published_or_final_version / abstract / Mechanical Engineering / Doctoral / Doctor of Philosophy

The relationship between some mechanical properties (tensile, creep and thermal fatigue behaviour) and microstructure has been investigated for lead sheet containing <0.06 wt % Cu (the eutectic composition), made either by a conventional milling (rolling) process, or by continuous casting (Direct Method). Milled lead sheet exhibits a recrystallised microstructure, the grain size of which decreases with increasing copper content; copper is present as particles of ≃1-5 μm long, formed from the initial needles or plates of the cast billet by spheroidisation, or by particle break-up during rolling. A comparison of materials produced by several manufacturers indicates that the copper distribution and final grain size are dependent on the thermo-mechanical history of the sheet. Direct Method (DM) sheet exhibits a cast cellular structure within grains which usually extend through the full thickness of the cast sheet; copper is present as a fine dispersion (particles ≃0.5 μm diameter) at cell boundaries. The copper distribution in both materials is stable to prolonged heating (100 hours at 200<SUP>o</SUP>C), but some grain growth occurred in the milled sheet. It was not possible to obtain a fully dispersed eutectic microstructure at the eutectic composition; primary lead dendrites (or cells) were always present. This is thought to be due to the difficulty of nucleating a copper particle at the very low copper concentrations used in this work. The tensile behaviour of specimens was investigated at various strain rates and temperatures. DM sheet exhibits an increase in UTS both with increasing copper content at each strain rate, and with increasing strain rate for each copper content; no systematic variation of strain with copper content was observed. The UTS of milled lead sheet (at ambient temperature and slow strain rates) was a maximum at 0.02 - 0.03 % copper. A steady increase in UTS with increasing copper content was obtained at higher strain rates (2.67 min<SUP>-1</SUP> and above) and low temperatures (≃200 K) and indicated that a time and temperature dependent softening process is active at ambient temperature and slower strain rates (up to 1.33 min^-1) which is thought to be grain boundary sliding, although no evidence for this has been detected in the recrystallised microstructure. Values of the work-hardening coefficient (n) and the strain-rate sensitivity (m) were determined for milled and DM sheet, and were found in all cases to be high. DM sheet exhibits good creep resistance, which increases with increasing copper content, owing to the large grains and stable grain boundaries at ≃90' to the direction of stress. Milled lead sheet was less creep resistant, exhibiting maximum creep resistance at 0.03% copper; this is thought to be due to competing processes of strengthening by copper (which pins grain boundaries to some extent) and grain boundary sliding, which increases with the corresponding decrease in grain size. Thermal fatigue tests have been conducted externally, using lead flashing lengths fixed to an outside wall, and in the laboratory using specially developed apparatus. The development of the apparatus, specimen shape and test cycle length is described. Cracking is usually multiple and intergranular; examination of the fracture surface indicates a combination of intergranular fatigue, creep processes (cavitation) and regions of ductile failure. The use of acetate replicas to trace crack growth has shown the migration of grain boundaries to directions of high stress in milled lead, and in 0.01% copper DM sheet. Grain boundary migration was not observed in the more stable DM microstructures of higher copper content.

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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

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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.