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Chow, Wing Hei = 白鑄鐵特殊的微結構--網絡圖案 / 周穎熙. / Thesis submitted in: November 2007. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2008. / Includes bibliographical references. / Text in English; abstracts in English and Chinese. / Chow, Wing Hei = Bai zhu tie te shu de wei jie gou -- wang luo tu an / Zhou Yingxi. / Abstract --- p.i / Acknowledgements --- p.iii / Chapter Chapter 1 --- Introduction / Chapter 1.1 --- Introduction --- p.1 / Chapter 1.2 --- Background of Thermodynamic --- p.3 / Chapter 1.3 --- Nucleation in pure metals --- p.4 / Chapter 1.4 --- Introduction of Growth --- p.7 / Chapter 1.5 --- Solidification of the Single Phase Binary Alloy --- p.9 / Chapter 1.6 --- Solidification of the Binary Eutectic Alloy --- p.12 / Chapter 1.7 --- Phase Separation by Nucleation and Growth --- p.15 / Chapter 1.8 --- How to obtain large undercooling --- p.19 / References --- p.20 / Figures --- p.22 / Chapter Chapter 2 --- Experiment / Chapter 2.1 --- Experimental Preparation --- p.31 / Chapter 2.2 --- Experimental Procedures --- p.32 / Chapter 2.3 --- Sample Analysis --- p.33 / References --- p.36 / Figures --- p.37 / Chapter Chapter 3 --- Network patterns in a white cast iron / Result --- p.38 / Reference --- p.58 / Figures --- p.64

Abstract not available

The development of nanostructured coatings which exhibit enhanced mechanical properties is currently of interest due to the importance of high performance coatings in a large range of applications. Single layer coatings have predominantly been used for these demanding applications, however the promising mechanical properties observed in multilayer coatings has shifted the focus of current research. In particular, there has been reports of the use of alternating materials with opposing mechanical properties, as seen in the abalone shell, which have exhibited hardness and toughness values significantly greater than either of their constituent materials. The main objective of this thesis was to fabricate Al/W nanostructured multilayers and determine if they exhibit enhanced mechanical properties. The Al/W nanostructured multilayers were fabricated using two different deposition techniques: pulsed magnetron sputtering and cathodic arc deposition. These two techniques differ in the energy of the depositing species and this results in significant differences in film properties. The indentation hardness of the coatings was measured using a Hysitron Nanoindenter. The relationship between the mechanical properties and microstructure was obtained using a range of characterisation techniques. Auger electron spectroscopy (AES), energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS) were used to determine the chemical composition and stoichiometry, while cross-sectional transmission electron microscopy (XTEM) and energy filtered transmission electron microscopy (EFTEM) were used to explore the microstructure. The findings of this thesis showed very different results for the two deposition techniques. Although sputtering successfully produced well defined multilayers, no evidence of enhanced hardness was found for periods between 5 and 200 nm. On the other hand, arc deposited samples with intended periods between 1 and 200 nm showed a hardness enhancement above that of pure W, however the samples of highest hardness did not contain Al layers for much of their thickness. Arc deposited samples with the finest nominal periods (1 and 2 nm) contained W-Al intermetallics and were soft. The hardening mechanism was not attributed to a multilayer structure, rather to the introduction of defects in the W layers which acted as pinning sites for dislocations. A modified Hall-Petch equation for hardness enhancement fitted the data for W films prepared by pulsed cathodic arc in which the grain diameter was replaced by the nominal multilayer period. The difficulty producing Al layers on W surfaces in the cathodic arc was overcom e by changing the film growth mechanism by introducing Ar or O2 at the W/Al interface. In the latter case, Al2O3/W multilayers were formed but again showed no hardness enhancements. Complete microanalysis and characterisation of the multilayer structures is vital in determining the mechanisms which govern the hardness enhancements. The evidence in this thesis suggests that the defect density, and not the presence of interfaces are responsible for the hardness enhancement effect.

The subject of this thesis is twofold: (i) to contribute to the understanding of the phase transformations and mechanical properties of TRIP-assisted multiphase steels; (ii) to show to what extent a new TRIP-assisted multiphase steel with a reduced silicon content could meet the performance requirements for high-strength formable steels. It enlightens the correlations between Processing, Microstructure and Properties. Furthermore, the different investigations carried out in this thesis will aim at comparing what can be called a conventional high silicon TRIP-assisted multiphase steel with a newly proposed low silicon TRIP-assisted multiphase steel.

The subject of this thesis is twofold: (i) to contribute to the understanding of the phase transformations and mechanical properties of TRIP-assisted multiphase steels; (ii) to show to what extent a new TRIP-assisted multiphase steel with a reduced silicon content could meet the performance requirements for high-strength formable steels. It enlightens the correlations between Processing, Microstructure and Properties. Furthermore, the different investigations carried out in this thesis will aim at comparing what can be called a conventional high silicon TRIP-assisted multiphase steel with a newly proposed low silicon TRIP-assisted multiphase steel.

Additions of boron in modified titanium alloys and Ti-B composites result in the in situ formation during high temperature processing of TiB reinforcement phases that improve the mechanical properties and wear resistance of unreinforced titanium alloys, while still utilizing the high strength-to-weight ratio and excellent corrosion resistance of titanium. Several boron-modified titanium alloys and Ti-B composites in a Ti-6Al-4V matrix have been investigated to determine the effect of processing parameters on the TiB reinforcement phases and resultant microstructures and mechanical properties. Using optical microscopy, scanning electron microscopy, conventional characterization techniques, and newly developed methodologies for three-dimensional visualization, the microstructures of these Ti-B materials have been studied. Observations included a similar anisotropic whisker morphology with roughly hexagonal cross-sections among all TiB phases; alignment of all TiB phases with extrusion, with the extent of alignment affected by thermomechanical processing parameters; brittle fracture behavior of TiB whiskers, with fracture down the length of whiskers not aligned in the tensile direction and across the width of whiskers aligned in the tensile direction; and discoveries of the anisotropic morphologies of the coarse primary TiB phase and the sub-micron precipitated TiB phase. It has been observed that extruded boron-modified alloys with compositions in the hypoeutectic regime of the quaternary system of titanium, alloying elements aluminum and vanadium, and boron, containing a unimodal size distribution of eutectic TiB whiskers, significantly improve the strength and stiffness compared to unreinforced Ti-6Al-4V alloy while also demonstrating tensile elongation to failure within the fracture-critical limits required for aerospace structural applications. Materials design methodologies have been developed using Ti-B materials, and they show promise for predicting the effects of processing parameters and the resultant microstructures and mechanical properties for boron-modified titanium alloys and Ti-B composites optimized for a variety of commercial and industrial applications.

¡@In this study, the plastic deformation of (001) single crystal silicon at 900 oC is investigated by uniaxial compression along [001]. Using optical microscopy and scanning electron microscopy to observe the sample surface, and analysis the corresponding microstructure of different strain deformed silicon by transmission electron microscopy, particularly dislocation, slip systems and work hardening behaviour. ¡@Experimental temperature condition was chosen which over the BDT temperature range of silicon. We were found similar stages of work hardening described in metals and use the result of analysis dislocations and slip systems to suggest a hardening process of plastic deformation. ¡@The observations are similar to the feature in stage II (dislocation tangled) and stage III (dislocation cell structure). Partial dislocation and dipole are formed in less strained sample. We suggest a hardening process of plastic deformation from these results.