Full metadata record
| DC Field | Value | Language |
|---|---|---|
| dc.contributor | Department of Industrial and Systems Engineering | en_US |
| dc.contributor.advisor | Yang, Xusheng (ISE) | en_US |
| dc.contributor.advisor | Chan K. C. (ISE) | en_US |
| dc.creator | Qian, Lei | - |
| dc.identifier.uri | https://theses.lib.polyu.edu.hk/handle/200/13407 | - |
| dc.language | English | en_US |
| dc.publisher | Hong Kong Polytechnic University | en_US |
| dc.rights | All rights reserved | en_US |
| dc.title | Atomistic simulations of enhanced mechanical properties of Cu-based hetero-nanostructures via boundary engineering | en_US |
| dcterms.abstract | Nanostructuring stands out as a prominent strategy widely employed to enhance the mechanical properties of both crystalline and amorphous metals. Nanograined metals, characterized by refined grain size and increased grain boundary density, benefit from Hall-Petch strengthening. However, homogeneous nanograins with extremely fine sizes can experience a strengthening-softening transition, known as the Hall-Petch breakdown. To address this, substantial efforts have been dedicated to introducing structural and/or chemical heterogeneity at the nanoscale to strengthen and stabilize nanograins, highlighting the significance of constructing heterogeneous nanostructures alongside grain refinement to improve the mechanical properties of nanostructured metals. | en_US |
| dcterms.abstract | Heterogeneous nanostructures typically involve spatial variations in structural characteristics (e.g., grain size, twin thickness and lamellar thickness) and/or chemical composition. Successful attempts to enhance mechanical properties have simultaneously leveraged the benefits of strong and stable heterogeneous nanostructures and optimized structural characteristics and/or chemical compositions. In this work, we propose three strategies for engineering boundaries to develop property-enhanced heterogenous nanostructures: (1) Embedding nanotwins into nanograins to construct nanograined-nanotwinned Cu with twin-modified grain boundaries; (2) Forming crystalline-amorphous (Cu-CuTa) nanocomposites with nanograins surrounded by nanosized amorphous grain boundaries; (3) Segregating Zr into nanograined Cu with extremely fine nanograins to induce structural transformation of grain boundary regions. Molecular dynamic simulations are employed to explore the size/composition-dependent mechanical responses and underlying mechanistic rationales of constructed heterogenous nanostructures. | en_US |
| dcterms.abstract | In the first part, a series of multi-temperature (300 K-800 K) creep tests at different sustained stress levels (0.2 GPa-2.0 GPa) were conducted by molecular dynamic simulations on twin-free nanograined Cu (grain size between 13.5-27 nm) and nanograined-nanotwinned Cu (grain size of 13.5 nm with twin thickness ranging 1.25 nm to 5 nm), respectively. The nanograined-nanotwinned structure can significantly enhance creep resistance relative to twin-free nanograined counterparts. Based on the classic Mukherjee-Bird-Dorn equation, the multi-temperature creep tests allow us to define and obtain the creep parameters (e.g. activation energy, activation volume, prestress exponent, and grain size/twin thickness exponent) and thus further build up the formula to describe the characteristic sizes (grain size/twin thickness)-, times, stress-, and temperature-dependent creep behaviors and corresponding plastic deformation mechanisms, which are also validated via the examination of atomic configurations, statistical analyses, and the summarized creep deformation maps. | en_US |
| dcterms.abstract | In the second part, we employ molecular dynamics simulations to investigate Cu-CuTa crystalline-amorphous nanocomposites with varying grain sizes and amorphous thicknesses. Our findings demonstrate significant strengthening effects in nanocomposites, effectively suppressing the Hall-Petch breakdown observed in traditional amorphous-free nanograined Cu. Intriguingly, we reveal a maximum strength followed by a strengthening-softening transition dependent on the amorphous thickness, as exemplified by a representative nanocomposite featuring a 12.5 nm grain size and a critical amorphous thickness of 4 nm. Inspired by observed shifts in atomistic mechanisms, we develop atheoretical model encompassing variations in grain size and amorphous thickness, providing valuable insights into the size-strength relationship for crystalline-amorphous nanocomposites. | en_US |
| dcterms.abstract | In the third part, we investigate the interplay between solute concentration, grain boundary structure, and strength performance in Zr-segregated nanograined Cu with extremely fine grain sizes by employing hybrid Monte Carlo/Molecular Dynamics simulations. Through varying segregation concentrations, sequentially obtained segregated grain boundaries and amorphous grain boundaries present distinct deformation mechanisms and strengthening effects. Notably, optimal strength for each grain size is attained when grain boundaries transit into a fully amorphous state. The maximum strength enhancement due to grain boundary amorphization increases with reducing grain size, thus extending Hall-Petch strengthening down to a grain size of ~2.5 nm. | en_US |
| dcterms.abstract | Building upon boundary engineering in nanograined Cu using molecular dynamic simulations, we have successfully designed and constructed several Cu-based heterogeneous nanostructures with significantly enhanced mechanical properties. These enhancements primarily focus on improving creep resistance to mechanical and thermal stimuli and achieving optimal strength performance when approaching the grain size limit. The key contributions and significance of this work are as follows: (1) revealing enhanced creep resistance and underlying mechanisms of nanograined-nanotwinned Cu and proposing corresponding theoretical model to depicting the characteristic sizes-, stress-, and temperature-dependent creep behaviors; (2) providing the guidance to alleviate the strength-ductility/creep resistance trade-off existing in nanograined metals, e.g. by incorporating nanotwins and tailoring its thickness in nanograined-nanotwinned metals; (3) unveiling the amorphous thickness-dependent strengthening-softening transition in crystalline-amorphous nanocomposites; (4) developing a theoretical model encompassing variations in grain size and amorphous grain boundary thickness to elucidate the size-strength relation in the crystalline-amorphous nanocomposites; (5) inspiring a promising pathway of segregation induced grain boundary amorphization to simultaneously exploit the benefits of strongest grain boundary structure and the smallest grain size for strengthening extremely fine nanograined metals. | en_US |
| dcterms.extent | xix, 178 pages : color illustrations | en_US |
| dcterms.isPartOf | PolyU Electronic Theses | en_US |
| dcterms.issued | 2024 | en_US |
| dcterms.educationalLevel | Ph.D. | en_US |
| dcterms.educationalLevel | All Doctorate | en_US |
| dcterms.LCSH | Nanostructured materials | en_US |
| dcterms.LCSH | Nanostructures | en_US |
| dcterms.LCSH | Grain boundaries | en_US |
| dcterms.LCSH | Hong Kong Polytechnic University -- Dissertations | en_US |
| dcterms.accessRights | open access | en_US |
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