| Author: | Chen, Kaiguo |
| Title: | Molecular dynamics modeling of interfacial plasticity in nanomaterials |
| Advisors: | Shi, San-qiang (ME) |
| Degree: | Ph.D. |
| Year: | 2015 |
| Subject: | Nanostructured materials. Interfaces (Physical sciences) Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Mechanical Engineering |
| Pages: | 189 pages : illustrations (some color) ; 30 cm |
| Language: | English |
| Abstract: | The interface has been confirmed over the last decade as playing a key role for nanomaterials, and has attracted widespread attention from research communities. It is not surprising that interfacial plasticity becomes much more pronounced in nanomaterial than in conventional materials due to the dramatic increase in interface atom fraction with decreasing geometric or structural material size to the nano scale. Boundless possibilities of interfacial plasticity have opened a window for developing materials with both high strength and ductility, which has been a long-standing goal for material scientists. Current knowledge obtained from different nanomaterials, such as nanocrystalline metals, nanotwinned materials, nanolaminates, nanowire and structural biomaterials, indicates that the ability of an interface to carry and transmit plastic events corresponds to the materials’ strengths and deformation homogeneity, respectively. However, the study of interfacial plasticity has remained quite challenging both experimentally and theoretically due to the complexity originating from the fact that the problem relies on both external loading conditions and internal microstructure, the latter have numerous possibilities. The interfacial plasticity in several typical nanomaterials has been investigated in this work by molecular dynamics (MD) simulation. First, a multi-mode deformation model was introduced to MD simulation of nanocrystalline copper. Abundant deformation twin (DT) lamellae developed during shearing following compression to the elastic limit. The DTs nucleated through two different mechanisms facilitated by Shockley partial slips. And we also found that DTs’ densities increase with increasing twin spacing. The interactions between DT and Shockley partials were observed in this simulation. Second, the tensile properties along the <111> direction of single crystalline copper with 90o nano twins are investigated by molecular dynamics simulation. The following results are observed. First, twin boundaries do not serve as a dislocation source at the very beginning of plastic deformation; instead, dislocations nucleate inside the crystal between the twin boundaries. Second, twin boundaries provide obstacles to the motion of dislocations on the inclined glide plane, and allow dislocations to move through under high stress. When the spacing of the twin boundaries is greater than 10 nm, deformation twinning starts to form during plastic deformation. Third, the flow strength of single crystal copper with nano twins increases as the twin spacing decreases, which resembles a Hall-Petch-like relationship. The strengthening mechanism is explained by a simple dislocation model. Third, the properties of Cu46Zr54 amorphous/crystalline interface and their effects on mechanical responses are studied via molecular dynamics simulation. Structural heterogeneity is observed in a Cu46Zr54 layer with both an as-quenched sample and a separately-quenched sample. A new multi-yielding scenario of interfacial sliding and the thickening of micro sliding bands is proposed by our simulations. During shear deformation, both samples yield first via the formation of shear transformation zones (STZs) in amorphous layers. After the STZ formation, micro sliding bands with highly localized atomic shear strain are developed in both samples via different interfacial mechanisms, sliding via STZs growth at ACIs for the separately-quenched sample and sliding via dislocation loop spreading at ACIs for the as-quenched sample. The thickening of micro sliding bands on an amorphous layer via internal friction is found to be a new plastic deformation mechanism when loading conditions are appropriate. The thickening rate in the as-quenched sample is higher than that in the separately-quenched sample. Finally, a crystalline layer yield via partial dislocation slip. An analytical model suggests that this new multi-yielding scenario should be expected to operate in bulk metallic glass based composites. Finally, by utilizing molecular dynamics, tensile deformation of copper nanowire with an amorphous Cu46Zr54 coating layer is simulated. Even though the presence of the amorphous coating layer on the surface of nanowire is quite common, its impact on mechanical response has not been discussed. Unprecedented size-dependent elasticity and plasticity in atomistic simulations is observed in core-shell nanowire. Results show that both Young’s modulus and the yield strength of copper nanowire are significantly reduced by the amorphous layer and decrease with decreasing nanowire diameter and increasing coating thickness. A simple core-shell model is presented to explain our results and may explain experimental results. After elastic response, the amorphous layer yields first via formation of shear transformation zones (STZs) and can further suppress the serrated flow of copper nanowire because STZs provide continuum dislocation nucleation sites, avoiding the establishment of dislocation starvation states in nanowire. This dissertation includes a diverse discussion of interfacial plasticity in four nanomaterial models, as revealed by molecular dynamics. The atomic details of several interface-mediated plastic deformation mechanisms are presented. A bridge between knowledge obtained by MD simulations and the macro mechanical properties of nanomaterials is shown. The results help us to further understand material behavior from an angle of interface-mediated plastic deformation. We believe that this dissertation may help to design high strength and high ductility materials via introducing appropriate interfacial plasticity. |
| Rights: | All rights reserved |
| Access: | open access |
Files in This Item:
| File | Description | Size | Format | |
|---|---|---|---|---|
| b28068397.pdf | For All Users | 6.35 MB | Adobe PDF | View/Open |
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