricated precisely into complex shapes on these lengthscales. In the fourth project, I employ MD simulations of MG resonators undergoing bending vibrations to quantify the intrinsic dissipation and loss mechanisms caused by thermal fluctuations and atomic rearrangements. I find a critical kinetic energy per atom, $K_r$, above which atomic rearrangements occur, and there is significant energy leakage from the fundamental mode to higher frequencies, causing finite quality factor. I find that decreasing the system size and cooling rate will increase $K_r$ and thus improve resonator performance.In the fifth project, I apply AQS cyclic training on model glasses and study the effects of cyclic training on the characteristic rearrangement strain and irreversibility strain, as a function of system size, cooling rate, strain amplitude and training protocol. I show that more slowly cooled glasses are more difficult to train since they already exist in low-lying minima in the PEL.

ISBN: 9798607313661Subjects--Topical Terms:

649730
Mechanical engineering.
Subjects--Index Terms:

Amorphous solid

Computational Studies of the Mechanical Behavior of Metallic Glasses.
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500 $a Advisor: O'Hern, Corey S.
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506 $a This item must not be sold to any third party vendors.
520 $a Metallic glasses (MG) are alloys with disordered atomic-scale structure. They possess promising mechanical properties, such as superior strength and hardness and a high elastic strain limit. Further, there is no theoretical framework to predict the complex spatiotemporal response of MG to applied deformations, and thus it is currently difficult to design and optimize their mechanical properties. In this thesis research, I use molecular dynamics (MD) simulations to study the mechanical properties of MG, such as the ductility, reversibility, elasticity, and yielding behavior on the atomic scale. In particular, this work identifies several key connections between atomic rearrangements and the macro-scale mechanical responses of MG. First, I characterize the nonlinear mechanical response of binary Lennard-Jones glasses subjected to athermal quasistatic (AQS) pure shear. I decompose the nonlinear stress versus strain into softening- and rearrangement-induced losses and study them as a function of strain, cooling rate and system size. I also characterize the structure of the potential energy landscape (PEL) in the shear strain direction, and find a dramatic change in the geometry of the landscape near the yielding transition.Mechanical yielding is poorly understood in MG. In the second project, I focus on shear strain near yielding. I identify several quantities that show significant changes as the strain is increased above yielding, including the system-size scaling of rearrangement size and frequency, and the decay exponent of energy drop distribution. Particularly, I find that the atomic rearrangement statistics can serve as order parameter for the yielding transition.Third, I identify the connection between rearrangement statistics and glass properties such as ductility and reversibility. I find that more rapidly cooled glasses show stronger rearrangements under applied AQS deformation, which further reduce stress accumulation and prevent catastrophic brittle failure. As a result, more rapidly cooled glasses are more ductile and less reversible. The intensity of the atomic rearrangements can thus be used to predict the brittleness of MG, even in the putative linear regime of stress versus strain. Metallic glasses are a promising materials class for micro- and nano-resonators since they are amorphous and can be fabricated precisely into complex shapes on these lengthscales. In the fourth project, I employ MD simulations of MG resonators undergoing bending vibrations to quantify the intrinsic dissipation and loss mechanisms caused by thermal fluctuations and atomic rearrangements. I find a critical kinetic energy per atom, $K_r$, above which atomic rearrangements occur, and there is significant energy leakage from the fundamental mode to higher frequencies, causing finite quality factor. I find that decreasing the system size and cooling rate will increase $K_r$ and thus improve resonator performance.In the fifth project, I apply AQS cyclic training on model glasses and study the effects of cyclic training on the characteristic rearrangement strain and irreversibility strain, as a function of system size, cooling rate, strain amplitude and training protocol. I show that more slowly cooled glasses are more difficult to train since they already exist in low-lying minima in the PEL.
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